What this calculator does
Fluid requirements scale roughly with body size, then rise with anything that increases losses: exercise, heat, altitude, illness. A common starting estimate is around 33 millilitres per kilogram of body weight, with additions on top for sweat losses.
This is a guideline rather than a prescription. Food contributes a meaningful share of daily fluid, and thirst is a reasonably reliable signal for most healthy adults. The calculation is most useful for people who exercise heavily, work in heat, or simply find a concrete target helpful.
The formula
A baseline is calculated from body weight, then roughly half a litre is added for each 30 minutes of exercise and a further allowance for warm or hot conditions. The result is total fluid, of which drinks are the main but not the only source.
| Term | Meaning |
|---|---|
| Baseline | Body weight × 0.033 litres: the resting requirement. |
| Exercise allowance | Approximately 0.5 L per 30 minutes of activity. |
| Climate allowance | An addition for warm or hot conditions. |
The inputs explained
| Field | What to enter |
|---|---|
| Weight (kg) | Body weight in kilograms. |
| Exercise (min/day) | Minutes of exercise on a typical day. Sweat rates vary widely between individuals, so treat this allowance as approximate. |
| Climate | The conditions you spend the day in. Heat raises losses substantially, particularly with any physical activity. |
When to use it
Setting a daily target
The glasses figure is often the easiest way to track it. Spreading intake across the day is more effective than drinking large volumes at once.
Adjusting for a training day
Raise the exercise minutes to see how much extra a long session demands. For sessions beyond about an hour, particularly in heat, electrolytes matter as well as volume.
Working in heat
Hot conditions can add a litre or more to daily requirements before any exercise is counted. Outdoor and industrial work in summer is where under-hydration is most consequential.
Checking your own hydration
Urine colour is a practical everyday indicator: pale straw generally suggests adequate hydration, dark yellow suggests more fluid is needed. It is more responsive than any formula.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Daily water target by body weight
Baseline requirements plus a 45-minute exercise allowance, across a range of body weights.
| Body weight | Daily target | Baseline from body weight | Glasses (250 ml) |
|---|---|---|---|
| 50 kg | 2.4 litres | 1.7 L | 10 |
| 60 kg | 2.7 litres | 2.0 L | 11 |
| 70 kg | 3.1 litres | 2.3 L | 12 |
| 80 kg | 3.4 litres | 2.6 L | 14 |
| 90 kg | 3.7 litres | 3.0 L | 15 |
| 110 kg | 4.4 litres | 3.6 L | 18 |
How exercise changes the requirement
A 75 kg person at different daily exercise durations.
| Exercise | Daily target | Glasses (250 ml) | Per waking hour |
|---|---|---|---|
| 0 min | 2.5 litres | 10 | 155 ml |
| 30 min | 3.0 litres | 12 | 186 ml |
| 45 min | 3.2 litres | 13 | 202 ml |
| 60 min | 3.5 litres | 14 | 217 ml |
| 90 min | 4.0 litres | 16 | 248 ml |
| 120 min | 4.5 litres | 18 | 280 ml |
Questions
Do I really need eight glasses a day?
The eight-glasses rule has no strong scientific basis and takes no account of body size, activity or climate. A weight-based estimate like this one is more defensible, and thirst is a reasonable guide for most healthy adults.
Does tea, coffee or food count?
Yes. Food typically supplies around 20 per cent of daily fluid, and caffeinated drinks are net hydrating despite a mild diuretic effect. The target here is total fluid from all sources.
Can you drink too much water?
Yes, though it is uncommon. Consuming very large volumes rapidly can dilute blood sodium: a serious condition called hyponatraemia, occasionally seen in endurance events. Drinking to thirst and spreading intake across the day avoids it.
How do I know if I am well hydrated?
Urine colour is the simplest everyday check: pale straw generally indicates adequate hydration. Persistent dark urine, headaches or dizziness are worth discussing with a doctor.
Do I need electrolytes as well?
For everyday activity, generally no: normal food supplies enough. For prolonged exercise, particularly over an hour in heat, replacing sodium alongside fluid becomes more important.
For energy expended in the same session, see calories burned. For daily energy needs, use the TDEE calculator.