Hydration Calculator

Methodology

1. Inputs

   Sex, body weight (lb or kg), minutes of exercise or active work (0 to 600), how hard it was (light, moderate or hard), the weather (mild, hot, or hot and humid), and whether you are eating less than usual today. Nothing you enter leaves the page.

2. Formula

   Litres to drink today = baseline + eating less + activity + heat. Each term is a low–high range; the low ends add up to the low end of the result and the high ends to the high end. Cups are US cups of 236.6 ml.

3. Baseline

   The National Academies’ adequate intake for total water is 2.7 litres a day for women and 3.7 litres for men, from drinks and food together, and about 2.2 and 3.0 litres of that comes from drinks.^[1] The drinks figure is the baseline: a mild day with little activity.

4. Eating less

   Food normally supplies the rest of the total, about 0.5 litres for women and 0.7 for men (roughly a fifth).^[1] If you are eating less than usual, 40–60% of that share moves to drinks: 0.2–0.3 litres for women and 0.28–0.42 for men. The 40–60% is a Zornava assumption, not a published value.

5. Activity

   Hours of activity × a sweat rate for the effort × a weather multiplier. Whole-body sweat rates in athletes typically run from about 0.5 to 2.0 litres an hour.^[2] The rates and multipliers below are Zornava assumptions set inside that span; light effort starts lower because an easy pace in someone who isn’t training produces less sweat.

   Sweat rate, litres an hour

   Effort	Mild	Hot	Hot & humid
   Light	0.2–0.4	0.24–0.56	0.26–0.64
   Moderate	0.4–0.8	0.48–1.12	0.52–1.28
   Hard	0.7–1.2	0.84–1.68	0.91–1.92

   The hot and hot-and-humid columns are the mild rates × 1.2–1.4 and × 1.3–1.6: the low rate is multiplied by the low multiplier, the high rate by the high one.

6. Heat outside exercise

   A whole-day allowance of 0.3–0.7 litres on a hot day and 0.5–1.0 litres on a hot and humid one. The National Academies say heat raises water needs but do not put a number on it,^[1] so these allowances are Zornava assumptions.

7. Sodium and potassium in sweat

   Sweat today (activity plus the heat allowance) × the concentration range. Whole-body sweat carries roughly 10–70 mmol of sodium per litre^[2] (230–1,610 mg at 23.0 mg per mmol) and about 2–8 mmol of potassium^[2] (78–313 mg at 39.1 mg per mmol). Results under 500 mg are rounded to the nearest 10 mg, larger ones to the nearest 50 mg. For context, the adequate intake for sodium is 1,500 mg a day and for potassium 2,600 mg (women) and 3,400 mg (men).^[3]

8. The 2% line

   2% of the body weight you enter. The American College of Sports Medicine sets the goal of drinking during exercise as preventing a water deficit of more than 2% of body weight.^[4]

9. Limits

   • It is an estimate for healthy adults. It does not apply if a clinician has limited your fluids, or to kidney or heart conditions, pregnancy or breastfeeding.
   • Sweat rate and sweat sodium vary several-fold between people;^[2] only a sweat test narrows them for one person. That is why every result is a range.
   • More is not better: drinking far beyond what you lose can dilute blood sodium.^[1]