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Health & Fitness Calculators

Health and fitness calculators turn height, weight, age and activity level into the numbers people actually train and eat by. They suit anyone starting a programme, tracking progress between weigh-ins, or simply checking whether a target they have been given is reasonable. The BMI Calculator is the usual starting point, giving a single figure and the band it falls into. It takes no account of muscle, so pair it with the Body Fat Calculator, which uses circumference measurements to estimate composition and gives a fairer picture for anyone who lifts. The Ideal Weight Calculator provides a range for your height rather than one number, which is more useful as a goal to move towards. For daily habits, the Calorie Calculator estimates the energy you burn and the intake that would maintain, lose or gain weight at a chosen rate, and the Water Intake Calculator suggests a fluid target based on body weight and how much you train. A sensible order is to measure first with BMI or body fat, set a target with the ideal weight range, then use the calorie figure to plan meals around it. Results are estimates rather than medical advice. Everything runs in your browser with no account and nothing recorded.

5 tools available

Showing 1–5 of 5

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BMI Calculator

Work out your body mass index from metric or imperial height and weight, with the matching category band.

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Health & Fitness Calculators 1,486 uses
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Calorie Calculator

Estimate BMR and daily calorie needs with Mifflin-St Jeor or Harris-Benedict, plus macro and meal splits.

Health & Fitness Calculators 34 uses
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Ideal Weight Calculator

Compare four established ideal body weight formulas against your height, sex and current weight.

Health & Fitness Calculators 402 uses
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Body Fat Calculator

Estimate body fat percentage from tape measurements using the validated US Navy circumference formula.

Health & Fitness Calculators 91 uses
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Water Intake Calculator

Estimate your daily water target from body weight, age, exercise, climate and life stage, with a drinking schedule.

Health & Fitness Calculators 276 uses

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How the daily target is assembled

The calculation starts with a base need scaled to body weight and age: 35 ml per kilogram for ages 14 to 30, 33 ml per kilogram for 31 to 55, and 30 ml per kilogram from 56 upward, reflecting the gradual decline in body water with age. Children under 14 use fixed reference values instead, since scaling by weight is unreliable at that stage.

Additions are then stacked on top. Exercise adds 8 ml per minute for light activity such as walking or yoga, 12 ml per minute for moderate work like jogging or a gym session, and 16 ml per minute for intense efforts such as running or HIIT. Climate adds 500 ml for hot and dry conditions, 750 ml for hot and humid, and 250 ml for cold dry air; a temperate climate adds nothing. High altitude and illness each add 500 ml. Pregnancy adds 300 ml and breastfeeding 700 ml, both following EFSA figures.

The total is held above a sensible adult minimum, capped at a safe ceiling, and rounded to the nearest 50 ml.

Worked example: 70 kg, age 28, a 45 minute gym session

A 28-year-old weighing 70 kg in a temperate climate falls in the 14 to 30 age band, so the base need is 70 x 35, which is 2,450 ml. A 45 minute moderate gym session adds 45 x 12, or 540 ml. A temperate climate adds nothing, and with no altitude, illness or pregnancy factors the running total is 2,990 ml. Rounded to the nearest 50 ml, the daily target is 3,000 ml, or three litres.

Choosing a 250 ml glass turns that into 12 glasses across the day. With a wake-up time of 07:00 and a bedtime of 23:00, that is a 16 hour waking window, so the schedule works out at roughly one glass every 80 minutes.

Change one input and the figure moves noticeably. Move the same person to a hot, humid climate and 750 ml is added, lifting the target to 3,750 ml, or 15 glasses. Raise the session to an intense hour and the exercise component alone becomes 960 ml.

Using the number sensibly

The target counts total fluid intake, not plain water alone. Tea, coffee, milk, juice and soup all contribute, and food supplies a meaningful share of daily water through fruit, vegetables and cooked grains. The tool also shows the EFSA and US National Academies reference values so you can see where your figure sits against published guidance, including the split between total water and the portion expected to come from drinks.

Pace matters as much as volume. The kidneys clear only around 0.8 to 1 litre per hour, and drinking faster than that over a sustained period risks diluting blood sodium, a condition called hyponatraemia that endurance athletes occasionally encounter. Spread intake through the day and use an electrolyte drink for long, sweaty sessions rather than water alone.

Several conditions change the right answer entirely, including kidney disease, heart failure and some medications, and a few require fluid restriction. Thirst and pale straw-coloured urine remain good day-to-day guides. This is general information, not medical advice.

Frequently Asked Questions

This calculator uses 35 ml per kilogram of body weight for ages 14 to 30, 33 ml for 31 to 55 and 30 ml from 56 upward, before adding for exercise and climate. A 70 kg 28-year-old therefore starts at about 2,450 ml, roughly 2.5 litres, before any activity adjustment.
Yes. The target covers total fluid intake, and tea, coffee, milk, juice and soup all count. The mild diuretic effect of caffeine does not outweigh the fluid in the drink at normal consumption levels. Food also contributes, particularly fruit, vegetables and anything cooked in water.
This tool adds 8 ml per minute for light activity, 12 ml per minute for moderate exercise and 16 ml per minute for intense sessions. A 45 minute moderate workout therefore adds 540 ml. For sessions over an hour, drink roughly 150 to 250 ml every 15 to 20 minutes and replace sodium lost in sweat.
Yes. Drinking faster than the kidneys can clear it, around 0.8 to 1 litre per hour, can dilute blood sodium and cause hyponatraemia, which is dangerous. The risk is highest during long endurance events where people drink heavily while losing salt in sweat. Spread intake out and include electrolytes for prolonged exertion.
The calculator adds 300 ml a day during pregnancy and 700 ml while breastfeeding, following EFSA recommendations. The breastfeeding figure replaces the fluid leaving the body in breast milk. These are general reference additions, so follow any specific guidance from your midwife or doctor in preference to them.