Water intake calculator
A practical daily water range based on your bodyweight, with extra for exercise and hot weather. This matters more during a fast, because food normally supplies a fair share of your fluid, so a fasting day can leave you drinking less than you think.
Aim for roughly
–
spread across the day, not in a few big gulps
This uses a common rule of thumb (30 to 35 ml per kg of body weight, plus extra for exercise and heat), not an official target. For comparison, the official European reference amounts for total daily water are about 2 litres for women and 2.5 litres for men, counting food, which usually supplies about a fifth. On a fast you miss that share, so let thirst and pale-yellow urine guide you. If you have heart or kidney disease, follow your doctor's fluid advice instead.
Related: do electrolytes break a fast and the troubleshooter.
How to use the water intake calculator
Pick kilograms, pounds, or stone and pounds, then enter your weight. Add the hours you exercise that day, in half-hour steps, and tick “Hot climate today” if it applies. The result is a daily range in litres. One litre is about 34 US fluid ounces.
How the range is worked out
The calculator multiplies your weight by 30 ml for the low end and 35 ml for the high end. It then adds 500 ml for each hour of exercise and 500 ml for a hot day. This 30 to 35 ml per kg method is our rule of thumb. No health body sets a water target per kilogram.
| Weight | Range on a quiet day | With 1 hour of exercise and heat |
|---|---|---|
| 60 kg (132 lb) | 1.8 to 2.1 L (61 to 71 fl oz) | 2.8 to 3.1 L |
| 80 kg (176 lb) | 2.4 to 2.8 L (81 to 95 fl oz) | 3.4 to 3.8 L |
| 100 kg (220 lb) | 3.0 to 3.5 L (101 to 118 fl oz) | 4.0 to 4.5 L |
Worked example: an 80 kg runner trains for an hour on a hot day. The low end is 80 × 30 = 2,400 ml, plus 500 for the run and 500 for the heat, so 3,400 ml. The high end is 80 × 35 = 2,800 ml plus the same 1,000 ml, so 3,800 ml. On screen that reads 3.4 to 3.8 L, about 115 to 128 fl oz.
The rule of thumb next to official water figures
Official figures are adequate intakes of total water, which means drinks plus the water in food.
| Body | Women | Men |
|---|---|---|
| EFSA (Europe), total water | 2.0 L a day | 2.5 L a day |
| US National Academies, total water | about 2.7 L | about 3.7 L |
About a fifth of that total normally comes from food. On EFSA’s figures, a woman’s drinks need to cover about 1.6 litres a day and a man’s about 2.0 litres. In UK guidance it comes out as 6 to 8 cups or glasses of fluid a day. For an 80 kg adult on a quiet day, the calculator’s 2.4 to 2.8 litres is more than EFSA’s drinks share for either sex. Treat the range as a rough guide and let thirst and urine colour set the pace.
Water intake on a fasting day
When you skip meals, you also skip the water those meals held. On a 16:8 day or a one-meal day, drinks have to make up that share. Sip steadily through the fasting hours instead of drinking a litre at once. The simplest check is the colour of your urine: aim for a clear, pale yellow. On a longer fast, a headache that lifts after a pinch of salt in water suggests sodium played a part. Our guide to salt and electrolytes on a fast covers how much is sensible.
Who should be careful
Some people need less water than the calculator suggests, and some need more. If you have heart failure or kidney disease, follow your doctor’s fluid limit instead of the calculator. If you are pregnant or breastfeeding, you may need more than usual. Very large amounts of water can dilute the sodium in your blood, which is dangerous, so spread your water across the day.
Related tools and guides
The guide on drinks that keep you fasted sorts tea, coffee, diet drinks and lemon water by calories. If headaches or tiredness keep coming back, the fasting troubleshooter walks through the usual causes. For fasts longer than a day, read our guide to water-only fasts first.
Frequently asked
How much water should I drink a day?
Official reference amounts count all water, from drinks and food together. In Europe the figure is 2.0 litres a day for a woman and 2.5 litres for a man. The US figures are higher, at about 2.7 and 3.7 litres. The water intake calculator gives a personal range from your weight, exercise and the weather.
Is water allowed during intermittent fasting?
Yes. Plain water has 0 calories, so it doesn't break a fast on any schedule, from 12:12 to one meal a day. Every plan on this site assumes you drink water through the fasting hours. Sparkling water and plain tea are also fine.
Does food count towards my daily water?
Yes. On a normal day, about 20% of the water you take in comes from food. On a fasting day you eat less, so more of your water has to come from drinks.
Can you drink too much water while fasting?
Yes. Drinking far more than you need can upset the balance of water and sodium in your body. If blood sodium falls too low (hyponatraemia), brain cells can swell, and that needs emergency care. Spread your water through the day and stop when you are no longer thirsty.
Why does the calculator add water for exercise and heat?
You lose extra water as sweat when you exercise or the weather is hot. The calculator adds about 500 ml for each hour of exercise and another 500 ml on a hot day. Both amounts are our rule of thumb, not official figures.