HydrateWise

How the hydration estimate works

See the formulas, assumptions and limitations behind HydrateWise daily water, weather, sweat-rate, electrolyte and race-planning calculations.

The daily calculator converts kilograms, pounds or stone into kilograms. Its central estimate begins at 33 ml per kilogram, while the displayed planning range uses 31–35 ml per kilogram. It then adds the selected activity and climate assumptions before converting the result into litres, millilitres and cup equivalents.

Daily activity, climate and current weather

Without specific exercise details, activity adds 0–1,000 ml across sedentary, light, moderate, high and intense-training settings. Warm conditions add 250 ml; hot or humid conditions add 500 ml. Optional current weather maps temperature and humidity into those same visible categories. The weather rules are broad planning assumptions—not measurements of sweat or a clinical prescription.

Optional exercise-duration adjustment

When duration is entered, it replaces the broad activity allowance. Easy, moderate and hard sessions use planning rates of 250, 500 and 750 ml per hour respectively, prorated for up to four hours. These deliberately simple rates do not claim to reproduce personal sweat loss; repeat athletes should use the measured sweat-rate tool.

Current drinks and remaining amount

Non-alcoholic prepared drinks entered in the calculator reduce the displayed remaining amount at their recorded volume. Alcohol is shown separately and does not reduce it. Drink entries are dated by the device’s local calendar day, stay in browser storage and are shared with the beverage audit; food moisture is not estimated.

Sweat-rate equation

The sweat-rate calculator follows the equation published in the National Athletic Trainers’ Association position statement: sweat loss equals body-mass loss in kilograms plus fluid consumed in litres minus urine passed in litres. Dividing that loss by exercise hours produces litres per hour.

Electrolyte and race-planning ranges

The electrolyte and race tools use 60–90% of a valid measured sweat rate as a broad starting range, rounded to 25 ml. The lower boundary is 50 ml per hour and the upper boundary is 1,000 ml per hour; the displayed range is kept below the measured loss rate. It remains adjustable for thirst, comfort and access rather than acting as a full-replacement instruction. When sweat rate is unknown, the race planner uses a clearly labelled 300–750 ml-per-hour contextual range that changes with the selected heat category.

Measured sodium loss is sweat loss multiplied by a supplied sweat-sodium concentration. When concentration is unknown, saltiness answers produce broad illustrative ranges only. Appearance cannot measure sodium concentration, and neither sodium nor an electrolyte product makes overdrinking safe.

History, target days and streaks

Planner totals are grouped by local calendar day. A consistent target day falls between 90% and 125% of the target saved with that day’s entries; missing days remain visible and break streaks without being described as measured zero intake. Higher totals are shown without a reward so the interface does not encourage overdrinking. Averages use logged days. History, custom drinks and reminder preferences remain in browser storage.

Important limitations

The models do not determine hydration status, measure food-derived water or electrolyte loss, diagnose illness, or know organ function. Decimal outputs are estimates. Repeat sweat-rate measurements in representative conditions and never treat observed loss as an instruction to force the same volume of fluid.

Why the settings remain editable

A cool rest day, a long workout and a hot travel day should not share one automatic target. The settings make assumptions visible, while the bottle planner turns the result into convenient checkpoints without creating rigid deadlines.