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.
Written and maintained by ReadoutKit / Takashi Yamazaki
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.
The healthy range, in pounds, by height
More useful than the BMI number itself is the weight band it corresponds to at your height. This is BMI 18.5 to 24.9 converted back into pounds; the last column shows, for reference, what 175 lb comes to as a BMI at each height.
| Height | Lower (BMI 18.5) | Upper (BMI 24.9) | Width of band | BMI at 175 lb |
|---|---|---|---|---|
| 60 in | 94.7 lb | 127.5 lb | 32.8 lb | 34.2 |
| 63 in | 104.4 lb | 140.6 lb | 36.1 lb | 31 |
| 66 in | 114.6 lb | 154.3 lb | 39.7 lb | 28.2 |
| 69 in | 125.3 lb | 168.6 lb | 43.3 lb | 25.8 |
| 72 in | 136.4 lb | 183.6 lb | 47.2 lb | 23.7 |
| 75 in | 148 lb | 199.2 lb | 51.2 lb | 21.9 |
The band widens as you go down the table: about 33 lb at five feet, about 51 lb at six foot three. Because BMI divides by height squared, one point of BMI buys more pounds the taller you are — so an identical half-point move on the scale means a smaller weight change for a shorter person.
The last column makes the same point from the other side. A fixed 175 lb reads as a BMI of 34.2 at five feet and 21.9 at six foot three: obese at one end of the table, comfortably mid-range at the other. A weight on its own carries almost no information, which is why comparing scale readings with other people tells you nothing.
What activity level does to the estimate
The same person — 5ft 9in, 175 lb, 40 — with only the activity multiplier moving. Resting expenditure is 1,694 kcal in every row; the multiplier is doing all the work.
| Activity level | Multiplier | Daily expenditure | Above sedentary |
|---|---|---|---|
| Sedentary | 1.2 | 2,033 kcal | 0 kcal |
| Lightly active (1–3 days) | 1.4 | 2,329 kcal | 296 kcal |
| Moderately active (3–5 days) | 1.6 | 2,626 kcal | 593 kcal |
| Very active (6–7 days) | 1.7 | 2,922 kcal | 889 kcal |
| Extra active | 1.9 | 3,219 kcal | 1,186 kcal |
From 2,033 kcal to 3,219 kcal is a spread of 1,186 kcal for one body on one day, decided entirely by which row you picked for yourself. This is the softest input in the whole calculation, and self-reported activity is known to skew high. If a deficit is not producing the loss the arithmetic promised, the first thing to try is the row above the one you chose.
Age barely moves the equation
The same person again, varying only age.
| Age | Resting (BMR) | Daily expenditure | Change from 25 |
|---|---|---|---|
| 25 yr | 1,769 kcal | 2,742 kcal | 0 kcal |
| 35 yr | 1,719 kcal | 2,665 kcal | -77 kcal |
| 45 yr | 1,669 kcal | 2,587 kcal | -155 kcal |
| 55 yr | 1,619 kcal | 2,510 kcal | -232 kcal |
| 65 yr | 1,569 kcal | 2,432 kcal | -310 kcal |
| 75 yr | 1,519 kcal | 2,355 kcal | -387 kcal |
Resting expenditure falls from 1,769 kcal at 25 to 1,519 kcal at 75: 250 kcal over half a century, or exactly 5 kcal a year, which is the coefficient Mifflin–St Jeor puts on age and nothing more. Daily expenditure follows it down from 2,742 to 2,355, a difference of 387 kcal.
Set that beside the previous table. One step up in activity is worth 296 kcal. A single change of habit outweighs fifty years of ageing in this model. That does not prove metabolism is unaffected by age — the equation only knows weight, height, age and sex, and it weights age the least of the four. It does mean that attributing midlife weight gain to the passage of years, rather than to what changes alongside it, has no support in the arithmetic on this page.
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
- What is a healthy weight for my height?
- The BMI 18.5–24.9 band works out at 94.7–127.5 lb at 5ft 0in, 114.6–154.3 lb at 5ft 6in, 125.3–168.6 lb at 5ft 9in and 136.4–183.6 lb at 6ft 0in. Notice that the band itself widens with height: about 33 lb at five feet, about 47 lb at six. BMI divides weight by height squared, so one point of BMI covers more pounds the taller you are. The table in this article runs from 5ft 0in to 6ft 3in.
- How much does metabolism really slow with age?
- Far less than the reputation suggests. Mifflin–St Jeor subtracts 5 kcal per year of age and nothing else, so for a 5ft 9in, 175 lb man the resting figure falls from 1,769 kcal at 25 to 1,519 kcal at 75 — 250 kcal across fifty years. Moving the same person up one activity band, from sedentary to lightly active, adds 296 kcal. One step of lifestyle outweighs five decades of ageing. Whatever drives midlife weight gain, the equation does not locate it in the age term.
- 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.
Sources
The formulas and rates used by this tool come from the documents below. Rules change; when a figure here disagrees with the source, the source is right.
Figures checked against the sources below on September 1, 2026.
- Obesity and overweight — fact sheetWorld Health Organization
- A new predictive equation for resting energy expenditure in healthy individuals (Mifflin–St Jeor)The American Journal of Clinical Nutrition (PubMed)