Fluid and sodium calculator
Fluids and sodium per hour from your weight, the weather and your sweat.
How does the fluid and sodium calculator work?
Without a sweat rate it estimates fluids from your body weight and the weather, from about 6 ml per kilogram an hour in the cold to about 12 ml in the heat. If you have measured your own sweat rate, that replaces the estimate. Sodium comes from the weather and how salty your sweat is, and both numbers rise at altitude.
To measure your sweat rate, weigh yourself before and after a one-hour run, add what you drank, and subtract any pee. The weight lost in grams, plus the drink in millilitres, is close to your sweat rate in millilitres an hour.
How to use the numbers
Fluids are what you aim to drink each hour, not what you must force down. Drink to thirst when you can, and use the number to check you are not far below it. Sodium can come from a drink mix, salt capsules or food. For a hot race, run the heat adjustment calculator too, because heat raises both numbers.
Frequently asked questions
How much sodium per hour in an ultra?
A common range is 300 to 800 mg an hour, with more for hot weather, heavy sweaters and altitude. The right amount for you is the one that keeps you feeling well on long training runs.
Can I drink too much in an ultra?
Yes. Drinking far more than you sweat, especially with little sodium, can lower blood sodium and make you ill. That is why the tool gives a target to aim for and not a number to exceed.
Why does altitude change the numbers?
At altitude the air is drier and you breathe faster, so you lose more water, and sodium needs rise a little with it. The tool adds a modest increase above about 2000 m.