Sweat Rate Calculator (Athlete Hydration)

What is your sweat rate?

Sweat rate is the volume of fluid you lose per hour during exercise — a personal number that depends on body size, fitness, sport, intensity, climate and individual physiology. Two athletes side by side can sweat at very different rates, so generic hydration advice ("drink eight glasses") is not useful for endurance training. The standard field test is to weigh yourself nude before and after a controlled session, recording any fluid you drank during it. Each lost kilogram is approximately one litre of water (sweat is mostly water plus electrolytes). Adding back the fluids you drank gives total loss; dividing by duration gives the rate. Knowing this number helps you target a drinking plan that replaces 70–80% of losses on long sessions, avoids the GI distress of overdrinking and matches your sodium needs.

How to run a sweat test

Weigh in dry — Empty your bladder, towel-dry, and weigh nude on an accurate scale. Use the same scale for the second weigh-in.
Train at your target intensity — Replicate the conditions you want data for — race-pace, climate, kit. Track every drop of fluid you drink during the session.
Weigh out fast — Strip, towel-dry, and weigh again within five minutes. Don’t empty bladder or eat first or the numbers shift.
Plug the numbers — Enter pre and post weight, the millilitres of fluid consumed and the duration. The card returns sweat rate, a hour-by-hour drink target and a sodium range to chase.

The math

Loss (L) = pre kg − post kg + fluids drunk (L). Sweat rate = Loss ÷ hours. The drink target replaces 70–80% of the rate; the sodium range applies 500–700 mg per litre of sweat (typical "average sweater" concentration).

Sweat rate categories

Rates above are for endurance work in moderate climate. Heat acclimation, high humidity and prolonged effort all push the number up; cool weather and casual paces drop it.

Rate	Profile	Hydration strategy
< 0.6 L/h	Low sweater	Sip on thirst, plain water is fine for sessions under 90 min.
0.6 — 1.2 L/h	Average sweater	Plan 500–700 ml/h with 500–700 mg sodium for sessions over 60 min.
1.2 — 1.8 L/h	Heavy sweater	Use a structured plan: 700–900 ml/h, electrolyte mix at 700–1000 mg/L sodium.
> 1.8 L/h	Extreme sweater	Train your gut to hold 1 L/h, 1000+ mg sodium, and consider IV recovery for ultras.

Frequently asked questions

Why doesn’t weight loss equal water loss exactly?

About 95% of acute training weight loss is fluid; the rest is glycogen, fat oxidised and respiratory water. For practical hydration planning, treat 1 kg ≈ 1 L of sweat — the error is well under 5%.

Should I aim to replace 100% of losses?

No. Replacing 70–80% during effort is enough; finish the rebalance after the session. Drinking to weight neutrality during exercise causes hyponatraemia in some athletes.

What is hyponatraemia?

A dangerous low blood-sodium state caused by drinking too much plain water during long efforts. Symptoms include headache, nausea and confusion. Salt tablets and a smarter pacing plan prevent it.

Do I need an electrolyte drink?

For sessions under 60 minutes, water is fine. Beyond that, a mix with 400–800 mg sodium per litre keeps you alert and reduces cramping risk in salty sweaters.

How does heat change the rate?

Sweat rate scales roughly linearly with temperature above 16 °C. A session at 30 °C can double the rate compared to 12 °C. Re-test in conditions that match your race day.

How often should I retest?

Once per training block (4–8 weeks) and again any time the conditions or your fitness change meaningfully. Save the spreadsheet — you’ll spot trends across seasons.

Sweat Rate Calculator (Athlete Hydration)