> ## Documentation Index > Fetch the complete documentation index at: https://docs.iotools.cloud/llms.txt > Use this file to discover all available pages before exploring further. # Reynolds Number Calculator > Calculate the Reynolds number for pipe or duct flow and see whether it is laminar, transitional or turbulent. Enter a velocity or a volumetric flow rate, pick a fluid preset (water, air, oil, glycerin) or your own density and viscosity, and work in metric or imperial units. [Try Reynolds Number Calculator in your browser →](https://iotools.cloud/tool/reynolds-number-calculator/) ## OpenAPI ````yaml https://api.iotools.cloud/v1/openapi post /v1/tool/reynolds-number-calculator openapi: 3.1.0 info: title: iotools.cloud API version: 1.0.0 description: >- Run any iotools.cloud tool over HTTP. Authenticate with `Authorization: Bearer iot_live_…`. **Only `POST /v1/tool/{slug}` costs credits.** Every GET here — the catalog, a tool's schema, your balance — is free. A tool call is charged its own weight or your plan's per-call minimum, whichever is larger; `x-iotools-credit-cost` on each operation is quoted at the free-tier minimum, and `GET /v1/tools/list` returns the exact figure for your key. `GET /v1/me/credits` reports what you have left, and `GET /v1/me/usage` reports what it went on. Failures are RFC 9457 problem documents — branch on `code`. servers: - url: https://api.iotools.cloud description: Production security: - bearerAuth: [] tags: - name: Catalog description: Find a tool and read its contract. Free. - name: Converters description: >- Convert between formats, encodings, and units — Base64, CSV and JSON, timestamps, and more. Fast, free, and processed right in your browser. - name: Formatters description: >- Format, beautify, minify, and validate code and data — JSON, HTML, CSS, SQL, and regex. Clean up messy input in one click, with nothing to upload. - name: Generators description: >- Generate exactly what you need and on demand — passwords, UUIDs, QR codes, hashes, random numbers, and more. Secure, instant, and free. - name: Calculators description: >- Crunch the numbers fast — from everyday math to specialized conversions and unit work. Free online calculators that run entirely in your browser. - name: Editors description: >- Edit and transform text, code, and images with quick, focused editors that run entirely in your browser — nothing to install, and no sign-up needed. - name: Utilities description: >- Everyday developer and web utilities — DNS and IP lookups, redirect and certificate checkers, and other quick diagnostics. Free and privacy-friendly. - name: Account description: Your key's allowance, limits and usage history. paths: /v1/tool/reynolds-number-calculator: post: tags: - Calculators summary: Reynolds Number Calculator description: >- Calculate the Reynolds number for pipe or duct flow and see whether it is laminar, transitional or turbulent. Enter a velocity or a volumetric flow rate, pick a fluid preset (water, air, oil, glycerin) or your own density and viscosity, and work in metric or imperial units. [Try Reynolds Number Calculator in your browser →](https://iotools.cloud/tool/reynolds-number-calculator/) operationId: run_reynolds_number_calculator requestBody: required: false content: application/json: schema: type: object properties: inputMode: enum: - velocity - flow description: Known Quantity velocity: type: number description: Mean Velocity minimum: 0 velocityUnit: enum: - mps - fps description: Velocity Unit flowRate: type: number description: Volumetric Flow Rate minimum: 0 flowUnit: enum: - lpm - m3h - gpm - cfm description: Flow Unit geometry: enum: - circular - rectangular - hydraulic description: Cross-section pipeDiameter: type: number description: Inside Diameter minimum: 0 ductWidth: type: number description: Duct Width minimum: 0 ductHeight: type: number description: Duct Height minimum: 0 hydraulicDiameter: type: number description: Hydraulic Diameter minimum: 0 lengthUnit: enum: - mm - in - m description: Length Unit fluid: enum: - water20 - water60 - seawater20 - air20 - air100 - ethanol20 - sae30 - glycerin20 - mercury20 - custom description: Fluid density: type: number description: Density (kg/m³) minimum: 0 viscosity: type: number description: Viscosity minimum: 0 viscosityUnit: enum: - cp - pas - cst description: Viscosity Unit required: [] examples: water_at_2_m_s_in_a_50_mm_pipe: summary: Water at 2 m/s in a 50 mm pipe value: inputMode: velocity velocity: '2' velocityUnit: mps geometry: circular pipeDiameter: '50' lengthUnit: mm fluid: water20 100_l_min_of_sae_30_oil_in_a_25_mm_pipe: summary: 100 L/min of SAE 30 oil in a 25 mm pipe value: inputMode: flow flowRate: '100' flowUnit: lpm geometry: circular pipeDiameter: '25' lengthUnit: mm fluid: sae30 air_in_a_300_200_mm_duct: summary: Air in a 300 × 200 mm duct value: inputMode: velocity velocity: '5' velocityUnit: mps geometry: rectangular ductWidth: '300' ductHeight: '200' lengthUnit: mm fluid: air20 responses: '200': description: Tool output content: application/json: examples: water_at_2_m_s_in_a_50_mm_pipe: summary: Water at 2 m/s in a 50 mm pipe value: tool: reynolds-number-calculator tool_version: 1.0.1 outputs: status: >- Re = 99,621 — turbulent flow. Water, 20 °C at 2 m/s, hydraulic diameter 0.05 m. Chaotic mixing — use Colebrook/Moody for the friction factor. result: - property: Reynolds number value: 99,621 - property: Flow regime value: Turbulent - property: Velocity value: 2 m/s - property: Hydraulic diameter value: 0.05 m - property: Density value: 998.2 kg/m³ - property: Dynamic viscosity value: 0.001002 Pa·s - property: Kinematic viscosity value: 1.004e-6 m²/s - property: Flow area value: 0.001963 m² - property: Volumetric flow value: 0.003927 m³/s (235.62 L/min) - property: Formula value: Re = ρVD/μ = 998.2 × 2 × 0.05 / 0.001002 regimeTable: - regime: Laminar reynolds-range: Re < 2300 behaviour: Smooth, orderly layers — friction factor f = 64/Re. this-flow: '' - regime: Transitional reynolds-range: 2300 – 4000 behaviour: >- Unstable — may switch between laminar and turbulent. Avoid designing here. this-flow: '' - regime: Turbulent reynolds-range: Re > 4000 behaviour: >- Chaotic mixing — use Colebrook/Moody for the friction factor. this-flow: ◄ This flow credits_used: 3 credits_remaining: null 100_l_min_of_sae_30_oil_in_a_25_mm_pipe: summary: 100 L/min of SAE 30 oil in a 25 mm pipe value: tool: reynolds-number-calculator tool_version: 1.0.1 outputs: status: >- Re = 943 — laminar flow. SAE 30 engine oil, 40 °C at 100 L/min (3.395 m/s), hydraulic diameter 0.025 m. Smooth, orderly layers — friction factor f = 64/Re. result: - property: Reynolds number value: '943' - property: Flow regime value: Laminar - property: Velocity value: 3.395 m/s - property: Hydraulic diameter value: 0.025 m - property: Density value: 875 kg/m³ - property: Dynamic viscosity value: 0.0788 Pa·s - property: Kinematic viscosity value: 9.006e-5 m²/s - property: Flow area value: 0.0004909 m² - property: Volumetric flow value: 0.001667 m³/s (100 L/min) - property: Formula value: Re = ρVD/μ = 875 × 3.395 × 0.025 / 0.0788 regimeTable: - regime: Laminar reynolds-range: Re < 2300 behaviour: Smooth, orderly layers — friction factor f = 64/Re. this-flow: ◄ This flow - regime: Transitional reynolds-range: 2300 – 4000 behaviour: >- Unstable — may switch between laminar and turbulent. Avoid designing here. this-flow: '' - regime: Turbulent reynolds-range: Re > 4000 behaviour: >- Chaotic mixing — use Colebrook/Moody for the friction factor. this-flow: '' credits_used: 3 credits_remaining: null air_in_a_300_200_mm_duct: summary: Air in a 300 × 200 mm duct value: tool: reynolds-number-calculator tool_version: 1.0.1 outputs: status: >- Re = 79,167 — turbulent flow. Air, 20 °C at 5 m/s, hydraulic diameter 0.24 m. Chaotic mixing — use Colebrook/Moody for the friction factor. result: - property: Reynolds number value: 79,167 - property: Flow regime value: Turbulent - property: Velocity value: 5 m/s - property: Hydraulic diameter value: 0.24 m - property: Density value: 1.204 kg/m³ - property: Dynamic viscosity value: 1.825e-5 Pa·s - property: Kinematic viscosity value: 1.516e-5 m²/s - property: Flow area value: 0.06 m² - property: Volumetric flow value: 0.3 m³/s (18000 L/min) - property: Formula value: Re = ρVD/μ = 1.204 × 5 × 0.24 / 1.825e-5 regimeTable: - regime: Laminar reynolds-range: Re < 2300 behaviour: Smooth, orderly layers — friction factor f = 64/Re. this-flow: '' - regime: Transitional reynolds-range: 2300 – 4000 behaviour: >- Unstable — may switch between laminar and turbulent. Avoid designing here. this-flow: '' - regime: Turbulent reynolds-range: Re > 4000 behaviour: >- Chaotic mixing — use Colebrook/Moody for the friction factor. this-flow: ◄ This flow credits_used: 3 credits_remaining: null schema: type: object required: - tool - tool_version - outputs - credits_used - credits_remaining properties: outputs: type: object properties: status: type: string description: Result result: type: array description: Breakdown items: type: object properties: property: type: string description: Property value: type: string description: Value x-iotools-columns: - property - value regimeTable: type: array description: Flow Regimes items: type: object properties: regime: type: string description: Regime reynolds-range: type: string description: Reynolds Range behaviour: type: string description: Behaviour this-flow: type: string description: This Flow x-iotools-columns: - regime - reynolds-range - behaviour - this-flow tool: type: string description: The tool's slug, echoing the {slug} in the request path. tool_version: type: string description: Output-contract version for this tool. credits_used: type: integer description: >- Credits this call consumed, after any settlement refund. 0 when metering is disabled. credits_remaining: type: - integer - 'null' description: >- Credits left in the current monthly allowance, or null when metering is disabled. request_id: type: string description: Correlation id, also sent as x-request-id. '400': $ref: '#/components/responses/ValidationError' '401': $ref: '#/components/responses/Unauthorized' '402': $ref: '#/components/responses/InsufficientCredits' '403': $ref: '#/components/responses/ToolNotAllowed' '404': $ref: '#/components/responses/ToolNotFound' '413': $ref: '#/components/responses/PayloadTooLarge' '429': $ref: '#/components/responses/RateLimited' '500': $ref: '#/components/responses/ToolFailed' '503': $ref: '#/components/responses/ToolDisabled' security: - bearerAuth: [] components: responses: ValidationError: description: Invalid request. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/validation_error title: type: string description: Short human-readable summary of the failure. examples: - Invalid request status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - validation_error detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. fields: type: - object - string additionalProperties: type: string description: >- What failed: a map of field name → message. Absent when the body itself is malformed; a single string when the failure isn't tied to one field. example: type: https://iotools.cloud/docs/errors/validation_error title: Invalid request status: 400 code: validation_error detail: One or more inputs are invalid — see `fields`. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 fields: inputString: Required Unauthorized: description: Missing or invalid API key. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/invalid_api_key title: type: string description: Short human-readable summary of the failure. examples: - Invalid API key status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - invalid_api_key detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. example: type: https://iotools.cloud/docs/errors/invalid_api_key title: Invalid API key status: 401 code: invalid_api_key detail: 'Provide ''Authorization: Bearer ''.' request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 InsufficientCredits: description: Monthly credit allowance exhausted. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/insufficient_credits title: type: string description: Short human-readable summary of the failure. examples: - Insufficient credits status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - insufficient_credits detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. credits_used: type: integer description: Always 0 — a refused call charges nothing. credits_remaining: type: integer description: Credits left in the allowance — fewer than this call costs. example: type: https://iotools.cloud/docs/errors/insufficient_credits title: Insufficient credits status: 402 code: insufficient_credits detail: This call costs 1 credit and 0 remain in this month's allowance. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 credits_used: 0 credits_remaining: 0 ToolNotAllowed: description: Tool exists but has no API surface. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/tool_not_allowed title: type: string description: Short human-readable summary of the failure. examples: - Tool not available over the API status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - tool_not_allowed detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. example: type: https://iotools.cloud/docs/errors/tool_not_allowed title: Tool not available over the API status: 403 code: tool_not_allowed detail: >- "Background Remover" is available on iotools.cloud but has no API endpoint. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 ToolNotFound: description: No such tool. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/tool_not_found title: type: string description: Short human-readable summary of the failure. examples: - Tool not found status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - tool_not_found detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. example: type: https://iotools.cloud/docs/errors/tool_not_found title: Tool not found status: 404 code: tool_not_found detail: No tool with that slug. See GET /v1/tools/list. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 PayloadTooLarge: description: Body too large. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/payload_too_large title: type: string description: Short human-readable summary of the failure. examples: - Payload too large status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - payload_too_large detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. example: type: https://iotools.cloud/docs/errors/payload_too_large title: Payload too large status: 413 code: payload_too_large detail: Request body exceeds this tool's size limit. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 RateLimited: description: Per-minute rate limit exceeded. headers: Retry-After: description: Seconds to wait before retrying (RFC 9110 delta-seconds). schema: type: integer content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/rate_limited title: type: string description: Short human-readable summary of the failure. examples: - Rate limit exceeded status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - rate_limited detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. retry_after: type: integer description: >- Seconds until the window resets — the same value as the `Retry-After` header. example: type: https://iotools.cloud/docs/errors/rate_limited title: Rate limit exceeded status: 429 code: rate_limited detail: Too many requests. Retry in 30s. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 retry_after: 30 ToolFailed: description: Tool failed to run. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/processing_error - https://iotools.cloud/docs/errors/internal_error title: type: string description: Short human-readable summary of the failure. examples: - Tool failed to run - Internal error status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - processing_error - internal_error detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. credits_used: type: integer description: >- Always 0 on `processing_error` — a failed run is refunded, floor included. credits_remaining: type: integer description: >- Credits left after the refund. Absent when metering is disabled. example: type: https://iotools.cloud/docs/errors/processing_error title: Tool failed to run status: 500 code: processing_error detail: The tool failed to run. Please try again. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 credits_used: 0 ToolDisabled: description: Tool temporarily disabled. content: application/problem+json: schema: type: object description: >- RFC 9457 problem document, served as application/problem+json. Branch on `code`; `title` is human prose and may be reworded without notice. Some failures add extension members — `fields` on validation errors, `retry_after` on 429s, `credits_used`/`credits_remaining` on billing-adjacent failures — documented on the responses that carry them. required: - type - title - status - code properties: type: type: string format: uri description: Stable documentation URI for this failure. examples: - https://iotools.cloud/docs/errors/tool_disabled - https://iotools.cloud/docs/errors/api_unconfigured title: type: string description: Short human-readable summary of the failure. examples: - Tool temporarily disabled - API not configured status: type: integer description: HTTP status code, matching the response's own status. code: type: string description: >- Stable machine-readable error code — branch on this, not `title`. enum: - tool_disabled - api_unconfigured detail: type: string description: Human explanation of this occurrence. request_id: type: string description: Correlation id, also sent as x-request-id. example: type: https://iotools.cloud/docs/errors/tool_disabled title: Tool temporarily disabled status: 503 code: tool_disabled detail: This tool is temporarily unavailable. Try again shortly. request_id: e4042b29-8f1e-4c7a-9b52-6f0d1a3c7e11 securitySchemes: bearerAuth: type: http scheme: bearer bearerFormat: iot_live_… ````