Water Intake Calculator: How Much Water Should I Drink a Day?

Short answer

A common rule of thumb is 30 to 35 ml of water per kg of body weight a day, plus extra on training days and in hot weather. Example with made-up numbers: a 70 kg person gets a base of 2.1 to 2.45 L. With 60 minutes of training, add 0.7 L. On a hot day, add another 0.5 L. That makes 3.3 to 3.65 L, about 13 to 15 glasses of 250 ml.

How the estimate is calculated

• Base = body weight (kg) × 30 to 35 ml
• Exercise = 0.35 L for every 30 minutes
• Hot weather = 0.5 L more
• Glasses = litres ÷ 0.25

Base range by body weight, before exercise and heat:

How this compares with official figures

The 30 to 35 ml per kg range is a rule of thumb, not a number taken from one study. For comparison, the European Food Safety Authority (EFSA) sets adequate intakes of 2.0 L a day for women and 2.5 L for men (EFSA, 2010). Those figures count water from all drinks and from food, and they assume a mild climate and moderate activity. EFSA notes that food usually supplies about 20 to 30% of the total, so the part you actually drink can be lower than the full figure.

The extra for exercise and heat is a rough allowance. Sweat rates vary a lot between people, so the American College of Sports Medicine recommends estimating yours by weighing yourself before and after training. The goal during exercise is to avoid losing more than 2% of body weight from water (ACSM, 2007). For a 70 kg person, 2% is 1.4 kg. If you lose more than a kilo in a session, you probably need more than the calculator adds.

Creatine and water

A common worry is that creatine dehydrates you or that you need to drink litres more while taking it. A review by creatine researchers found that experimental and clinical studies don't support the idea that creatine causes dehydration or muscle cramps, and it doesn't give an extra amount of water to drink (Antonio et al., 2021). So the calculator adds nothing for creatine. Mix it with water or any drink you like and keep drinking as usual.

Practical tips

• Spread drinks through the day instead of catching up at night.
• Have a drink with each meal and around your training.
• Coffee, tea, milk and soups count towards the total, as does the water in fruit and vegetables.
• Thirst is a useful signal for most healthy people in normal conditions.

Limits

This is an estimate for healthy adults, not medical advice. If you have kidney or heart disease, have been told to limit fluids, take diuretics, or are pregnant or breastfeeding, follow your doctor's advice. EFSA, for example, sets higher intakes during pregnancy and breastfeeding. Drinking far more than you need isn't safer either, so don't force down large amounts in one go.