# Sweat Rate Calculator

> Calculates sweat loss and sweat rate from body weight before and after exercise, corrected for fluid drunk and urine passed, plus the percentage of body mass lost, a dehydration classification and the volume to drink afterwards (150 % of the deficit), following the ACSM position stand on exercise and fluid replacement.

- Calculator id: `sweat-rate` · Category: Sports & Fitness (`fitness`) · Tool name: `calculate_sweat_rate`
- Canonical page: https://tttkmbb.com/fitness/sweat-rate · This document: https://tttkmbb.com/fitness/sweat-rate.md · JSON definition: https://tttkmbb.com/fitness/sweat-rate.json

## Purpose

Calculates sweat loss and sweat rate from body weight before and after exercise, corrected for fluid drunk and urine passed, plus the percentage of body mass lost, a dehydration classification and the volume to drink afterwards (150 % of the deficit), following the ACSM position stand on exercise and fluid replacement.

**Use when:** An athlete weighed themselves before and after a session and wants their sweat rate, how dehydrated they became, or how much to drink during and after similar sessions.

**Do not use when:** You want general daily fluid needs (use water-intake), the weights were taken with wet clothing or after eating, or for clinical dehydration assessment; results are informational, not medical advice.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `pre_exercise_weight_kg` | number | kg | required | Nude (or minimal dry clothing) body weight before exercise, after emptying the bladder. (> 0, max 300) |
| `post_exercise_weight_kg` | number | kg | required | Body weight after exercise, towelled dry, same clothing and scale. (> 0, max 300) |
| `fluid_intake_ml` | number | mL | optional, default 0 | Volume of fluid consumed during the session. (min 0, max 10000) |
| `urine_output_ml` | number | mL | optional, default 0 | Urine volume passed between the two weighings (0 if none). (min 0, max 5000) |
| `duration_minutes` | number | min | required | Exercise duration between the two weighings. (> 0, max 1440) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `sweat_loss_liters` | number | L | (pre − post) + fluid drunk − urine, in litres (1 kg of body mass ≈ 1 L). |
| `sweat_rate_l_per_hour` | number | L/h | Sweat loss divided by the duration in hours. |
| `sweat_rate_ml_per_hour` | number | mL/h | The same rate in millilitres per hour. |
| `body_mass_loss_percent` | number | % | (pre − post) / pre × 100; negative when weight was gained. |
| `dehydration_status` | string |  | < 2 % minimal; 2–3 % performance impaired; > 3 % significant (ACSM thresholds). |
| `fluid_to_replace_ml` | number | mL | 150 % of the net body-mass deficit, drunk over the hours after exercise (ACSM). |
| `recommended_intake_ml_per_hour_next_session` | number | mL/h | Hourly intake that would match the sweat rate in the same conditions (an upper bound: ACSM advises keeping the loss below 2 % of body mass without drinking more than you sweat). |

## Formula

`sweat_loss_liters = (pre_exercise_weight_kg − post_exercise_weight_kg) + fluid_intake_ml / 1000 − urine_output_ml / 1000; sweat_rate_l_per_hour = sweat_loss_liters / (duration_minutes / 60); body_mass_loss_percent = (pre − post) / pre × 100; fluid_to_replace_ml = 1.5 × (pre − post) × 1000`

Body-mass change is the standard field measure of sweat loss (ACSM 2007); it ignores respiratory water loss and metabolic mass loss, which are small for sessions under about 3 hours. Drinking should not exceed the sweat rate, because over-drinking causes exercise-associated hyponatraemia. Informational, not medical advice.

## Data Sources

- Sawka MN et al. (2007) American College of Sports Medicine position stand: Exercise and fluid replacement, Med Sci Sports Exerc 39:377-390 — https://pubmed.ncbi.nlm.nih.gov/17277604/ (peer_reviewed, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/sweat-rate?pre_exercise_weight_kg=…&post_exercise_weight_kg=…&duration_minutes=…`
- `POST https://tttkmbb.com/api/v1/calculate/sweat-rate` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/sweat-rate · OpenAPI operationId `calculate_sweat_rate` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "sweat-rate", "inputs": {…}}`

## Example

- 70 → 69.2 kg, 500 mL drunk, 60 min: inputs `{"pre_exercise_weight_kg":70,"post_exercise_weight_kg":69.2,"fluid_intake_ml":500,"duration_minutes":60}` → `{"sweat_loss_liters":1.3,"sweat_rate_l_per_hour":1.3,"sweat_rate_ml_per_hour":1300,"body_mass_loss_percent":1.14,"dehydration_status":"Minimal (< 2 % body mass loss)","fluid_to_replace_ml":1200,"recommended_intake_ml_per_hour_next_session":1300}`
- 62 → 60.5 kg, 750 mL drunk, 100 mL urine, 90 min: inputs `{"pre_exercise_weight_kg":62,"post_exercise_weight_kg":60.5,"fluid_intake_ml":750,"urine_output_ml":100,"duration_minutes":90}` → `{"sweat_loss_liters":2.15,"sweat_rate_l_per_hour":1.43,"body_mass_loss_percent":2.42,"dehydration_status":"Performance impaired (2–3 % body mass loss)","fluid_to_replace_ml":2250}`

```
GET https://tttkmbb.com/api/v1/calculate/sweat-rate?pre_exercise_weight_kg=70&post_exercise_weight_kg=69.2&fluid_intake_ml=500&duration_minutes=60
```

## Limitations

You want general daily fluid needs (use water-intake), the weights were taken with wet clothing or after eating, or for clinical dehydration assessment; results are informational, not medical advice. Body-mass change is the standard field measure of sweat loss (ACSM 2007); it ignores respiratory water loss and metabolic mass loss, which are small for sessions under about 3 hours. Drinking should not exceed the sweat rate, because over-drinking causes exercise-associated hyponatraemia. Informational, not medical advice. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Why is fluid intake added and urine subtracted?**

Weight loss understates sweating when you drank (the drink replaced some sweat) and overstates it when you urinated, so both are corrected to isolate sweat.

**Should I drink the full sweat rate during exercise?**

Not necessarily: ACSM suggests limiting the loss to under 2 % of body mass, which for many people means drinking less than the full sweat rate; drinking more than you sweat is harmful.

**Why 150 % for recovery?**

Urine production continues after exercise, so restoring fluid balance within a few hours needs about 1.5 L for every kilogram lost, ideally with sodium.

## Related

- [Water Intake Calculator](https://tttkmbb.com/health/water-intake.md) — Daily baseline fluid needs outside training.
- [Calories Burned Calculator](https://tttkmbb.com/fitness/calories-burned.md) — Energy cost of the same session.