How the hydration estimate works

See how HydrateWise works out daily fluid, sweat rate, electrolyte and race estimates, and what each result can tell you.

Your daily starting point

The calculator first converts your weight to kilograms. It starts at 33 ml per kilogram and shows a range of 31 to 35 ml per kilogram. It adjusts those figures for your activity and the weather, then gives the result in litres, millilitres and cups.

On a quiet day, the calculator adds nothing for activity. The most active setting adds 1,000 ml. Warm weather adds 250 ml, and hot or humid weather adds 500 ml. If you use current weather, the calculator uses temperature and humidity to pick one of those settings.

When you add a workout

Enter how long and hard you plan to exercise to use a workout estimate in place of the general activity allowance. The calculator uses 250 ml per hour for easy sessions, 500 ml for moderate sessions and 750 ml for hard sessions. It works out a share of that hourly figure for the time you enter, up to four hours.

What you’ve already drunk

The daily calculator, drink checker, Water Left Today and tracker use the same list of drinks saved for today on your device. The calculator counts the full amount of each non-alcoholic drink towards the estimate. It shows alcoholic drinks separately and leaves water from food out of the calculation.

Measuring sweat rate

The sweat rate calculator follows the formula from the National Athletic Trainers’ Association. Start with the weight you lost in kilograms, add the litres you drank and subtract any urine you passed in litres. Divide the result by the number of hours you exercised to estimate how much you lost per hour.

Save a few tests with their conditions so you can pick one that matches the workout or event you’re planning.

Race and electrolyte estimates

When you add a measured sweat rate, the race and electrolyte calculators start with 60% to 90% of it. They round to the nearest 25 ml and aim to keep the estimate between 50 ml and 1,000 ml an hour and below the measured loss. If you don’t know your rate, the race planner starts with a broader range of 300 to 750 ml an hour and adjusts for heat.

If you know how much sodium is in each litre of your sweat, the calculator multiplies that figure by the estimated litres lost. Salt marks can give you a clue that your sweat is salty. A sweat-sodium test gives you a number you can use in the calculator. Electrolyte products cannot make overdrinking safe.

Tracker totals and streaks

The tracker groups drinks by the calendar date on your device. It counts a target day when you log between 90% and 125% of the target saved for that day. Averages use the days you logged. A day without entries breaks a streak and appears as unlogged.

Use the estimates to plan and adjust for thirst, comfort and where you can get a drink. Follow any personal advice from your healthcare team. The bottle planner can help you work out how many refills you’ll need.