Health & Fitness Calculators

Free health and fitness calculators

Our health calculators help you understand your body with clear, science-based estimates you can use to set realistic goals. From body mass index to daily calorie needs, each tool explains not just your number but what it actually means — and its limits. They're free, require no sign-up, and work in both metric and imperial units.

Know your numbers

The BMI calculator gives a quick screening of whether your weight sits in a healthy range for your height, while the BMR calculator estimates the calories your body burns at rest. Building on that, the calorie (TDEE) calculator works out your full daily calorie needs based on how active you are — the starting point for any weight-loss, maintenance or muscle-gain plan.

Use them as a starting point

These tools give population-level estimates, not a diagnosis. They're most useful as a first step — a way to understand where you stand and set sensible targets — rather than the final word on your health. For anything concerning or personal, a doctor who knows your full situation is always the better guide.

Three numbers that measure three different things

BMI, BMR and TDEE get grouped together because they all appear on health sites, but they answer completely unrelated questions. Confusing them is the single most common reason people end up with a calorie target that doesn't work.

BMI — mass relative to height

Body mass index divides your weight in kilograms by your height in metres squared. That is the whole calculation. It produces one number that places you on a scale — broadly, below 18.5 is classed as underweight, 18.5 to 24.9 as a healthy range, 25 to 29.9 as overweight and 30 or above as obesity for adults. What it does not do is look inside that weight. BMI cannot see whether the mass is muscle or fat, and it has no idea where on your body it sits, which matters because abdominal fat carries different health implications than the same weight distributed elsewhere.

BMR — what your body burns doing nothing

Basal metabolic rate estimates the energy your body would use over twenty-four hours if you did absolutely nothing: keeping your heart beating, your lungs working, your brain running, your kidneys filtering and your temperature stable. For most people this accounts for somewhere around 60 to 70 percent of everything they burn in a day. Our calculator uses the Mifflin-St Jeor equation, which takes your weight, height, age and sex — a formula derived from measured populations, so an individual can sit meaningfully above or below what it predicts.

TDEE — the full day, including you moving around

Total daily energy expenditure takes BMR and multiplies it by an activity factor, from roughly 1.2 for a mostly seated life up to around 1.9 for heavy physical work or serious daily training. That multiplier is meant to absorb everything BMR leaves out: deliberate exercise, walking to the bus, fidgeting, and the energy spent digesting food, which is itself around 10 percent of intake.

How they fit together

BMR is the floor and TDEE is the whole building. Because TDEE is calculated from BMR, the two always move together, and the gap between them is entirely down to how active you say you are. Eat at roughly your TDEE and weight tends to hold steady; eat consistently below it and you're in a deficit; above it, a surplus. The familiar rule of thumb — a deficit of about 500 calories a day producing roughly a pound, or just under half a kilo, of loss per week — is a reasonable starting estimate, though it overstates results over long stretches, since a smaller body burns less and your TDEE falls as you lose.

Picking the right one for your goal

If the question is “is my weight in a sensible range for my height?”, BMI is the quick screen — with the caveats below firmly in mind. If the question is “how much should I actually be eating?”, TDEE is the number to build on, because it reflects your real daily output rather than your resting baseline. BMR is rarely the number you eat to; its value is as the input TDEE is built from, and as a floor — sustained eating below your basal rate is a red flag rather than a strategy, and worth discussing with a professional before you attempt it.

Where these estimates fall down

• BMI can't tell muscle from fat. A well-trained rugby player can be classified “overweight” while carrying very little fat, and someone with low muscle mass can land inside the healthy range while carrying more fat than is ideal.
• The adult thresholds don't apply to everyone. Children and teenagers are assessed on age-and-sex percentile charts instead, the standard categories aren't meaningful during pregnancy, and research suggests health risk begins at different BMI points across some ethnic groups.
• The activity multiplier is the weakest link in TDEE. Most people overestimate how active they are, and one step up the activity scale can move a target by several hundred calories.
• Body composition is invisible to Mifflin-St Jeor. Two people of identical height, weight, age and sex get the same BMR from the formula even if one carries far more lean mass, which burns more at rest.
• Medical factors aren't modelled at all. Thyroid conditions, some medications, pregnancy and breastfeeding all shift energy needs in ways no general formula captures.

The sensible way to use any of these outputs is as a hypothesis. Take the number, hold it for two to four weeks, track what your weight actually does, and adjust from the real result rather than the prediction. Your own data beats the formula every time.