Sweat Rate Calculator

Most athletes guess their hydration. They drink when they remember, grab whatever is on the table, and hope it works out. The problem is that fluid loss varies enormously between people. One runner loses half a litre an hour, another loses well over two. Drinking the same amount as your training partner is a coin flip. Your sweat rate is the number that turns guessing into a plan.

What sweat rate actually is

Sweat rate is how much fluid your body loses per hour of exercise, measured in litres per hour. It is driven by intensity, heat, humidity, your fitness, and plain genetics. It is not fixed either. A cool easy run and a hot threshold session can produce wildly different numbers, which is exactly why you test in conditions close to the ones you care about.

The calculator above does the math for you. You need four things: your body weight before, your body weight after, how much fluid you drank during the session, and how long it lasted.

How to measure it properly

Run the test once and you have a number you can use for years.

• Weigh yourself with as little clothing as possible, ideally right before you start. Note the exact figure.
• Train for about 60 minutes at the effort you want data for. Race pace gives you race numbers.
• Track every drop you drink during the session, in millilitres.
• Towel off and weigh yourself again in the same minimal clothing.

Plug the four numbers in above. The result is your sweat rate in litres per hour, plus the percentage of body mass you lost and a target for how much to drink next time.

What your number means

Most endurance athletes land somewhere between 0.5 and 2.0 litres per hour. Above 2.0 is common in hot conditions or for bigger, harder-working athletes. There is no good or bad sweat rate, it is simply your baseline. What matters is the body mass you lose over a session.

Lose more than about 2 percent of your body weight and performance starts to drop, your heart rate drifts up, and effort feels harder for the same pace. That is the line worth defending on long or hot days.

How to use it

Once you know your rate, you have a drinking target. Aim to replace a sensible share of what you lose during the session, and rehydrate with around 150 percent of the deficit afterwards to account for ongoing losses. Add sodium when sessions run long or hot, because you lose salt in sweat too and water alone will not hold.

Do not overdrink either. Gaining weight during a session means you are taking on more than you lose, which carries its own risks. The goal is to finish a little down, not soaked and sloshing.

Your sweat rate is one of the cleanest inputs you can give a coach. If you want the full picture on tracking fluid, electrolytes, and what the trend tells you over a training block, read the hydration tracking guide.