curl --request POST \
  --url https://api.iotools.cloud/v1/tool/heart-rate-zone-calculator \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "age": "30",
  "maxHrFormula": "haskell",
  "useKarvonen": "",
  "targetIntensity": "75"
}
'

{
  "tool": "heart-rate-zone-calculator",
  "tool_version": "1.0.1",
  "outputs": {
    "summary": [
      {
        "metric": "Max Heart Rate",
        "value": "190 BPM"
      },
      {
        "metric": "Formula",
        "value": "Haskell-Fox (220 - age)"
      },
      {
        "metric": "Target Heart Rate (75%)",
        "value": "143 BPM"
      }
    ],
    "zones": [
      {
        "zone": "Zone 1 — Very Light",
        "intensity-of-max-hr": "50–60%",
        "bpm-range": "95–114 BPM",
        "training-purpose": "Recovery, warm-up, cool-down"
      },
      {
        "zone": "Zone 2 — Light",
        "intensity-of-max-hr": "60–70%",
        "bpm-range": "114–133 BPM",
        "training-purpose": "Fat burn, base endurance"
      },
      {
        "zone": "Zone 3 — Moderate",
        "intensity-of-max-hr": "70–80%",
        "bpm-range": "133–152 BPM",
        "training-purpose": "Aerobic capacity, stamina"
      },
      {
        "zone": "Zone 4 — Hard",
        "intensity-of-max-hr": "80–90%",
        "bpm-range": "152–171 BPM",
        "training-purpose": "Lactate threshold, speed"
      },
      {
        "zone": "Zone 5 — Maximum",
        "intensity-of-max-hr": "90–100%",
        "bpm-range": "171–190 BPM",
        "training-purpose": "VO₂ max, peak power"
      }
    ]
  },
  "credits_used": 3,
  "credits_remaining": null
}