How do I measure my sweat rate?

Weigh yourself before and after a session in minimal clothing, note any fluid you drank, and compute: sweat lost = (before weight − after weight) + fluid drunk. About 1 kg (2.2 lb) of body-mass loss equals roughly 1 litre of sweat. Divide by the hours you exercised to get litres per hour.

Can a smartwatch or fitness tracker measure sweat?

No wrist device directly senses sweat. Trackers estimate fluid loss from calories burned via a heat-balance model, which runs materially off — and in a hot room most sweat drips rather than evaporates, so it cools nothing and the model can't account for it. A $25 scale used correctly is far more accurate.

How much sweat is one pound of body weight?

Roughly one pound of weight lost during exercise equals about 16 ounces (about 470 ml) of sweat, since sweat is almost entirely water. One kilogram is about one litre.

How to measure your sweat rate

The one number a smartwatch can't sense.

Your sweat rate — how much fluid you lose per hour — is the input everything else depends on. No wrist device can measure it; they model it from calories and run well off. But a single before-and-after weigh-in gets you within a few percent, and it takes one session. Here's exactly how.

Do it in the weigh-in calculator →

The one-weigh-in method.

Weigh in. Right before your session, empty your bladder and weigh yourself in as little clothing as possible. Write it down.
Train normally, and count your fluid. Do your usual workout. Note roughly how much you drink during it — that water stayed in you, so it has to be added back.
Weigh out. Towel the sweat off your skin and weigh again in the same clothing, right after finishing.
Do the math. Your sweat loss is the weight you dropped plus whatever you drank. About 1 kg (2.2 lb) of loss is 1 litre of sweat. Divide by the hours you trained for your rate.

sweat lost (L) = (weight before − weight after) + fluid drunk
sweat rate (L/hr) = sweat lost ÷ hours trained

Prefer not to do arithmetic in a sweaty parking lot? Put the two weights into the precise weigh-in mode and it returns your rate, and turns it straight into your sodium, potassium and magnesium.

Why the scale beats the wrist.

Every watch that shows a "sweat loss" number is guessing from a heat-balance model: it knows roughly how many calories you burned and assumes a share of that heat left as evaporated sweat. Two problems. First, the calorie estimate itself carries error. Second — and this is the big one — in a hot, humid room most of your sweat drips onto the floor instead of evaporating. Dripping sweat cools nothing, so a heat-balance model simply can't see it. That's how a wearable can read a fraction of what a scale plainly shows.

The instrument that actually knows is a scale. A basic $25 kitchen or bathroom scale, used correctly, lands within a few percent of your true fluid loss, because it measures the thing itself — mass that left your body — rather than inferring it. It is the single highest-value $25 in your training kit.

One honest caveat. A weigh-in measures the water you lost, not the salt in it — and sweat sodium varies about tenfold between people (roughly 200–2,000 mg per litre). So the scale nails your volume; how salty you run is the second dial. The calculator asks you for both, and you can refine the salt setting over a few sessions.

Turn the rate into a dose.

A sweat rate on its own is trivia. It becomes useful the moment you multiply it by how salty you sweat and get an actual replacement target:

sodium to replace ≈ sweat rate (L/hr) × your sweat sodium (mg/L) × hours

That's exactly what the calculator does — plus potassium and magnesium, plus the cheap real-food-plus-bulk-minerals mix that hits the number. Measure once, and it keeps your rate for every session after.

Do it now

Measure it once.

One weigh-in beats a season of guessing — on your own device.

Open the weigh-in calculator →

The one number a smartwatch can't sense.

The one-weigh-in method.

Why the scale beats the wrist.

Turn the rate into a dose.

Weigh-in calculator

Why salt, not just water

Save & converge

Measure it once.