Sweat Rate & Hydration Calculator for Athletes

How to use this tool

1. Weigh yourself (in kg or lb) immediately before exercise.
2. Record how much fluid you drank during exercise.
3. Weigh yourself immediately after exercise.
4. Enter the exercise duration and see your sweat rate and rehydration target.

How to calculate sweat rate

The standard method used by sports scientists and the American College of Sports Medicine:

1. Weigh yourself immediately before exercise (in kg, without clothing).
2. Record exactly how much fluid you drink during the session (in litres).
3. Weigh yourself immediately after exercise (same conditions).
4. Apply the formula: Sweat rate (L/hr) = (pre-weight − post-weight + fluid intake) ÷ exercise duration (hours).

For example: pre-weight 75 kg, post-weight 74 kg, fluid intake 0.5 L, duration 1 hour -> (75 − 74 + 0.5) / 1 = 1.5 L/hr sweat rate.

Sweat rate norms

Hydration strategy

The ACSM recommends drinking 150–250% of sweat losses to fully restore body water within 4–6 hours post-exercise. For a 1-hour workout at 1.5 L/hr sweat rate, that means drinking 2.25–3.75 L of fluid during recovery.

During exercise, aim to drink at regular intervals (every 15–20 minutes) rather than waiting until you are thirsty. Thirst is a lagging indicator - by the time you feel thirsty, you may already be 1–2% dehydrated.

Electrolytes and hyponatremia

Sweat contains sodium, potassium, and other electrolytes in addition to water. Drinking large amounts of plain water to replace sweat losses dilutes blood sodium concentration - a dangerous condition called hyponatremia (literally "low sodium in blood").

For sessions under 60 minutes, plain water is usually adequate. For sessions lasting 60–90 minutes or more, especially in heat, use a sports drink or electrolyte supplement to replace sodium alongside fluid. Salty foods (pretzels, pickles) can also help maintain sodium balance during long training days.