Hydration Calculator
Estimate your daily water intake based on bodyweight, exercise, and climate.
Enter your details to see your water target
Your daily fluid intake target, scaled to bodyweight with adjustments for exercise and climate.
How to use this calculator
- 1
Pick your units
Imperial or metric. The weight input swaps over instantly.
- 2
Enter your bodyweight
The base calculation uses ~35 ml per kg of bodyweight.
- 3
Add adjustments
Minutes of moderate-to-vigorous exercise per day (+~500 ml per 30 minutes), and whether your climate is hot or humid (+500 ml).
- 4
Read your results
Total daily target in liters, fluid ounces, and approximate cups, plus a visual glass count.
How much water do you actually need?
The classic "8 glasses a day" advice is a rough rule of thumb that doesn't scale with body size. 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 liters per day for men and 2.7 liters for women. Food typically provides 20–30% of this, so the beverage share works out to roughly 3.0 L for men and 2.2 L for 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) — approximates typical daily drinking-water needs (i.e., the amount you cover by drinking), not total water turnover, 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.
How is hydration calculated?
Total daily fluid target =
(weight (kg) × 35 ml)
+ (exercise minutes / 30 × 500 ml)
+ (500 ml if hot climate)
This produces a working estimate. It's deliberately simple. Real hydration needs vary with kidney function, sweat rate (which differs by 2–3x between individuals), medications, dietary sodium, altitude, and many other factors. The simplest day-to-day checks are thirst and urine color: pale yellow means you're well hydrated, dark amber means drink more.
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). A number of conditions and circumstances require personalized 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 or breastfeeding, or you're calculating for an infant, young child, or 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 or water-loading behavior, or a clinician has told you to restrict fluids.
For everyone else: use this as a rough target, listen to thirst and watch urine color, and adjust around exercise and weather.