Estimate how much fluid you lose through sweat during exercise, in litres per hour, using the standard sports-science body-mass-balance method. Weigh in before and after your session to build a personalised hydration strategy instead of guessing.
Sweat rate is worked out from the change in your body weight before and after exercise, adjusted for any fluid you drank and any urine you passed during the session. Sweat loss (L) = (pre-exercise weight − post-exercise weight, in kg) + fluid intake during exercise (L) − urine volume during exercise (L). Dividing that figure by your exercise duration in hours gives your sweat rate in litres per hour. One kilogram of body-mass lost during exercise is treated as roughly one litre of sweat, since sweat is almost entirely water — this is the standard assumption used in sports science and by bodies like the American College of Sports Medicine (ACSM).
| Metric | Value |
|---|
This is a single-session estimate. Sweat rate changes with heat, humidity, intensity, clothing and how heat-acclimatised you are — test across a few sessions in conditions similar to your event for a more reliable personal figure.
The calculation is a simple mass balance: whatever body weight you lose during exercise, once you account for fluid you drank and any urine you passed, is treated as sweat. This works because sweat is almost entirely water, so a change in body mass during a session (excluding food, since you're not eating mid-workout) closely tracks fluid loss. It's the same method used in ACSM's position stand on exercise and fluid replacement and routinely applied by sports dietitians working with athletes.
Athletes and regular exercisers measure sweat rate to move from generic fluid advice ("drink plenty of water") to a number based on their own body and the conditions they train in. Sweat rate varies enormously between individuals — genetics, body size, fitness, heat acclimatisation, clothing and the specific session all play a role — so two people doing an identical session side by side can lose very different amounts of fluid. Knowing your own figure lets you plan roughly how much to drink before, during and after exercise, which helps you avoid the performance and health costs of significant dehydration on one end, and the risk of overhydration and exercise-associated hyponatremia (dangerously diluted blood sodium) on the other.
Accuracy depends mostly on careful weigh-ins. Use the same scale for both readings, weigh in nude or in dry, minimal clothing (wet kit adds weight that isn't sweat lost from your body), and towel off visible sweat before the post-exercise weigh-in. Record exactly how much you drank during the session and, if relevant, how much urine you passed. For the most useful result, test during a session that reflects real training or race conditions — similar pace or effort, similar clothing, and ideally similar heat and humidity — since a sweat rate measured on a cool, easy day won't tell you much about a hot race.
| Step | What to do |
|---|---|
| 1. Before | Weigh in nude or in dry, minimal clothing immediately before starting |
| 2. During | Track total fluid drunk (mL or fl oz) and any urine passed during the session |
| 3. After | Towel off sweat and weigh in again immediately after finishing, same scale, same or equivalent clothing |
| 4. Calculate | Enter your figures above to get sweat loss, sweat rate and % body weight lost |
| 5. Repeat | Test again in different conditions (heat, intensity, duration) to build a fuller picture |
A widely used general starting point in sports-hydration guidance is to aim to replace roughly what you lose — using your measured sweat rate to estimate how much to drink over a similar future session, spread out gradually rather than consumed all at once. For example, someone with a measured sweat rate of about 1 L/hour might aim to drink somewhere in the vicinity of 500–1000 mL per hour during a similar session, adjusted down for gut comfort and practicality, and topped up afterwards based on any remaining weight deficit. This is general, educational guidance rather than an individual prescription — exact fluid and electrolyte needs (including sodium replacement in longer or hotter sessions) vary by person and event, so endurance athletes and anyone training intensely in heat should work with a sports dietitian or physician to build a personalised plan.
Sweat rate is not a fixed personal constant — it shifts session to session with several factors, which is why a single test only tells you about that specific session:
This calculator applies the standard body-mass-balance formula exactly as described above; it does not add any hidden adjustments. Its accuracy depends entirely on the quality of your inputs — an inconsistent scale, wet clothing left on for the post-exercise weigh-in, or unrecorded fluid intake will all skew the result. It also assumes body-mass change during the session reflects sweat loss alone; over unusually long sessions where you eat solid food, that assumption becomes less precise. This tool is for general educational and training-planning purposes and is not a substitute for individualised advice from a sports dietitian, physician or accredited sports scientist, particularly for endurance athletes, people training in extreme heat, or anyone with a medical condition affecting fluid or electrolyte balance.
What is sweat rate and why should I measure it?
Sweat rate is how much fluid your body loses through sweat during a set period of exercise, usually expressed in litres per hour. Knowing your own sweat rate lets you build a personalised hydration plan rather than relying on generic fluid intake advice, helping you avoid both dehydration and overdrinking during training or competition.
How is sweat rate calculated?
Sweat rate is calculated using the body-mass-balance method: weigh yourself immediately before and after exercise, then work out sweat loss as (pre-exercise weight − post-exercise weight) + fluid intake during exercise − urine output during exercise. Dividing that sweat loss by your exercise duration in hours gives your sweat rate in litres per hour. One kilogram of body-mass lost during exercise is treated as roughly one litre of sweat, since sweat is almost entirely water.
How do I measure my sweat rate accurately?
For the most accurate reading, weigh yourself nude or in minimal, dry clothing on the same scale immediately before and immediately after exercise, towel off sweat before the post-exercise weigh-in, and record any fluid you drank and any urine passed during the session. Test under conditions similar to what you're training or competing for (same intensity, clothing, and if possible, similar heat and humidity), since sweat rate is highly condition-specific.
What counts as a high sweat rate?
Sweat rates vary enormously between individuals — from under 0.5 litres per hour to more than 2.5 litres per hour in some athletes exercising hard in the heat. There's no single "normal" figure; your own baseline, established over a few sessions, is far more useful than comparing yourself to a generic benchmark or another athlete.
Does sweat rate change with heat and humidity?
Yes, significantly. Higher temperatures and humidity generally increase sweat rate because your body relies more heavily on sweat evaporation to stay cool, and humid air makes that evaporation less efficient, prompting even more sweat production. Exercise intensity, clothing, body size and heat acclimatisation all shift your sweat rate as well, which is why a single test session only reflects that specific set of conditions.
How much fluid should I drink to replace sweat losses?
A common general starting point from sports-hydration literature is to aim to replace roughly what you lose — broadly, drinking an amount similar to your calculated sweat loss over the course of or after a session, adjusted for practicality and gut comfort. This is general guidance rather than a personalised prescription; endurance athletes and anyone training intensely in heat should work with a sports dietitian to fine-tune a fluid and electrolyte plan for specific events.
Can drinking too much during exercise be harmful?
Yes. Drinking well beyond your fluid losses, especially over several hours, can dilute blood sodium levels and lead to a condition called exercise-associated hyponatremia, which can be serious. This is a key reason sports-hydration guidance now emphasises replacing roughly what you lose rather than "drinking as much as possible" — your sweat rate calculation helps you avoid both extremes.