A general estimate for healthy adults — total fluid from all drinks and food, not just plain water. Not medical advice; if you have a heart or kidney condition, follow the fluid limit your doctor sets.
How your water target is worked out
The calculator starts from your bodyweight, using the common rule of about 30 to 40 millilitres of water per kilogram — so it shows a low-to-high band with the midpoint at 35 ml/kg rather than one exact figure. It then adds roughly 350 ml for every 30 minutes of exercise to replace what you sweat out, and scales the whole total up in warm (+10%) or hot and humid (+20%) conditions. The result is your total daily fluid, shown in litres, US cups and glasses so you can pick whichever is easiest to track.
Why it is a range, not a single number
Hydration needs genuinely vary between people and from day to day — with your size, how hard you train, the weather, altitude, illness and even the food you eat. A single hard number would give a false sense of precision, so aim for the middle of the band and adjust by how you feel. The simplest real-world check is the colour of your urine: pale straw means you are well hydrated, while dark yellow means drink more.
Fitting water around the rest of your day
Remember that about a fifth of your fluid comes from food, and that tea, coffee, milk and juice all count — so you rarely need to drink the whole figure as plain water. Spread it across the day rather than gulping it late, keep a bottle within reach, and drink a little more around workouts. Pair this with the TDEE Calculator to set your calories and the Protein Calculator to round out the nutrition side of your plan.