Sweat Rate, Measured and Turned Into a Drinking Plan
After reading this you can measure your own sweat rate in liters per hour from two weigh-ins, express your fluid loss as a percent of body weight, and build a per-hour drinking plan that replaces most of what you lose.
What sweat rate is and why the number matters
Your sweat rate is how much fluid you lose per hour of exercise, measured in liters per hour (L/h). It is not a fixed personal constant. It rises with heat, humidity, intensity and body size. A cool 8 km jog might cost you 0.6 L/h. The same runner racing a hot half marathon can lose 1.8 L/h or more.
Here is the hook. Suppose you weigh 70 kg and finish a one hour session at 69.2 kg, having drunk 500 ml. You did not "lose 0.8 kg of fat". You lost 0.8 kg of water plus the 0.5 kg you drank, so 1.3 L of sweat in an hour. That is a sweat rate of 1.3 L/h. Knowing that number lets you plan drinks instead of guessing, which is the whole point of the measurement.
When to run this test, and when the number lies
Run the test when you want a drinking plan for a specific condition: a hot race, an indoor trainer session, a long run at goal pace. Repeat it in each condition, because the result only describes the session you measured.
The number lies when the session is short. A 20 minute test multiplies any weigh-in error by three. A 0.1 kg towel-drying error over 20 minutes reads as 0.3 L/h of phantom sweat. Over a full hour the same error is only 0.1 L/h. Aim for 60 minutes or more.
This test measures fluid loss, not fat loss. The scale drop after a workout is almost all water and returns within a day as you rehydrate. Do not read a post-run weigh-in as progress on body composition.
The formula and the intuition behind it
Think of your body as a bucket during the session. Fluid enters when you drink. Fluid leaves as sweat and as urine. Your net weight change is what the bucket did on balance. Rearrange that bookkeeping to isolate sweat:
Here S is total sweat loss in liters, m_{\text{pre}} and m_{\text{post}} are body weight in kilograms before and after, V_{\text{in}} is fluid drunk during the session in liters, and V_{\text{urine}} is any urine passed during it in liters. The step that lets you mix kilograms and liters is the density of water: 1 kg of sweat or drink equals 1 L, close enough for this purpose.
The intuition: if you drank 0.5 L and still ended up 0.8 kg lighter, the drink did not vanish, it left as sweat too. So sweat is the weight you lost plus everything you put back in. Divide by hours to get the rate:
R is sweat rate in L/h and t is duration in hours. Percent dehydration compares your net loss to your starting weight:
D is the percent of body weight lost as net fluid. Note that D uses net weight change, not total sweat, because your body only feels the fluid it failed to replace.
Worked example with the demo numbers
Use the tool's default data: pre 70 kg, post 69.2 kg, 500 ml drunk, 0 ml urine, duration 1:00:00.
- Weight change:
70 - 69.2 = 0.8kg, which is0.8L of net loss. - Add fluid drunk:
0.8 + 0.5 = 1.3L of sweat produced. Urine is 0, so nothing to subtract. - Divide by duration:
1.3 / 1.0 = 1.3L/h sweat rate. - Percent dehydration:
0.8 / 70 = 0.01143, so1.143%of body weight lost. - Drinking plan at 60 to 90% replacement:
0.6 \times 1.3 = 0.78L/h up to0.9 \times 1.3 = 1.17L/h. Call it roughly 780 to 1170 ml per hour.
You drank only 500 ml this session. To hit the low end of the plan next time you would need about 280 ml more per hour.
Reading the result and building the plan
Two numbers drive your decisions: the sweat rate R and the dehydration percent D. Sweat rate sets how much to carry and drink per hour. Dehydration percent tells you whether your current drinking is keeping up.
The plan replaces 60 to 90% of losses, not 100%. Full replacement is hard to drink and risks sloshing or overdrinking. Some short-term deficit is normal and safe for most people. The table shows the plan for a range of measured rates.
| Sweat rate (L/h) | 60% target (ml/h) | 90% target (ml/h) |
|---|---|---|
| 0.6 | 360 | 540 |
| 1.0 | 600 | 900 |
| 1.3 | 780 | 1170 |
| 1.8 | 1080 | 1620 |
| 2.4 | 1440 | 2160 |
At the high end you can see the practical ceiling. Most people struggle to absorb much beyond 1000 to 1200 ml/h during hard exercise, so a 2.4 L/h sweater running a hot marathon will finish with a real deficit no matter how disciplined the plan. That is expected. The goal is to keep the deficit small, not zero.
Common mistakes that corrupt the number
The math is trivial. The measurement is where errors creep in.
- Wet clothes and sweat on skin
- Weigh nude and towel off both times. Soaked kit holds 0.3 to 0.5 kg of sweat, which the scale would credit back to you and shrink your sweat rate by that much.
- Guessing fluid intake
- Weigh your bottle before and after instead of estimating sips. A 200 ml misjudgment over one hour is a 0.2 L/h error in the result, roughly 15% off a 1.3 L/h reading.
- Ignoring urine
- If you urinate mid-session, that fluid left your body but is not sweat. Subtract it. Most sessions have zero, which is why the field defaults to 0.
- Testing too short
- A 15 minute test turns a 0.1 kg towel error into 0.4 L/h of noise. Go 60 minutes or longer.
- Treating one test as universal
- A cool indoor result badly underestimates a hot race. Test in the conditions you will race in.
Related tools
Once you know your sweat rate, pair it with fuel. The Marathon Fueling Calculator sets carbs per hour and a gel schedule, and your drink volume determines whether you take those carbs as gels or in the bottle. If heat is what drives your sweat up, the Heat & Altitude Pace Adjuster shows how much to slow down in the same conditions.
For the rest of race planning: build even or negative splits with the Race Split Calculator, predict finish times with the Race Time Predictor, set training paces with the VDOT Running Calculator, and stack swim, bike and run into one total with the Triathlon Time Calculator.
Frequently asked questions
How much fluid loss is too much?
Losing more than about 2 to 3% of body weight as net fluid tends to hurt endurance performance in the heat. For a 70 kg athlete, 2% is 1.4 kg and 3% is 2.1 kg. The demo session sat at 1.14%, comfortably inside that range.
Why replace only 60 to 90% and not everything?
Drinking your full sweat rate is hard on the gut and can lead to overdrinking. A small deficit during exercise is normal and you top up afterward. The plan targets most of the loss, not all of it.
Does the drink have to be water?
For rates above about 1.2 L/h or sessions over 90 minutes, add sodium and carbohydrate rather than plain water, because you lose salt in sweat too. The volume math is the same; the contents change.
Why did drinking more raise my calculated sweat rate?
Because sweat equals weight lost plus fluid taken in. If two sessions end at the same weight but you drank more in one, you must have sweated more to offset the extra drink. The formula is doing correct bookkeeping.
Can I use pounds instead of kilograms?
Convert first: 1 lb is about 0.4536 kg. A 1.5 lb loss is 0.68 kg, so 0.68 L of net loss before adding fluid drunk.
This is an educational estimate for sport and training. It is not professional coaching or medical advice. If you feel unwell during exercise, stop and seek help.
Related tools
Race Time Predictor
Predict your 10K, half and marathon time from any recent race result.
VDOT Running Calculator
Daniels VDOT and easy/marathon/threshold/interval training paces from a race result.
Age Grading Calculator
Compare race times across ages: age-graded percentage and open-class equivalent.
Heat & Altitude Pace Adjuster
Adjust your goal pace for hot weather and race altitude.
Marathon Fueling Calculator
Carbs per hour, total gels and a fueling schedule for your race duration.
Race Split Calculator
Per-km or per-mile split table for a goal time, with even or negative split pacing.