Sweat Rate & Hydration Calculator

Measure your sweat rate from body weight before and after exercise and learn how much fluid to drink to rehydrate. Example: losing 2 lb during a 60 minute run while drinking 16 oz gives a sweat rate of about 47 oz (1.4 L) per hour.

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Units

Weight before exercise (lb or kg)

Weight after exercise (lb or kg)

Fluid drunk during exercise (oz or mL)

Exercise time (minutes)

How to use the Sweat Rate & Hydration Calculator

Enter the values in the fields.
Press Calculate to see the result with the steps shown.

Formula and method

Sweat loss = (weight before – weight after) + fluid consumed. Sweat rate = sweat loss / exercise hours. Replacement = 150% of body weight lost (about 24 oz per pound).

Dehydration by percent of body weight lost

Weight lost	Effect
Under 1%	Normal
1 to 2%	Thirst, mild performance drop
2 to 3%	Reduced endurance, higher heart rate
Over 3%	Heat illness risk; stop and rehydrate

Typical sweat rates

Conditions	Sweat rate per hour
Cool, easy effort	0.3 to 0.6 L
Moderate, warm	0.6 to 1.2 L
Hard, hot, humid	1.2 to 2.5 L

Sodium and electrolytes

Sweat contains about 500 to 1,500 mg of sodium per liter. Long hot sessions call for salted foods or an electrolyte drink.

Cautions

This calculator gives estimates for education and planning, not medical advice. Check with your doctor before major changes to diet, exercise or medicines, especially if you have a medical condition.

Frequently asked questions

How do I measure my sweat rate?

Weigh before and after, add fluids drunk.

How much should I drink after exercise?

About 1.5 times the weight lost.

Is drinking only to thirst enough?

For most sessions under an hour.

What is hyponatremia?

Low blood sodium from over-drinking.

How much fluid in one hour of running?

Often 400 to 800 mL.

Does heat matter?

Yes, it raises sweat rate.

Sources

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Effortless Math Team

12 minutes ago