About
Three inputs, three multipliers, one range. The simplifications are listed, not hidden.
What the page computes
Three inputs, one range: a baseline from body weight, an addition for exercise, and a percentage for heat. Only the total is rounded; the breakdown under the result shows each step so the sum can be reproduced with a calculator.
| Step | Formula | Worked example (70 kg, 30 min, hot) |
|---|---|---|
| Baseline | weight × 30 to weight × 35 ml | 2,100–2,450 ml |
| Exercise | (minutes ÷ 30) × 350 to 500 ml | +350–500 ml |
| Heat | subtotal × 1.10 to subtotal × 1.20 | +245–590 ml |
| Result | rounded to the nearest millilitre | 2,695–3,540 ml |
The cups line divides the result by 250 ml and rounds, which is why the two figures can disagree slightly: 2.7 L is 10.8 cups, shown as 11.
Input handling
- Weight is accepted from 20 to 300 kg. Outside that range the result is blank rather than absurd — a typing error should not produce a confident answer.
- Exercise minutes are proportional: 90 minutes adds exactly three times the 30-minute amount. The field accepts 0 to 480.
- Heat is a checkbox, not a temperature. It applies the 10–20% addition to the baseline plus exercise subtotal.
- Recalculation happens on every keystroke and every toggle; there is no submit button and no page reload.
Why these coefficients
The 30–35 ml/kg baseline is a clinical rule of thumb, not a regulatory standard. It lands near the reference totals published by EFSA and the US Institute of Medicine for average adults, and it scales with body size in a way a fixed "eight glasses" cannot. It is a starting point for a healthy adult, and the page says so.
The exercise addition of 350–500 ml per half hour corresponds to roughly 0.7–1.0 L of sweat per hour, which is mid-range for moderate activity in temperate conditions. Real sweat rates span about 0.5–2.0 L/hour, which is why the page explains the weigh-in method on the how-much-water page instead of pretending one coefficient fits everyone.
The heat addition is a percentage rather than a fixed volume because losses scale with body size and with what is being worn. It is a blunt instrument: humidity, wind, shade and acclimatisation all matter, and none of them is an input.
What the calculator does not know
- Your body composition and health history. Muscle and fat differ in water content, and kidney, heart and liver disease change the whole calculation.
- Medication. Diuretics, lithium and some other drugs alter fluid and sodium balance.
- Pregnancy and breastfeeding. Requirements rise, and the baseline here does not model it.
- Sodium intake and sweat sodium concentration. Both matter for long events and heavy sweaters.
- What you eat. The result is total water, including food; without a diet record the split between food and drink is an assumption.
- What you actually drink. The page estimates a target; it does not track intake, and it has no memory from one visit to the next.
Sources behind the numbers
The reference intakes quoted on this site are the 2004 US Institute of Medicine adequate intakes (3.7 L/day for men, 2.7 L/day for women, total water) and the 2010 EFSA reference values (2.5 L and 2.0 L). The exercise guidance follows the general shape of the American College of Sports Medicine position on fluid replacement: replace losses, include sodium for long or heavy sweating, and avoid drinking beyond need. The NIOSH-style exposure arithmetic does not apply here; nothing on this site is a workplace or clinical standard.
How the pages are built
Four static pages, one stylesheet and one script, served from this domain. No framework, no third-party script, no analytics, no cookie. The formula and the formatting live in one shared script and are covered by tests outside the browser, so the calculator and this description cannot drift apart.
Corrections
If a coefficient is wrong or a boundary is handled badly, the fix belongs in the shared script, not in the page text. The tests check the worked example above and the input boundaries.