ForHosting KIT · Developer Utilities

Target heart rate Karvonen calculator

The Karvonen formula estimates a personal training heart rate from three simple numbers: your age, your resting heart rate, and the intensity you want as a percentage of heart-rate reserve.

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

Unlike a flat percentage of maximum heart rate alone, the reserve method first subtracts resting rate from estimated maximum, then scales that reserve by intensity and adds resting rate back, so people with different resting baselines get different targets at the same intensity label. This capability applies HR max equals two hundred twenty minus age, computes reserve, multiplies by intensity, and returns structured JSON with maximum HR, reserve, target bpm, and both percent and fraction forms of intensity. The path is pure arithmetic with no network and no random seed, so coaching apps, PE labs, and prepaid API clients get identical results every time. When resting heart rate meets or exceeds the estimated maximum, the request fails with invalid input because reserve cannot be zero or negative for a meaningful Karvonen target. The browser widget runs free on your device; the prepaid API charges $0.002 per successful item when servers and SDKs need the same contract.

How to use it

Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.

Check your inputs

Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.

Use it again or automate it

Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.

What you can do with it

Get an answer now

Enter one set of values and see the result without building a spreadsheet or script.

Compare scenarios

Change one value at a time and rerun the calculation to understand what affects the result.

Automate repeated work

Use the API when the same calculation needs to run inside your product or workflow.

FAQ

How do I use this capability?

Complete the fields above and run it on this page. The form highlights anything that needs attention.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/health/target-hr-karvonen

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/health/target-hr-karvonen \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"age":30,"resting_hr":60,"intensity":70}'

const res = await fetch("https://api.kit.forhosting.com/health/target-hr-karvonen", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "age": 30,
    "resting_hr": 60,
    "intensity": 70
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/health/target-hr-karvonen",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "age": 30,
    "resting_hr": 60,
    "intensity": 70
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/target-hr-karvonen", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"age":30,"resting_hr":60,"intensity":70}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"age":30,"resting_hr":60,"intensity":70}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/target-hr-karvonen", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "age": 30,
  "resting_hr": 60,
  "intensity": 70
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "health.target_hr_karvonen",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →

Run — free