Working out your daily water needs
Water keeps your blood flowing, your joints cushioned, your temperature steady and your kidneys flushing out waste. You lose it constantly — through breathing, sweat and urine — so it has to be topped up every day. This calculator turns three simple inputs (your weight, how long you exercise and whether it’s hot) into a sensible starting estimate for total daily fluid.
How the math works
The estimate is built from three parts that are added together:
- Baseline:
0.033 L × body weight in kg— bigger bodies need more water. - Exercise:
+0.35 L for every 30 minutesof activity (0.35 × minutes ÷ 30). - Hot climate:
+0.5 Lwhen you tick the hot-weather box.
Imperial weights are converted first: 1 lb = 0.45359 kg. The total litres are then
converted to other units — 1 US cup = 236.6 ml and 1 US fl oz = 29.5735 ml.
A worked example
Take a 70 kg adult who runs for 45 minutes on a hot day. The baseline is
0.033 × 70 = 2.31 L. Exercise adds 0.35 × 45 ÷ 30 = 0.525 L, and the hot
climate adds 0.5 L. The total is 2.31 + 0.525 + 0.5 ≈ 3.3 L — about
3,335 ml, or roughly 14 US cups / 113 fl oz
of total fluid for the day from all food and drink.
What changes your needs
Climate & heat
Hot, humid or high-altitude conditions increase sweat loss. Air-conditioned and heated indoor air can dry you out too.
Activity level
Longer or more intense exercise means more sweat. Replace fluid before, during and after long sessions.
Diet & drinks
Water-rich food, tea, coffee and milk all count. About 20–30% of intake typically comes from food, not the glass.
Life stage & health
Pregnancy, breastfeeding, fever, vomiting and some medicines raise needs. Certain conditions require limits instead.
Quick reference by weight
| Body weight | Baseline (no exercise) | Approx. US cups |
|---|---|---|
| 50 kg (110 lb) | 1.65 L | ~7 cups |
| 60 kg (132 lb) | 1.98 L | ~8 cups |
| 70 kg (154 lb) | 2.31 L | ~10 cups |
| 80 kg (176 lb) | 2.64 L | ~11 cups |
| 90 kg (198 lb) | 2.97 L | ~13 cups |
Baselines use 0.033 L per kg before any activity or heat adjustment. They represent total fluid from all sources, not plain water alone. The best day-to-day guide is simple: drink when you’re thirsty and check that your urine is pale straw-coloured.