What this calculator works out
This calculator builds a daily fluid target from body weight, then adjusts it for sex, age, activity level, exercise, temperature and how much caffeine and alcohol you have had.
It is the more detailed of the two hydration tools here. The adjustments are broad allowances rather than measured physiology, but they capture the factors that genuinely move fluid needs.
Hydration details
Before you use the result
- Enter exercise in minutes done today, not a typical week.
- Count caffeinated drinks consumed today, not cups of any drink.
- Choose the temperature band for where you actually are, which for indoor workers may differ from the weather outside.
- The result is total fluid from all sources — every drink and the water in food.
Alcohol suppresses the hormone that helps the body retain water, so it increases urine output well beyond the volume drunk. That is why the alcohol adjustment here is much larger per drink than the caffeine one, and why replacing fluid alongside alcohol makes a noticeable difference the following morning.
How this calculator works
A base figure from weight, then a series of additions and subtractions:
Base = weight in kg × 35 ml
Less 150 ml if female · Less 100 ml if over 65
Plus activity level, plus temperature band
Plus 10 ml per minute of exercise
Plus 40 ml per caffeinated drink · Plus 150 ml per alcoholic drink
Subject to a minimum of 1,200 mlThe floor of 1,200 ml exists so that a very light adult with no adjustments does not produce an implausibly low target.
These allowances are approximations drawn from general guidance rather than individually measured losses. Actual sweat rates vary several-fold between people doing identical exercise.
Worked example: 80 kg, mostly sitting, 30 minutes of exercise
Using the default figures — 80 kg, male, age 45, mostly sitting, 30 minutes of exercise, cool conditions, two caffeinated drinks, no alcohol:
- Base: 80 × 35 = 2,800 ml
- Exercise: 30 × 10 = 300 ml
- Caffeine: 2 × 40 = 80 ml
- Total: 3,180 ml, or about 3.2 litres
Add three alcoholic drinks and hot conditions and the figure rises to 4.1 litres. That 900 ml swing from two variables is the useful part of this tool: it shows that fluid needs are driven far more by what you do and where you are than by your body weight.
Common mistakes
- Reading the total as water to drink in addition to everything else. It is the total from all sources.
- Entering weekly exercise instead of today's. The allowance is per session, not per week.
- Ignoring a clinical fluid restriction. Advice from a clinician overrides any calculator.
- Assuming a higher figure is better. There is no benefit to exceeding your needs.
- Using it during illness with vomiting or diarrhoea. Losses are much larger and rehydration solutions may be needed.
Frequently asked questions
Does caffeine dehydrate you?
Much less than commonly believed. Caffeine has a mild diuretic effect, but caffeinated drinks still deliver more fluid than they cause you to lose, and regular consumers develop tolerance to the effect. NHS guidance counts tea and coffee towards fluid intake for exactly this reason. The small allowance here is conservative rather than necessary.
Why does the female adjustment reduce the target?
It reflects average differences in body composition — lean tissue holds more water than fat tissue, and average body composition differs between sexes at the same weight. It is a population average applied as a flat offset, not a measurement of you, which is why the adjustment is small.
How much extra should I drink when exercising?
The allowance here is 10 ml per minute, or 600 ml an hour. Actual sweat rates vary enormously — from under half a litre to more than two litres an hour depending on intensity, heat, humidity and the individual. Weighing yourself before and after a session gives a personal figure: each kilogram lost is roughly a litre of fluid.
What about during illness?
Vomiting and diarrhoea cause losses well beyond anything modelled here, along with salts that plain water does not replace. Oral rehydration solutions are designed for this and are available from any pharmacy. Contact a GP or NHS 111 if someone cannot keep fluids down, particularly a young child or an older adult.
Is what I enter stored?
No. Everything is processed in your browser and never transmitted or retained.
Related tools
References
- NHS — fluid intake guidance, dehydration signs and rehydration advice
- GOV.UK — Scientific Advisory Committee on Nutrition advice on water requirements
Sources are checked at publication and can change — how I choose and check references.