Hydration Calculator

How much water do you actually need?

The classic "8 glasses a day" advice is a rough heuristic with no specific scientific backing. Real needs depend on bodyweight, activity, climate, and individual physiology.

The Institute of Medicine's Adequate Intake (AI) for total water (including water from food) is about 3.7 litres per day for men and 2.7 litres for women. Food typically provides 20–30% of this — so direct fluid intake is closer to 2.5 L men / 2.0 L women as a baseline.

The practitioner heuristic of ~35 ml per kg bodyweight produces similar numbers for average adults but scales better across body sizes. We use that as the base, plus adjustments for exercise and hot climates.

The breakdown

• Base: ~35 ml × bodyweight (kg) — covers daily respiration, perspiration, and urinary losses for most healthy adults.
• Exercise bonus: ~500 ml per 30 minutes of moderate- to-vigorous activity to replace sweat losses (per ACSM position stand).
• Climate bonus: ~500 ml in hot or humid climates where sweat rates are higher even at rest.

When this calculator isn't for you

This calculator produces a general guideline for healthy adults under typical conditions. It is not medical advice and is not designed for clinical use. Fluid intake can matter clinically in either direction — too little or too much — and a number of conditions and circumstances require personalised guidance from a qualified professional rather than a heuristic.

Talk to a healthcare professional rather than relying on this calculator if any of the following apply:

• Kidney disease, heart failure, liver disease, or another condition that affects fluid balance.
• You take diuretics, lithium, or other medications that affect fluid or electrolyte balance.
• You're pregnant, breastfeeding, an infant, a young child, or a frail older adult.
• You're an endurance athlete, working in extreme heat, or sweating heavily for sustained periods — fluid and electrolyte planning matters and a flat per-kg number is not enough.
• You have a history of disordered eating, water-loading behaviour, or have been told by a clinician to restrict fluids.

For everyone else: use this as a rough target, listen to thirst and urine colour, and adjust around exercise and weather.