curl --request POST \
  --url https://api.iotools.cloud/v1/tool/spring-rate-calculator \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "units": "metric",
  "material": "music-wire",
  "shearModulus": "",
  "wireDiameter": "2",
  "coilDiameter": "16",
  "activeCoils": "8",
  "load": ""
}
'

{
  "tool": "spring-rate-calculator",
  "tool_version": "1.0.1",
  "outputs": {
    "results": [
      {
        "metric": "Spring Rate",
        "value": "4.840 N/mm (27.64 lbf/in)"
      },
      {
        "metric": "Spring Index (C = D/d)",
        "value": "8.000 — good (4–12 is the practical range)"
      },
      {
        "metric": "Wahl Correction Factor (Kw)",
        "value": "1.184"
      },
      {
        "metric": "Shear Modulus Used",
        "value": "79,300 N/mm² (11,501,493 psi)"
      },
      {
        "metric": "Active Wire Length (π·D·n)",
        "value": "402.1 mm (15.83 in)"
      }
    ]
  },
  "credits_used": 3,
  "credits_remaining": null
}