Shed

Free tool · Hydration

Sweat Rate Calculator

Weigh yourself before and after a run, add what you drank, and find how much fluid you actually lose per hour. It is the only way to know your own number, and the average is not it.

Weight unit

Your sweat rate

1700ml/hr

In these conditions. It changes considerably with heat and humidity.

What it means

  • Body mass lost

    Above about 2 percent, performance starts to decline

    1.7%

  • Drink per hour

    Around three quarters of your sweat rate. You are not meant to replace all of it

    1300 ml

  • Over a 3 hour race

    Total fluid at that rate

    3900 ml

Reading

Within the normal range. Losing up to about 2 percent of body mass is expected and not a problem.

Stop calculating. Start training.

These numbers are the first 30 seconds of what Shed does.

Shed reads your Apple Health, grounds your VDOT in your actual training — not one race — and writes the whole block: every workout, every pace, adjusted as life happens. The calculator gives you a snapshot; Shed gives you the plan.

Get your plan on iPhone

How to run the test properly

Pick a run that resembles the conditions you care about: similar effort, similar temperature, ideally similar time of day. An hour is enough. Weigh yourself immediately before, in dry kit or nothing, after using the toilet. Track exactly how much you drink, which means carrying a measured bottle rather than estimating from a fountain.

Afterwards, towel dry thoroughly. Wet kit weighs a surprising amount and will make your sweat rate look lower than it is. Weigh in the same state you started.

Two percent is the number to know

Losing up to about 2 percent of your body mass over a race is normal, expected and not worth fighting. Performance decline becomes measurable somewhere past that, and it gets steeper as the loss grows, particularly in heat.

The opposite error is the dangerous one. Drinking beyond your sweat rate over several hours dilutes blood sodium and can cause exercise-associated hyponatremia, which has killed marathon runners in a way that mild dehydration has not. If you are gaining weight during a race, you are drinking too much.

Frequently asked

How do I do a sweat rate test?
Weigh yourself before a run, naked or in dry kit and after using the toilet. Run for a known time at a representative effort, tracking exactly how much you drink. Towel off and weigh again in the same clothing state. Fluid lost is the weight difference plus whatever you drank.
What is a normal sweat rate?
Most runners fall between 500 and 1500 ml per hour, with individual and conditional variation that is enormous. A large runner in heat can exceed 2500. There is no correct number; the point of measuring is that yours is not the average.
Should I replace everything I sweat?
No. Aiming to replace roughly three quarters is the standard guidance. Losing up to about 2 percent of body mass during a race is normal and does not meaningfully hurt performance. Drinking to replace 100 percent risks hyponatremia, which is genuinely dangerous in a way that mild dehydration is not.
Does my sweat rate change?
Substantially. Heat, humidity, intensity, altitude and heat acclimatisation all move it. A test in March tells you very little about a race in July. Test in conditions similar to your race, and retest if the forecast changes.
What if I finished heavier than I started?
You drank more than you sweated. That is the direction to avoid. Persistent over-drinking during long efforts dilutes blood sodium, which is the mechanism behind exercise-associated hyponatremia. Drink to thirst plus a plan, not to a rigid schedule that ignores conditions.

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