How to measure your sweat rate in 90 minutes

If you’re going to measure one thing about yourself for endurance running, measure this. Your sweat rate is the input that everything downstream depends on — hydration, sodium dose, race-day fueling, whether the calculator on this site tells you something useful or just makes up numbers.

I do this test 3–4 times a year, mostly in shoulder seasons when conditions are changing. The protocol is dumb-simple and takes about 90 minutes from start to finish.

You need

• A bathroom scale that reads in 0.1 kg or 0.2 lb increments
• A water bottle of known capacity
• A target effort you can sustain for 60+ minutes at race intensity
• A towel
• A pen and the calculator app on your phone

The protocol

1. Weigh nude, just after using the bathroom. Record to the nearest 0.1 kg. Call this W₁.
2. Train for at least 60 minutes at race effort. The longer the test, the better the signal — 90 minutes is ideal. Note the duration to the nearest minute.
3. Track every milliliter of fluid you drink during the session. This is the part everyone forgets. Pre-fill your bottle, finish it or measure what’s left.
4. Don’t pee mid-test. If you have to, weigh yourself first and add that to a “void” tally — but really, just don’t drink so much that you have to.
5. Towel off immediately after. Get the surface sweat off your skin and hair. Don’t change yet.
6. Weigh nude again. Same scale, same conditions. Call this W₂.

The math

Sweat rate (L/hr) = ( (W₁ − W₂) + fluid intake in L ) ÷ hours

So if you started at 70.0 kg, finished at 68.8 kg, drank 500 mL over 80 minutes:

(70.0 − 68.8) + 0.5 = 1.7 L over 80 min
1.7 ÷ (80/60) = 1.275 L/hr

You’re a heavy sweater. Plug 1.28 into the calculator and it’ll tell you exactly how much salt you’re losing — and how much to put back in.

Things people get wrong

They don’t actually do it at race intensity. A zone-2 jog is not a race. Your sweat rate at marathon pace in summer heat is not the rate you saw during a chilly easy run. Do the test at the effort and conditions you actually care about.

They forget the fluid in. This is the most common error and it always biases the result low. If you sweated out 1.5 L and drank 1.0 L during the session, your net weight loss is only 0.5 kg — but your actual sweat rate is 1.5 L/hr, not 0.5.

They weigh in clothes. Sweat-soaked shorts can weigh 200–400 g. That goes straight into your error bar. Nude before and after, or in identical dry clothes.

They do it once. A single test is a data point, not a model. Do this 3–4 times across a range of conditions — cool, mild, hot — and you’ll start to see the shape of your personal sweat curve. That’s the real prize.

What “normal” looks like

For context, here’s where most adults land:

Sweater type	L/hr	Description
Light	< 0.6	Small, fit, cool weather
Moderate	0.6 – 1.0	Most recreational runners
Heavy	1.0 – 1.5	Larger athletes, warm weather
Very heavy	1.5 – 2.0	Hot weather, hard effort
Elite	> 2.0	Pros, marathoners in heat

Tour de France stage winners hit 2.5+ L/hr. Don’t be surprised if your hot-day number is double your cool-day number. Plan accordingly.

For what it’s worth: I land in the “very heavy” bucket on hot days and “heavy” on cooler ones. The salt-rim hat does not lie.