CALCULATOR

Sweat Rate Calculator

Weigh before and after a session to find your real sweat rate, how much to drink during training, and how much sodium you lose.

Sweat and replacement

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Sweat rate–

Total sweat lost–

Body mass lost–

Drink during next session–

Post-session rehydration–

Estimated sodium lost–

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Measuring your own sweat rate

Sweat rate varies enormously between people, from under half a litre to well over two litres per hour, and generic hydration advice cannot account for that. Measuring your own takes one session: weigh yourself before and after, account for what you drank, and the difference tells you what you lost.

The core relationship is that one kilogram of body mass lost during exercise is approximately one litre of sweat. Adding the fluid you drank during the session gives total sweat produced.

The practical threshold that matters is 2 percent of bodyweight. Losses beyond that are associated with measurable declines in endurance performance and increased cardiovascular strain, particularly in heat. Most people who train indoors in cool conditions stay comfortably under it; anyone training hard in heat frequently does not.

The calculation

Sweat ratesweat lost (ml) = (pre-weight kg – post-weight kg) x 1000 + fluid drunk (ml)
sweat rate (ml/hr) = sweat lost / (minutes / 60)

Example: 75.0 to 74.2 kg, 500 ml drunk, 60 min
(0.8 x 1000) + 500 = 1300 ml/hr

Rehydration after trainingreplace 1.25 to 1.5 litres per kg of bodyweight lost

The rehydration multiplier exists because drinking exactly what you lost does not fully restore fluid balance. Some of the intake is excreted as urine, so replacing 125 to 150 percent of the deficit, alongside sodium, restores balance more reliably.

What different losses mean

Body mass lost	Effect
Under 1%	No meaningful performance impact
1 to 2%	Minor effects, mainly in heat
2 to 3%	Measurable endurance decline, higher perceived effort
Above 3%	Substantial performance loss, rising heat strain

Typical sweat rates by activity

Activity	Conditions	Typical rate
Walking	Temperate	300-500 ml/hr
Gym session	Air conditioned	500-800 ml/hr
Running	Temperate	800-1200 ml/hr
Running	Hot and humid	1200-2000 ml/hr
Cycling	Hot, low airflow	1000-1800 ml/hr
Team sport	Hot	1000-2500 ml/hr

Sodium, and who actually needs it

Sweat sodium concentration ranges from roughly 400 to 1500 mg per litre, and it varies more between individuals than sweat rate does. Salty sweaters, who notice white residue on clothing and equipment, sit at the top of that range and lose substantially more sodium per litre than light sweaters at the same volume.

Under 60 to 90 minutes in cool conditions: water is generally sufficient.
Beyond 90 minutes, or in heat: sodium replacement helps retain fluid and maintain performance.
Salty sweaters in long events: the group with the clearest case for deliberate sodium intake.
Do not over-drink plain water during very long events. Diluting blood sodium is a real risk in endurance settings.

Thirst remains a reasonable guide for most people in most sessions. Structured replacement matters most in long, hot or competitive settings where the losses accumulate faster than thirst signals them.

Frequently asked questions

What is a normal sweat rate?

Roughly 0.5 to 2.0 litres per hour, with wide individual variation depending on fitness, heat, humidity and intensity.

How much should I drink during exercise?

Enough to keep losses under about 2 percent of bodyweight. Matching 80 to 100 percent of your measured sweat rate is a practical target.

How much fluid after training?

Around 1.25 to 1.5 litres per kilogram of bodyweight lost, since some of what you drink is excreted rather than retained.

Do I need electrolytes or just water?

For sessions under an hour in cool conditions, water is generally sufficient. Longer sessions, heat and heavy sweating make sodium replacement more relevant.

Sources

Sawka MN et al. American College of Sports Medicine position stand: exercise and fluid replacement. Medicine and Science in Sports and Exercise, 2007.
Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 2017.
Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate, 2005.
National Institutes of Health, Office of Dietary Supplements. Nutrient fact sheets. ods.od.nih.gov
NHS. Nutrition and health guidance. nhs.uk
US Departments of Agriculture and Health and Human Services. Dietary Guidelines for Americans, 2025-2030. dietaryguidelines.gov

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General information, not medical advice. Consult a qualified professional before changing your diet, especially with a medical condition.