How the daily target is worked out
For anyone 14 or over it starts from body weight — 0.67 fl oz per pound for men, 0.5 for women — then adds for activity and a hot climate. Children go by standard amounts for their age instead.
| Adds to the base | Per day |
|---|---|
| Light activity | + 6 fl oz (180 ml) |
| Moderate | + 12 fl oz (350 ml) |
| High | + 24 fl oz (700 ml) |
| Extreme | + 32 fl oz (950 ml) |
| Hot / tropical climate | + 12 fl oz (350 ml) |
Other fluids are taken off the total at roughly a cup of tea or coffee = 240 ml, a glass of juice or milk = 210 ml, a can of soda = 325 ml, a serving of fruit or vegetables = 120 ml.
Common questions
Why does it ask about tea, coffee and juice?
Those count toward your fluid total, so the calculator subtracts what you already typically drink from the recommended amount to show how much plain water is actually still needed.
Is the child guidance a rough guess?
No — it uses fixed age-band amounts (for example, roughly 40 oz for ages 4–8) rather than the weight-based formula used for teens and adults, matching standard pediatric guidance rather than scaling down the adult rule.
General targets for healthy people. Illness, fever, pregnancy and breastfeeding raise fluid needs; restrict fluids only on medical advice.