Sweat rate and hydration calculator

How to do the weigh-in properly

The whole thing rests on two numbers, so it is worth spending five minutes getting them right. Weigh yourself immediately before the session in as little clothing as is decent, after using the bathroom. Train. Then towel yourself thoroughly dry, strip off anything sweat-soaked, and weigh again on the same scale. Sweat trapped in a soaked shirt weighs exactly as much as sweat in your body and will make your loss look smaller than it was.

Track what you drink during the session — a marked bottle is easiest. If you use the bathroom mid-session, you either need to measure it or accept that the result will underestimate your sweat rate. The formula from the National Athletic Trainers' Association position statement is simply the weight you lost, plus the fluid you drank, minus any urine you passed, divided by the hours you trained.^[1] A kilogram of weight lost is treated as a litre of sweat, which is close enough to true for this purpose.

Why one measurement is not enough

Sweat rate is not a fixed personal trait. It moves with temperature, humidity, intensity, clothing, altitude, how heat-acclimatised you currently are, and to a lesser degree with caffeine and some medications. The same athlete can sweat 0.8 litres an hour on a cool morning and 2.2 litres an hour in August. So the useful move is to measure two or three times across the conditions you actually train and compete in, and keep those numbers rather than a single average.

What is reasonably stable is the shape of your response — if you are a heavy sweater relative to your training partners in one condition, you will be in most conditions. Once you have measured a couple of times you can interpolate sensibly for the days in between without weighing every session.

The two percent line, and why it matters

The position statement frames the goal as keeping fluid losses under about two percent of bodyweight, because that is where the evidence starts showing meaningful decrements in performance and thermoregulation.^[1] Past three percent, the concern shifts from performance to exertional heat illness risk.

What happens physiologically is that plasma volume drops, which means less blood returning to the heart per beat, which means heart rate rises to hold cardiac output steady. At the same time, less blood is available to shunt to the skin for cooling, so core temperature climbs faster. Perceived effort goes up for the same power output. None of this feels like thirst — it feels like a bad day.

What to drink during, and how much

The position statement's practical guidance is 200 to 300 mL — roughly 7 to 10 fluid ounces — every 10 to 20 minutes, adjusted to your own sweat rate.^[1] The calculator above works out your specific number by taking your measured sweat rate, projecting it over your planned session length, and subtracting the two percent your body can afford to lose.

Pre-loading helps: 500 to 600 mL in the two to three hours before, and another 200 to 300 mL in the ten to twenty minutes before you start. Starting a hot session already down a litre is a hole you will not dig out of while you are working.

One thing worth saying plainly: you do not have to replace every drop. Trying to drink at the full rate of a 2 L/h sweat rate is usually impossible and often counterproductive, because gastric emptying tops out somewhere around 1.0 to 1.2 litres an hour and the surplus just sloshes. Replace what you can, stay inside the two percent, and make up the rest afterwards.

Sodium, and the "salty sweater" question

Sweat sodium concentration varies enormously between people, roughly 10 to 90 millimoles per litre in published measurements, which is about 230 to 2,070 milligrams per litre. It is substantially genetic, only modestly trainable, and it does not track neatly with sweat rate — you can be a light sweater with very salty sweat, or the reverse.

The practical consequence shows up in long, hot events. Someone losing two litres an hour at 1,400 mg of sodium per litre is down nearly 3,000 mg an hour, and if they replace that with plain water they dilute what sodium remains. That is the mechanism behind exercise-associated hyponatraemia, which is rare but genuinely dangerous and, importantly, is caused by overdrinking rather than by underdrinking. Anyone who finishes an event heavier than they started needs to change their drinking strategy, not add electrolytes to the same volume.

For rehydration drinks specifically, the position statement suggests adding 0.3 to 0.7 grams of sodium chloride per litre when exercise runs beyond a few hours, when access to meals is limited, or during the first days of heat acclimatisation.^[1] For a normal ninety-minute session with a meal afterwards, ordinary food handles the sodium replacement without any product involved.

Rehydrating afterwards

Replace about 150 percent of the deficit over the following four to six hours.^[1] The extra fifty percent exists because a chunk of what you drink leaves as urine before it is absorbed into the tissues that need it — drinking exactly your deficit leaves you short. Including sodium with the fluid, whether from a drink or from food eaten alongside it, meaningfully improves how much you retain.

Spreading it out beats slamming it. A litre drunk in one go triggers a large urine response; the same litre over a couple of hours is largely retained.

Common questions

Isn't drinking to thirst good enough?

For most training under about an hour in temperate conditions, yes, and the research supports it. Thirst becomes less reliable in three situations: long events where you are distracted and drinking is inconvenient, hot conditions where sweat rates outrun the thirst signal, and older athletes, in whom the thirst response is blunted. Knowing your sweat rate tells you which situation you are in.

Do I need an electrolyte product?

Usually not for sessions under about ninety minutes, if you eat normal meals with normal salt in them. It becomes genuinely useful for long hot sessions, multi-hour events, back-to-back sessions in the heat, and for people whose measured sweat losses are high. If you are buying one, look at the milligrams of sodium per serving rather than the marketing — many popular products carry very little.

Does coffee dehydrate me?

Not meaningfully at habitual doses. The mild diuretic effect of caffeine is more than offset by the fluid the coffee itself contains, and regular consumers develop tolerance to the diuretic effect anyway. Your morning coffee counts toward your fluid intake.

What about urine colour?

Useful and free, but crude. Pale straw suggests you are reasonably hydrated, dark amber suggests you are not. It is thrown off by B vitamins, beetroot, some medications, and by the fact that the first urine after drinking a lot of water is pale regardless of your actual state. Use it as a daily sanity check, not as a measurement.

Can I use this for a sauna or hot yoga session?

The arithmetic works the same way and the fluid replacement advice holds. What changes is that passive heat exposure produces sweat without the cardiovascular work, so you can rack up large losses while feeling like you have not done much. Weigh in and out of the sauna at least once — the number surprises people.

Sources for this page

1. McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training, 2017;52(9):877–895. Full text (PDF)
2. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. JAND, 2016;116(3):501–528. PMID 26891166
3. Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 2017;47(Suppl 1):111–128.
4. Hew-Butler T, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine, 2015;25(4):303–320.