curl --request POST \
  --url https://api.iotools.cloud/v1/tool/belt-length-calculator \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "unit": "mm",
  "smallDiameter": "100",
  "largeDiameter": "250",
  "centerDistance": "500",
  "beltType": "open",
  "driverRpm": ""
}
'

{
  "tool": "belt-length-calculator",
  "tool_version": "1.0.1",
  "outputs": {
    "results": [
      {
        "quantity": "Belt length (L)",
        "value": "1561.03 mm",
        "other-units": "156.103 cm · 61.4578 in"
      },
      {
        "quantity": "Wrap angle — small pulley",
        "value": "162.746°",
        "other-units": "2.84046 rad"
      },
      {
        "quantity": "Wrap angle — large pulley",
        "value": "197.254°",
        "other-units": "3.44273 rad"
      },
      {
        "quantity": "Speed ratio (D₂ : D₁)",
        "value": "2.5 : 1",
        "other-units": "driven speed = 0.4 × driver speed"
      },
      {
        "quantity": "Belt span between pulleys",
        "value": "494.343 mm",
        "other-units": "49.4343 cm · 19.4623 in"
      }
    ],
    "breakdown": "Open belt drive\nL = 2C + (π/2)(D₁ + D₂) + (D₂ − D₁)² ÷ 4C\nD₁ = 100 mm   D₂ = 250 mm   C = 500 mm\nL = 1561.03 mm (156.103 cm · 61.4578 in)\nWrap angles: 162.746° on the small pulley, 197.254° on the large one.\nThis is a theoretical length — round up to the nearest stock belt size and check the manufacturer's pitch-length table before ordering."
  },
  "credits_used": 3,
  "credits_remaining": null
}