Exercise Hydration Calculator

Why hydration matters during exercise

A loss of about 2% of body weight is where it starts to show. In the 2017 review by NATA (the National Athletic Trainers’ Association), performance in prolonged aerobic exercise fell consistently once the deficit reached 2%, and heat makes it worse.
Strength and power hold up a little longer. Most of the muscle-function studies in that review found the drop at 3 to 4%.
Bigger deficits push core temperature higher, and the risk of heat illness rises with it.

Sweat rate varies by individual

Adult sweat rates run from 0.5 to 4.0 litres an hour (NATA 2017), depending on:

• Body size (larger athletes sweat more)
• Exercise intensity
• Air temperature and humidity
• Acclimatization to heat, which raises sweat rate rather than lowering it
• Fitness level

The published guidelines

Before: start the session already hydrated. NATA’s original 2000 statement put that at 17 to 20 oz (500 to 600 mL) two to three hours out, then 7 to 10 oz (200 to 300 mL) in the last 10 to 20 minutes.
During: about 200 mL (7 oz) every 15 to 20 minutes is what the 2017 update calls ideal for most people. The aim is to finish less than 2% down, not to replace every drop.
After: 100 to 150% of what you lost (NATA 2017). At the top of that range it is 24 oz for each pound still down, and the extra half covers what leaves again as urine.

When electrolytes are needed

For sessions over an hour, and sooner in the heat, add sodium. Sweat carries anywhere from 10 to 100 mmol of sodium a litre, roughly 230 to 2,300 mg, so a salty sweater (white streaks on a dark cap) can lose ten times what a training partner does. Sports drinks, electrolyte tablets and salty snacks all work. Sweat carries far less potassium and very little magnesium, so sodium is the one to plan for.

Hyponatremia (low blood sodium) during long events comes from drinking more than you sweat, not less. Finishing heavier than you started is the warning sign, and it is why this plan never asks you to replace more than three quarters of your loss while you are still moving.

How this calculator estimates your sweat rate

Sweat rate scales with body size, so the estimate works in fluid ounces per pound per hour.
A moderate session is set at 0.21, which puts a 160 lb person at about 34 oz an hour, or almost exactly 1 litre.
Light is half that, high is one and a half times, very high is double, and hot or humid conditions add 30% on top.
Those are population averages and individual variation is genuinely large, so treat the number as a starting point rather than a prescription.

The absorption ceiling nobody mentions

Your stomach empties fluid at roughly 1 to 1.2 litres an hour at best, near 10 oz every 15 minutes, and plenty of people manage less. That limit does not rise just because you are sweating faster.
A hard session in the heat can lose fluid quicker than you can possibly take it in. When it does, the plan caps the during figure at that ceiling, moves the rest to afterwards, and tells you how far down you will finish.
Forcing more during only leaves it sloshing in your stomach.

The one test that beats this page

Weigh yourself before and after a typical session, in the same clothes and before you drink. Every pound lost is about 16 oz of fluid gone, plus whatever you drank on the way. Do it twice, once in cool weather and once in the heat, and you know your own sweat rate better than any estimate here.
For easy workouts under an hour, drinking to thirst is enough.

Signs of dehydration

• Dark yellow or amber urine
• Dry mouth and lips
• Headache or lightheadedness
• Muscle cramps
• Reduced sweat rate late in the session