ForHosting KIT · Health & fitness

Sweat rate calculator

Weigh in before and after training, add what you drank and get your hourly sweat loss: 72.5 kg before, 71.8 kg after and 0.5 L drunk in 60 minutes means 1.2 litres, or 1200 mL, per hour. Free in your browser.

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Worked example

If you enter

Weight pre: 72.5
Weight post: 71.8
Fluid intake: 0.5
Duration min: 60

You get

Weight pre kg: 72.5
Weight post kg: 71.8
Weight unit: Kg
Fluid intake l: 0.5

Knowing how much you sweat per hour is the foundation of a personal hydration plan for running, cycling, team sports, and heat acclimation. Coaches, sports dietitians, and self-coached athletes weigh in before and after a session, log every millilitre of fluid consumed, and convert the mass change into an hourly sweat rate so the next workout can replace what was lost without overdrinking. This sweat rate from weight loss calculator accepts pre-exercise weight, post-exercise weight, fluid intake, optional urine volume, and session duration in minutes or hours, then returns fluid loss and sweat rate in litres per hour and millilitres per hour using the standard field approximation that one kilogram of body mass equals about one litre of fluid.

How sweat rate is calculated from body weight change

During exercise you lose water mainly through sweat and, to a smaller extent, through breathing and any urine produced mid-session. When you weigh yourself nude or in the same dry kit before and after a bout of work, the drop in body mass is mostly water that left the body. Adding back the fluid you drank during the session, and subtracting any urine volume you measured, recovers an estimate of total fluid loss in litres under the practical rule that one kilogram of mass equals one litre of water.

Dividing that fluid loss by the session duration in hours yields sweat rate in litres per hour, which is the number sports scientists and coaches use to plan replacement drinks for similar conditions. Use consistent scales, empty the bladder before the pre-weight when possible, towel off before the post-weight, and record fluids carefully so the estimate stays trustworthy.

Who measures sweat rate, and why

Endurance coaches batch-test squads after hard sessions and store each athlete hourly rate so race-day bottles match real losses instead of one-size-fits-all charts. Sports dietitians use repeat measures across seasons to show how heat acclimation and fitness change fluid needs without guessing. Research assistants and field studies need a deterministic oracle that rejects incomplete weigh-ins instead of silently assuming zero intake.

Content sites targeting sweat rate calculator and sweat rate from weight loss benefit from a live tool whose free browser path matches the paid automation path byte for byte on the same pure module. Failed validation does not consume balance. Keep requests small, pin the rounded rates in tests, and you get reproducible hydration planning numbers for LMS plugins, coaching dashboards, and edge workers without standing up a full sports-science stack of your own.

What you can do with it

Personal race hydration plans

Weigh before and after a long run, enter drinks consumed and session minutes, and set bottle volumes from your own litres-per-hour sweat rate.

FAQ

How do I calculate sweat rate from weight loss?

Subtract post-exercise body weight from pre-exercise weight (kg), add fluid intake in litres, subtract urine in litres if measured, then divide by session duration in hours. The result is sweat rate in L/h (also returned as mL/h).

Why is sweat rate negative?

Usually a measurement issue: wet kit at the post-weight, incomplete towel-off, or under-recorded fluid.

How should I weigh myself for an accurate sweat rate?

Weigh nude or in the same dry kit on the same scale. Empty your bladder before the first weigh-in, towel off before the second and write down every drink, so the change in mass reflects sweat rather than errors.

Should I count urine passed during the session?

Yes, if you can measure it. Its volume is subtracted from the fluid loss, because it left the body without being sweat. If you leave it out, the calculator treats it as zero.

How do I use my sweat rate?

Use it to plan drinks for sessions in similar conditions, replacing what you lose without overdrinking. Repeat the test across seasons, since heat acclimation and fitness change how much you sweat.

Related tools

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/sweat-rate

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/sweat-rate \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"weight_pre":72.5,"weight_post":71.8,"fluid_intake":0.5,"duration_min":60}'

const res = await fetch("https://api.kit.forhosting.com/health/sweat-rate", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "weight_pre": 72.5,
    "weight_post": 71.8,
    "fluid_intake": 0.5,
    "duration_min": 60
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/health/sweat-rate",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "weight_pre": 72.5,
    "weight_post": 71.8,
    "fluid_intake": 0.5,
    "duration_min": 60
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/sweat-rate", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"weight_pre":72.5,"weight_post":71.8,"fluid_intake":0.5,"duration_min":60}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"weight_pre":72.5,"weight_post":71.8,"fluid_intake":0.5,"duration_min":60}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/sweat-rate", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "weight_pre": 72.5,
  "weight_post": 71.8,
  "fluid_intake": 0.5,
  "duration_min": 60
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "health.sweat_rate",
  "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.

Technical notes

Missing required fields and non-physical values fail with invalid_input instead of inventing a rate.

Input contract, units, response fields, and errors

Optional urine defaults to zero and uses the same fluid unit. Optional precision controls decimal places from zero to fifteen and defaults to four. Negative sweat rate can appear if the athlete gains more mass than fluid intake explains—for example when clothing is still wet at the post-weight—so treat the sign as a data-quality flag rather than a physiological miracle. Send a JSON body with four required quantities: weight_pre and weight_post as finite positive body weights in the unit named by weight_unit (kilograms by default, or pounds converted with the international pound factor), fluid_intake as a non-negative volume in the unit named by fluid_unit (litres by default, or millilitres converted by dividing by one thousand), and duration_min as a finite positive session length in minutes. You may supply duration_hours instead of duration_min when you prefer hours; minutes win if both are present. On success the response echoes normalised values: weight_pre_kg, weight_post_kg, fluid_intake_l, urine_l, duration_min, duration_hours, weight_loss_kg, fluid_loss_l, sweat_rate_l_per_h, sweat_rate_ml_per_h, method, formula, and precision. Missing required fields, non-finite numbers, non-positive weights or duration, or negative fluid volumes fail with invalid_input and are not billed. Pair results with your own notes on temperature, humidity, and intensity; this endpoint returns only the arithmetic of the weight-and-fluid method.

Integration use cases

Team and club testing days

Coaches collect pre and post weights plus fluid logs for a squad and store each athlete sweat_rate_l_per_h for heat and intensity blocks.

Training apps and spreadsheets

Call the API from a training log or macro so every completed session can attach a deterministic fluid-loss estimate without manual arithmetic.

Technical questions

Which fields are required?

weight_pre, weight_post, fluid_intake, and duration_min (or duration_hours). Missing required fields fail with invalid_input and are not billed. urine is optional and defaults to zero.

Can I enter weight in pounds and fluids in millilitres?

Yes. Set weight_unit to lb (or pound/pounds) and fluid_unit to ml. Weights convert with 1 lb = 0.45359237 kg; millilitres divide by 1000 to litres.

What does the sweat rate calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →

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