Sweat rate calculator

What your number means

The honest headline from the ACSM position stand on fluid replacement: most athletes lose 0.5–2.0 litres per hour, with the full population range spanning roughly 0.3 to over 3.0 L/hr. That tenfold spread is why generic advice — drink 500ml an hour, a bottle every 45 minutes — is a number picked from the middle of a range wide enough to be meaningless at the edges. Rough bands:

Sweat rate	Reading	Practical implication
Under 0.8 L/hr	Low	Typical for easy running in cool air. Aid stations are plenty; carrying is optional.
0.8–1.5 L/hr	Middle of the range	Carry something on runs past 75–90 minutes in warmth. Thirst remains the guide.
1.5–2.0 L/hr	High	Plan access to fluid on long runs. Past two hours, sodium starts to matter too.
Over 2.0 L/hr	Very high	Worth a weigh-in to confirm. Long, hot races need a deliberate carry-and-refill plan.

How the estimate works: the reference chart

Running costs close to 1 kcal per kilogram per kilometre regardless of fitness, and nearly all of it becomes heat — so your weight and pace set the heat you have to shed. In cool air, moving through it carries much of that heat away for free; as the temperature climbs, more of it has to leave as evaporated sweat, and one litre evaporated removes about 580 kcal. The dew point then sets the exchange rate: on dry air sweat evaporates efficiently, on saturated air it drips off having cooled nothing, so your body produces more of it for less return. Sun adds load on top. The chart shows the model for a 70kg runner at 5:30/km (8:51/mile) in part sun.

Conditions (temp / dew point)	Estimated rate	Over 90 min	Body mass at 90 min, drinking nothing

Treat it as an estimate, not a measurement: at the same heat load, individual sweating varies by roughly ±25%, and body size and acclimation both move it. What the estimate is for is the decision you actually face — whether to carry a bottle, and whether aid every 5km is enough for you on this day. A single weigh-in then tells you how your body runs against the model.

The weigh-in, if you want the real number

1. Weigh before the run. Empty bladder, minimal clothing — note what you did so you can repeat it.
2. Run 60–90 minutes at a typical effort, tracking the volume of anything you drink.
3. Towel dry and weigh again in the same clothing state. Sweat left in your kit reads as fluid you still have.
4. Enter both numbers above. The maths is (weight lost + fluid drunk) ÷ hours, with 1kg of body mass ≈ 1 litre of sweat — and the page shows how far your measured rate sits from the estimate.
5. Repeat in different weather. The step everyone skips, and the one that matters most — your hot-day rate can be double your cool-day rate.

For the full story — why runners guess half their real loss, why drinking to thirst beats a replacement schedule, and what the number is actually for — read How Much Fluid Do You Lose Running? To see what the same air costs your pace, run the conditions through the heat-adjusted pace calculator.

Frequently asked questions

Should I drink back everything I lose? No. Losing up to ~2% of body mass during a run is normal and well tolerated; the evidence supports drinking to thirst, with full replacement across the following 24 hours through food and normal drinking. Overdrinking is the more dangerous error — drinking beyond thirst is the primary cause of exercise-associated hyponatraemia.

Is finishing heavier than I started a good sign? The opposite. A gain means you drank beyond your losses, which is the direction that causes hyponatraemia. Sodium tablets and sports drinks do not protect against it — the fix is letting thirst set the rate.

Does this apply to cycling? The weigh-in method is identical — weigh, ride, weigh. The estimate is built for running; on the bike, airflow improves evaporative cooling so rates run lower at the same effort outdoors, while hot indoor sessions on the trainer can be the highest numbers you ever record.

Fuelling the same long run? The carb loading planner covers the food side.