Sweat rate calculator

How to measure your sweat rate properly

The arithmetic is trivial — weight lost, plus what you drank, minus what you passed, divided by hours. The measurement is where people go wrong, and a sloppy weigh-in can move the answer by half a litre an hour. The protocol below is the one used in exercise physiology labs, minus the equipment you do not have.

1. Weigh nude, or in the same dry kit both times. Sweat-soaked shorts and socks can hold 300–500 g of fluid. If you weigh clothed at the end, that retained sweat is counted as body mass you still have, and your rate comes out too low.
2. Towel dry thoroughly before the second weigh-in. Skin-surface sweat is fluid you have already lost.
3. Use the same scale, on the same hard floor. Digital scales that read to 0.1 kg or 0.2 lb are fine. Carpet is not — it changes the reading between weigh-ins.
4. Record every drop you drink. Weigh or mark your bottles. Gels and sports drink count; so does the mouthful you spat out, which does not.
5. Note urine separately. If you stopped for a toilet break, that volume is not sweat. Guessing 300 mL for a normal void is better than ignoring it.
6. Do it in conditions you care about. A rate measured on a cool morning tells you almost nothing about racing in August.

Two known limitations, since no calculator can fix them: respiratory water loss and metabolic mass loss (the fuel you burned) are both folded into your weight change, which inflates the sweat figure slightly — roughly 1–2% of body mass loss per hour of hard, long work. For sessions under two hours the error is small enough to ignore.

Why 2% is the number everyone quotes

The American College of Sports Medicine’s position stand on exercise and fluid replacement puts the practical line at about 2% of body mass. Below it, most people perform normally. Above it, endurance performance tends to fall, perceived effort rises, heart rate drifts upward for the same pace, and the body’s ability to shed heat degrades — which matters more the hotter it is. For a 70 kg athlete, 2% is 1.4 kg, or about 1.4 litres of sweat.

It is a threshold, not a cliff edge. Plenty of elite marathoners finish 3–4% down and run very fast. But they are acclimatised, trained, and doing it deliberately. If you are 3% down after a club session in mild weather, the honest reading is that you drank too little, not that you are unusually tough.

Why your rate moves so much

Reported sweat rates in athletes commonly span 0.5 to 2.0 litres per hour, with figures above 3 L/h recorded in large, heat-acclimatised people working hard in extreme conditions. The variation is not noise — it is the thing worth tracking:

• Heat and humidity. Humidity is the underrated one. Sweat only cools you when it evaporates; in humid air it drips instead, so you lose the fluid without the benefit and keep producing more.
• Acclimatisation. Ten to fourteen days of training in heat makes you sweat earlier and more, while conserving sodium. Your fluid needs go up as you adapt, which surprises people who assume adaptation means needing less.
• Intensity and body size. Sweat rate tracks heat production, so it rises with pace, power and how much of you there is to cool.
• Kit. Anything that traps air — a pack, a rash guard, a helmet, full football pads — pushes the rate up.

Salt, and when it starts to matter

Sweat sodium concentration varies roughly five-fold between people, in the region of 10–70 mmol/L, or somewhere around 230–1,600 mg of sodium per litre. That range is why blanket electrolyte advice is useless. For a one-hour session, plain water and a normal diet cover it for almost everyone. The people who plausibly need sodium in the bottle are those combining a high sweat rate with long duration — over two or three hours — and especially those who finish sessions with gritty white marks on their skin and kit, cramp repeatedly, or race in heat. If that is you, a drink with sodium is a reasonable tool, not a miracle. Everyone else is buying flavoured water.

Measure more than once

A single weigh-in gives you one number in one set of conditions. The value comes from the log: the same session in 12°C and in 28°C, or the same route before and after a fortnight of heat training. That is why this page keeps a history and a chart rather than throwing your result away when you close the tab. Pair it with the daily water intake calculator for your baseline, and the hiking water calculator when the question is how much to carry rather than how much you lost.