How to calculate your sweat rate

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

It's worth doing, because sweat rate varies hugely between people. The 2017 Sports Medicine review by Lindsay Baker pulled together whole-body sweat rates across athlete groups and found them ranging from roughly 0.5-2.0 litres per hour, with sweat sodium from around 10-90mmol/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 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 whole kit list.

Pick a session that looks like your race, at a similar intensity and room temperature. A 60-minute Zone 2 run in February tells you very little about what a Hyrox in a stuffy 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. Wear as little as possible and record the number to the nearest 0.1kg. That's 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 under 45 minutes gets noisy. Try not to wee during the session. If you do, you'll need to measure it and add it back in, which is a miserable job.

4. Towel off properly and weigh again. Sweat still on your skin or soaked into your kit counts as fluid you've lost, so dry off well and weigh in the same clothing 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 drunk, so 1.6 litres of sweat in 60 minutes. Your sweat rate is 1.6L/h.

Repeat it two or three times in 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 their performance. Above it, endurance capacity starts to drop, and it gets worse in the heat.

For a 78kg athlete, 2% is about 1.56kg. At 1.6L/h, you cross that line in roughly an hour of hard work if you drink nothing. That's a Hyrox, and it'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. Aim to replace most of what you lose. Replacing all of it, or more, creates a different problem.

Sodium is the harder half

You can measure fluid loss at home. Sodium loss needs a lab sweat test or a patch kit. You can still read the signs, though.

White salt crusting on your kit and a salty taste on your face after a session point to the higher end of that 10-90mmol/L range. Take a rough midpoint of 40mmol/L, and 1.6 litres of sweat carries about 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 won't put that back. This is where an electrolyte drink earns its place. If you want 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 1.4L/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 for most people the gut absorbs fluid well below their sweat rate. A workable plan looks more like this:

• Pre-load. 400-600ml with sodium in the 2 hours before the start, finished at least 30 minutes out so you're not queuing for the toilet at the start line.
• During. Take what the race format allows. In a Hyrox that's realistically a few hundred millilitres across the runs, and it won't be enough. Accept the deficit and plan for it.
• After. Replace roughly 125-150% of the weight you lost, with sodium, over the next few hours. Fluid without sodium mostly ends up in the toilet.

For short, intense races, the pre-load is where the real gains are. 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 your body weight shifts noticeably, and otherwise leave it alone.

The athletes who get hydration right did the test once and wrote the number down. Then they moved on.

Interval's Electrolyte+ has 1500mg of electrolytes per stick, with 500mg lion's mane and 500mg maca, for the sessions where you need more than water. It's £19.99 for 20 sticks, and it's worth pairing with your own number.