How much should I drink during exercise?

Aim to drink to thirst and roughly match your sweat rate — often 0.4–0.8 L per hour for most people. After a session, drink about 1.5× the fluid you lost (weight before vs after) to fully rehydrate, since you keep sweating and don't absorb every drop.

How do I measure my sweat rate?

Weigh yourself naked before and after an hour of exercise, accounting for any fluid you drank. Each kilogram of weight lost is roughly one litre of sweat. Divide by the hours trained to get your litres-per-hour sweat rate.

How much sodium is lost in sweat?

Sweat sodium typically runs about 500–1000 mg per litre, so a litre of sweat loses roughly 800 mg on average. Heavy, salty sweaters (white residue on kit) can lose more and may benefit from extra sodium in long or hot sessions.

Do I need electrolytes or just water?

For sessions under about an hour, plain water is usually fine. For longer, hotter, or salty-sweat sessions, adding sodium (via a sports drink or electrolyte mix) helps you retain fluid and avoid dilutional hyponatraemia from drinking large volumes of plain water.

What are signs of dehydration?

Thirst, dark urine, headache, dizziness, a falling pace, and a higher heart rate at the same effort. Losing more than about 2% of bodyweight in a session can impair performance. Persistent or severe symptoms warrant medical attention.

Hydration Calculator

Estimate how much fluid and sodium to replace after a training or endurance session. This hydration calculator turns your exercise time and sweat rate into the sweat lost, the fluid to drink to fully rehydrate, and the sodium to put back.

How it works

The calculator multiplies your session length by a typical sweat rate for the chosen intensity to estimate total sweat loss in litres. To fully rehydrate it recommends drinking about 1.5× the loss— you keep sweating after you stop and don't absorb every drop, so matching the loss exactly leaves you short. It then estimates sodiumlost at roughly 800 mg per litre of sweat. Your bodyweight is optional and only feeds the daily baseline (≈33 mL/kg) shown for context.

Sweat-rate presets

Intensity sets the litres-per-hour rate. The right column shows what you'd lose in a single hour at each rate.

Intensity	Sweat rate (L/hour)	Loss in 1 hour
Light	0.5 L/h	0.5 L
Moderate	1.0 L/h	1.0 L
Heavy	1.5 L/h	1.5 L
Very heavy	2.0 L/h	2.0 L

Worked example

A 2-hour ride at a moderatesweat rate (1 L/h) loses 2 h × 1 L/h = 2 Lof sweat. To fully rehydrate, drink about 2 L × 1.5 = 3 L(≈101 oz) over the following hours, and replace roughly 2 L × 800 mg/L = 1,600 mg of sodium.

Reading the numbers

Output	What it means
Sweat loss	Estimated fluid lost during the session.
Fluid to rehydrate	Drink this over the hours after — about 1.5× the loss.
Sodium	Approximate sodium to replace via food or electrolytes.
Daily baseline	Routine daily fluid from bodyweight, for context only.

For your everyday target rather than a session top-up, use the Water Intake Calculator.

Frequently asked questions

How much should I drink during exercise?: Aim to drink to thirst and roughly match your sweat rate — often 0.4–0.8 L per hour for most people. After a session, drink about 1.5× the fluid you lost (weight before vs after) to fully rehydrate, since you keep sweating and don't absorb every drop.
How do I measure my sweat rate?: Weigh yourself naked before and after an hour of exercise, accounting for any fluid you drank. Each kilogram of weight lost is roughly one litre of sweat. Divide by the hours trained to get your litres-per-hour sweat rate.
How much sodium is lost in sweat?: Sweat sodium typically runs about 500–1000 mg per litre, so a litre of sweat loses roughly 800 mg on average. Heavy, salty sweaters (white residue on kit) can lose more and may benefit from extra sodium in long or hot sessions.
Do I need electrolytes or just water?: For sessions under about an hour, plain water is usually fine. For longer, hotter, or salty-sweat sessions, adding sodium (via a sports drink or electrolyte mix) helps you retain fluid and avoid dilutional hyponatraemia from drinking large volumes of plain water.
What are signs of dehydration?: Thirst, dark urine, headache, dizziness, a falling pace, and a higher heart rate at the same effort. Losing more than about 2% of bodyweight in a session can impair performance. Persistent or severe symptoms warrant medical attention.

Related calculators

Disclaimer. This is general guidance, not medical advice. Sweat rates vary widely with heat, fitness, and the individual, so these are estimates. Drinking far more than you lose can cause dangerously low blood sodium (hyponatraemia) — sip to thirst and don't over-drink. If you have a medical condition, consult a professional.