BMI & calorie needs
Find your BMI, resting metabolism and daily calorie needs, with the formulas shown rather than hidden.
Runs entirely in your browser. Nothing you enter is uploaded.
How this works
What each number means
This tool produces three figures that are often confused with one another. They answer different questions.
- BMI compares your weight to your height: weight in kilograms divided by height in metres squared. It is a ratio, not a measurement of body fat.
- BMR — basal metabolic rate — is roughly the energy your body would use lying still all day: breathing, circulation, temperature, brain function. For most adults it is 60–70% of total daily energy use.
- Daily need — total daily energy expenditure — is BMR multiplied by an activity factor. It is the figure that matters for eating decisions.
The formulas, in full
BMI is straightforward:
BMI = weight(kg) / height(m)²
BMR uses the Mifflin–St Jeor equation, which is what most dietetic guidance now recommends for healthy adults:
male: 10 × weight + 6.25 × height − 5 × age + 5female: 10 × weight + 6.25 × height − 5 × age − 161
Weight is in kilograms, height in centimetres, age in years. Daily need multiplies that result by an activity factor between 1.2 for a sedentary week and 1.9 for someone doing hard physical work.
Where these numbers stop being useful
The activity multiplier is the weakest link in the chain, and it is worth knowing that before you act on the output. The bands are broad, self-assessment is generous, and the difference between “light” and “moderate” on a 1,500 kcal BMR is around 270 kcal a day — enough to turn a planned deficit into maintenance.
A practical way to handle this: take the estimate, eat to it consistently for three weeks, and watch what your weight actually does. If it holds steady, that figure is your real maintenance level regardless of what the equation predicted. The calculator gives you a starting point; your own data corrects it.
Setting a rate of change
A kilogram of body mass carries roughly 7,700 kcal, so a 500 kcal daily deficit works out at about 0.45 kg a week in theory. In practice the loss is faster in the first fortnight, as glycogen and its associated water leave, and then slows to something closer to the arithmetic.
| Weekly rate | Daily deficit | Usually appropriate for |
|---|---|---|
| 0.25 kg | ~275 kcal | Small amounts to lose, or preserving muscle |
| 0.5 kg | ~550 kcal | The common default |
| 0.75 kg | ~825 kcal | Higher starting weight, short periods only |
The tool warns if your target falls below roughly 1,200 kcal a day, which is the level below which most guidance says medical supervision is needed. That warning is not decoration — sustained intakes that low make it hard to meet basic nutrient requirements at all.
Please read this part
This is a calculator, not care. It does not know your medical history, your medications, whether you are pregnant, or whether you have a history of disordered eating. BMI in particular is a crude instrument that was never designed to assess an individual, and treating it as a personal score does more harm than good.
If you are making a substantial change to how you eat, or if any of the numbers here concern you, talk to a doctor or a registered dietitian. They can look at things this page cannot.
Common questions
- Is BMI reliable for me personally?
- BMI is a population screening tool, not a personal diagnosis. It cannot tell muscle from fat, so a trained athlete can land in the overweight band while carrying very little fat, and an inactive person can sit in the normal band with high body fat. It is also less accurate for people who are very tall or very short, for older adults who have lost muscle, and during pregnancy. Use it as one number among several, not as a verdict.
- Why does the calorie estimate differ from my fitness tracker?
- Trackers estimate energy expenditure from movement and heart rate, which is a different method with its own error. Both are approximations of a number that varies day to day by several hundred calories. If they disagree by 10–15%, that is normal. The useful test is not which is right but whether your weight actually moves as predicted over three or four weeks.
- Which BMR equation does this use?
- Mifflin–St Jeor, which is the equation most dietetic bodies now recommend for healthy adults. It replaced the older Harris–Benedict equation because it tends to be more accurate in modern populations, though it still carries an error of roughly ±10% for any individual.
- Why does the calculator ask for sex at birth?
- The equation was derived from populations grouped that way, and it uses the variable as a rough stand-in for average lean body mass, which drives resting metabolism. It is a limitation of the formula rather than a judgement, and it is one reason the result should be treated as an estimate.
- Is a bigger deficit faster?
- Briefly, and then usually not. Very low intakes are hard to sustain, cost lean tissue alongside fat, and are followed by rebound more often than not. A deficit of 15–20% below your daily need is the range most practitioners work in, which usually means about 0.25 to 0.5 kg a week.