Water Intake Calculator
It estimates a daily water target from body weight, then adds for exercise and hot climates.
Daily fluid target
2.79 L
Liters
2.79
Fluid ounces
95
Cups (240 ml)
11.6
Glasses (250 ml)
11.2
From exercise
+350 ml
From climate
+0 ml
General guidance only, not medical advice. Kidney, heart and liver conditions can change fluid requirements substantially — follow your clinician's advice over any calculator.
What it calculates
It estimates a daily water target from body weight, then adds for exercise and hot climates.
Why it matters
Fixed advice like 'eight glasses a day' ignores body size entirely — a 55 kg and a 95 kg person have very different needs.
Who it's for
Anyone tracking hydration, athletes planning fluid replacement, and people in hot climates.
Formula
- w
- Body weight in kilograms
- 30–40
- Milliliters per kilogram, the usual guideline range
- +350
- Milliliters per half hour of exercise
Worked example
70 kg, no exercise, temperate climate
- 170 × 35 ml = 2,450 ml
- 2No exercise or heat adjustment
About 2.45 liters, or 83 fl oz, a day
How the water intake calculator works
The common clinical guideline is 30–40 ml of fluid per kilogram of body weight per day, and the midpoint of 35 ml/kg is used as the target. Roughly 350 ml is added for each half hour of exercise to replace sweat losses, and about 500 ml for a consistently hot climate. This is total fluid intake — around a fifth of it typically comes from food.
The common clinical guideline is 30–40 ml of fluid per kilogram of body weight per day. The midpoint of 35 ml/kg is used as the target, with the full range shown alongside it.
This is total fluid, not just plain water — tea, coffee, milk and food all count, and roughly a fifth of a typical intake comes from food. Pale straw-colored urine remains the simplest practical check.
Common mistakes
- Treating 'eight glasses' as universal regardless of body size or activity.
- Counting only plain water; tea, coffee, milk and food all contribute.
- Drinking very large volumes rapidly during endurance events, which risks hyponatraemia.
Tips and best practice
- Pale straw-colored urine is the simplest practical indicator of adequate hydration.
- During long endurance efforts, replace electrolytes as well as fluid.
Frequently asked questions
How much water should I drink a day?
Roughly 30–40 ml per kilogram of body weight. For a 70 kg adult that is about 2.1–2.8 liters of total fluid, before adjusting for exercise and heat.
Does coffee or tea count?
Yes. The mild diuretic effect of caffeine does not offset the fluid in the drink, so both contribute to your daily total.
How much extra should I drink when exercising?
About 350–700 ml per hour of exercise, more in heat. Weighing yourself before and after gives the most accurate figure for your own sweat rate.
Can you drink too much water?
Yes. Consuming very large volumes quickly can dilute blood sodium — hyponatraemia — which is dangerous. It is rare but does occur in endurance events.
Does food count toward the total?
Yes, roughly 20% of daily fluid intake typically comes from food, especially fruit and vegetables.
Related calculators
Methodology & trust
- Formula source
- Commonly cited 30–40 ml/kg clinical guideline for daily fluid intake.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.