Know Your Sweat Rate: How to Calculate Your Personal Hydration Needs

You have heard the advice: "sip 500ml every hour," or "just drink to thirst." Then you finish a humid Saturday long run beside a friend who is soaked through while you are barely damp. You did not sweat the same amount, so why would you drink the same amount?

Your sweat rate is easy to measure with a bathroom scale and one run. Here is how to work it out, and what to do with the number.

Sweat Rates at a Glance

The One-Hour Weigh-In Test

Weighing yourself before and after exercise is the simplest and most accurate way to measure sweat rate. All you need is a reliable digital scale, a towel and one steady run.

Those corrections matter more than you might think. In one study, sweat trapped in clothing made sweat loss look 8–10% lower than it was, while uncorrected urine loss made it look 16% higher in warm conditions.

The Formula and a Worked Example

Sweat rate (ml per hour) = (weight before − weight after, in kg) × 1,000 + fluid drunk (ml)

• Weight Before: 70.0kg
• Weight After: 69.2kg, a drop of 0.8kg
• Fluid Drunk: 400ml from a soft flask
• Sweat Rate: (0.8 × 1,000) + 400 = 1,200ml, or 1.2 litres an hour in those conditions

What to Do with Your Number

You do not need to drink back every millilitre. You need to stay inside safe limits on both sides.

Do Not Lose Too Much

The American College of Sports Medicine sets the line at losing more than 2% of your body weight. For a 70kg runner, that is 1.4kg. Take the runner above on a two-hour long run: at 1.2 litres an hour they sweat out about 2.4 litres, so they need to drink roughly a litre along the way to stay inside the line.

Do Not Finish Heavier

Overdrinking is the other risk, and it is the more dangerous one. Hyponatremia researchers advise drinking according to thirst rather than to fixed targets, because sweat and urine losses vary so much from person to person. Most runners who become ill with hyponatremia gained weight during their event.

When Singapore researchers weighed 1,887 runners, they concluded that race-day drinking should aim for the right amount, not the most.

Test in More Than One Condition

Your sweat rate is not one fixed number. It shifts with effort, weather, heat acclimation, fitness and body size, so repeat the test a few times:

• An Easy Early-Morning Run: Your baseline in the coolest part of the day.
• A Hard Tempo Session: More effort usually means more sweat.
• A Hot Long Run: The closest match to a humid race morning.

Dial In Your Numbers with SUPERnatural+ on ATHLO+

Once you know how much you sweat, carrying the right amount is the easy part. The 500ml SUPERflask is a soft flask that shrinks as you drink, and SUPERnatural+ SUPERsalty puts 1,000mg of sodium in a single sachet sized for it. For more on what you lose besides water, read Electrolytes 101.

Ready to learn the warning signs of drinking too little or too much? Read Dehydration vs Hyponatremia next.