How to calculate your sweat rate

You can work out how to calculate your sweat rate in one training session, with a set of bathroom scales and about 90 seconds of arithmetic. Then you stop guessing at hydration for the rest of your race season.

Worth doing, because sweat rate varies wildly between people. The 2017 Sports Medicine review by Lindsay Baker pulled together whole-body sweat rates across athlete populations and found them spanning roughly 0.5 to 2.0 litres per hour, with sweat sodium concentrations ranging from around 10 to 90 mmol/L. Two athletes in the same gym, doing the same session, can lose four times as much fluid as each other and close to ten times as much sodium.

Your number is yours. A generic 500ml an hour is a coin flip laid on top of it.

What you need to calculate sweat rate

Bathroom scales that read to 0.1kg, and a drinks bottle you can measure. That's the kit list.

Pick a session that looks like your race: similar intensity, similar room temperature. Testing a 60 minute Zone 2 run in February tells you very little about what a Hyrox in an unventilated hall in July will do to you. If you race in heat, test in heat.

The protocol, step by step

1. Empty your bladder, then weigh yourself. Minimal clothing, and record the number to the nearest 0.1kg. Call this your pre-weight.

2. Weigh or measure your drink. Note exactly how much fluid is in the bottle before you start. 1 litre of water weighs 1kg, which keeps the maths simple.

3. Train for a measured block. 60 minutes is ideal. Anything shorter than 45 minutes gets noisy. Don't urinate during the session. If you do, you'll need to measure that volume and add it back in, which is a miserable job.

4. Towel off thoroughly and weigh again. Sweat still sitting on your skin or soaked into your kit counts as fluid you lost, so dry properly and weigh in the same clothing state as before.

5. Do the sum.

Sweat loss (litres) = (pre-weight kg minus post-weight kg) + fluid drunk in litres

Then divide by the hours you trained to get litres per hour.

A worked example. You start at 78.4kg, finish at 77.5kg, and drank 700ml over the hour. That's 0.9kg lost plus 0.7 litres consumed, so 1.6 litres of sweat in 60 minutes. Your sweat rate is 1.6 L/h.

Repeat it two or three times across different conditions. One test gives you a number. Three tests give you a range, which is more honest and more useful.

What the number actually tells you

The ACSM position stand on exercise and fluid replacement sets the practical threshold at 2% of body mass. Below that, most athletes hold performance. Above it, endurance capacity starts to drop, and the effect gets worse in the heat.

For a 78kg athlete, 2% is about 1.56kg. At a sweat rate of 1.6 L/h, you cross that line in roughly an hour of hard work if you drink nothing. That's a Hyrox. That's most of what you do in a training block.

The same position stand advises against drinking more than you sweat, which pushes body mass up during exercise and raises the risk of hyponatraemia. Replacing most of what you lose is the target. Replacing all of it, or more, is a different problem.

Sodium is the harder half

Fluid loss you can measure at home. Sodium loss you can't, not without a lab sweat test or a patch kit. But you can read the signal.

White salt crusting on your kit, and a persistently salty taste on your face after a session: these point to the higher end of that 10 to 90 mmol/L range. Take a rough midpoint of 40 mmol/L, and 1.6 litres of sweat carries roughly 1.5g of sodium out with it. A heavy salty sweater at the top end could lose double that in the same hour.

Water alone doesn't put that back. This is where an electrolyte drink earns its place. If you want the detail on what to look for in a formula, we covered it in isotonic vs electrolyte drinks for Hyrox.

Turning the number into a race plan

Say you test at 1.4 L/h and your race takes 90 minutes. You're looking at roughly 2.1 litres of sweat loss.

Drinking all of that mid race isn't practical and probably isn't wise, because gut absorption caps out well below sweat rate for most people. A workable plan looks more like this:

• Pre-load. 400 to 600ml with sodium in the 2 hours before the start, finishing at least 30 minutes out so you're not queuing for the toilet on the start line.
• During. Take what the race format allows. For Hyrox that's realistically a few hundred millilitres across the run legs, and it won't be enough. Accept the deficit and plan for it.
• After. Replace roughly 125 to 150% of the mass you lost, with sodium, over the following few hours. Fluid without sodium mostly gets urinated out.

The pre-load is where the real gains are for short, intense races. Start properly hydrated and accept the mid-race deficit.

Test it, then stop thinking about it

One hour of measuring gives you a number you'll use for the next 12 months. Retest when the seasons change or when your body mass shifts meaningfully, and otherwise leave it alone.

The athletes who get hydration right did the test once and wrote the number down. Then they stopped thinking about it.

Interval's Electrolyte+ is built around a calcium-heavy formula with Lion's Mane and Maca, designed for the sessions where water alone stops being enough. Worth pairing with your own number.