Sweat rate varies fourfold between athletes. The runner next to you might lose 0.4 L per hour; you might lose 1.8 L per hour. Using their hydration protocol — or worse, a generic “drink to thirst” rule — is why so many race-day hydration plans fail. The 90-minute test below gives you a number you can plan around.
The one-sentence answer
Weigh yourself nude before a 60-minute run, again after, add any fluid you drank, subtract any urine, and divide by hours. That is your sweat rate in litres per hour.
Why this matters
Fluid intake during racing is bounded on both sides. Drink too little and dehydration past 2% body-mass loss costs you 5–15% performance. Drink too much — especially water alone — and you tip toward hyponatremia, which is more dangerous than dehydration. The right intake is bounded by your actual sweat loss, which is bounded by your physiology, not by what works for someone else.
What you need
- A scale that reads to 0.1 kg or 0.2 lb
- A measured water bottle
- A towel
- A 60-minute run at race-pace effort
- Conditions similar to your goal race
The protocol
- Use the bathroom. Strip down. Weigh yourself and record to 0.1 kg.
- Run for 60 minutes at the effort you plan to race. Track fluid consumed exactly — note the bottle volume before and after.
- Back home, do not pee, do not drink. Strip, towel off any sweat, weigh yourself again.
- Subtract the after weight from the before weight. Add the fluid you drank (1 L of water = 1 kg). The result is sweat loss in kg — which converts directly to litres.
- Divide by hours run. That is your sweat rate per hour.
Worked example
Before: 72.4 kg. After: 71.1 kg. Fluid consumed during the run: 0.5 L (= 0.5 kg). Time: 1 hour.
Sweat loss = (72.4 − 71.1) + 0.5 = 1.8 kg = 1.8 L. Sweat rate = 1.8 L/hr.
How to interpret your number
- Under 0.5 L/hr: very light sweater. 400–500 mL/hr replacement is plenty.
- 0.5–1.0 L/hr: moderate. 500–750 mL/hr target.
- 1.0–1.5 L/hr: heavy. 700–1000 mL/hr target.
- Above 1.5 L/hr: very heavy. 800–1200 mL/hr target, plus aggressive sodium — see our sodium guide for replacement math.
Note: target replacement is 70–80% of sweat loss, not 100%. The gut cannot absorb at 100% rates, and trying invites GI distress and hyponatremia risk.
Test in different conditions
Sweat rate is highly condition-dependent. A cool-weather test underestimates race-day need in heat by 50% or more. Run two tests: one near goal-race conditions, one in significant heat. Use the higher number when racing in heat — see our hot-weather marathon fueling guide.
Sodium is a separate test
Sweat-rate testing tells you fluid loss. It does not tell you sodium loss — that requires a lab patch test or a heuristic estimate based on symptoms (salty face, white deposits on a black shirt, frequent cramping). Most runners can use a sweat-rate test plus heuristic sodium estimates and be 90% there.
When to retest
Every 6 months, or after significant fitness change, weight change, or a move to a different climate. A weight loss of 5+ kg shifts sweat rate meaningfully.
Frequently asked questions
How often should I retest? Twice a year is plenty.
Do I need a lab sweat-sodium test? Not for most runners. Heuristics get you 90% of the way.
What if I cannot weigh nude? Weigh in the same clothing, dry both times — but towel inside the clothes to remove trapped sweat. Accept ±5% error.
Does humidity matter? Yes — humidity raises sweat rate without raising evaporation. Test in race-day humidity if you can.