Measure your own sweat rate: the arithmetic

Every hydration recommendation you have ever read treats you as an average. Sweat rates differ several-fold between athletes doing identical work in the same room, and the difference decides whether a sauna session takes off half a kilo or two.

Your own number takes one training session and a set of scales to find. Almost no fighter has it.

The arithmetic, in full

The measurement rests on a simple fact: over a training session, essentially all of your body mass change is fluid. You do not lose meaningful fat or muscle in 90 minutes.

A worked example. You start at 78.4 kg, finish at 77.1 kg, and drank 600 ml during a 90-minute session.

Sweat loss is 78.4 minus 77.1, which is 1.3 kg, plus the 0.6 litres you drank, giving 1.9 litres. Across an hour and a half that is roughly 1.27 litres per hour.

Getting it right rather than approximately right

• Towel off properly. Sweat left on skin and hair is weight the scale still counts, and it is the single biggest source of error in this measurement.
• Weigh naked both times. A soaked singlet can hold several hundred grams. Weighing in kit before and after is not the same as weighing naked before and after.
• Bathroom before, not during. If you urinate mid-session, the measurement is finished unless you can measure and add that volume too.
• Use the same scales. Consistency matters more than accuracy for this purpose.
• Record the conditions. Room temperature, humidity, what the session actually was. The number is meaningless without them.

Why one measurement is not enough

Sweat rate moves with temperature, humidity, clothing, intensity, and how heat-adapted you currently are, so it is never a fixed property of a person.

So the useful thing is a small set of figures, each attached to a condition.

That third row is the one that matters most in a cut, and it is the one nobody measures in advance. A fighter plans a sauna session to remove a kilogram and has no idea whether his own rate makes that 15 minutes or 45.

What the number changes

Through camp: it tells you how much to drink during and after sessions, in litres rather than in "plenty of water". Two fighters in the same gym can need double the replacement of each other, and the one with the higher rate is the one whose training quality quietly degrades if he follows generic advice.

In fight week: it tells you how long a sweat session needs to be, which means you stop when the number is reached rather than when you feel finished. Sweating to a stopwatch and sweating to a target are different activities with different risks.

The limits that should bound either of them are in sauna and hot baths.

After the scale: it feeds the refill. You cannot replace what you have not measured, and the recovery target is a proportion of what actually left. The structure is in rehydration after the weigh-in.

Sodium, which the scale cannot see

The weight measurement gives you volume. It tells you nothing about concentration, and sweat sodium varies enormously between people: reported concentrations run from around 10 to over 90 millimoles per litre, with most athletes somewhere in the 20 to 70 range.

That means two fighters losing the identical 2 litres can lose 4 or 5 times as much sodium as each other. A salty sweater has visible white marks on dark kit, stings in the eyes, and a taste on the lips, and he needs a very different replacement strategy from someone who does not.

You cannot measure that at home. What you can do is notice the signs and act on them, which is covered in electrolytes and ORS.

The percentage that matters more than the litres

Convert the loss to a percentage of your starting body mass, because that is the figure the performance literature is built on.

In the worked example above, 1.3 kg lost from 78.4 kg is about 1.7 percent, before accounting for what he drank. Losses in that region are already associated with increased perceived effort. Larger deficits progressively affect strength, power and decision-making, which is the reason a managed cut has an upper limit rather than a target.

Those thresholds, and where the line sits for a fight, are in the weight cut calculator and set out phase by phase in the Professional Weight Cut Protocol.

10 minutes, once, and you stop guessing

Everything else on this site is about rules, evidence and structure. This is the one page that asks you to go and generate a piece of data about yourself.

It costs one session, a set of scales and a towel. And it replaces the vaguest part of every hydration plan, the part where somebody tells you to drink to thirst or aim for a number invented for the average of a population you are not in.

Do it on Tuesday. Write it in the log next to everything else.