BMI Calculator
Body mass index, the WHO category it falls in, and the healthy weight range for your height.
| BMI | 23.7 Normal weight |
|---|---|
| Category | Normal weight |
| Healthy weight range | 58.6 kg (129.2 lb) to 79.2 kg (174.6 lb) BMI 18.5 to 25 for 178 cm |
| Difference to range | already inside the range keep this within a few kilos |
| BMI Prime | 0.95 BMI divided by 25; below 1.0 is inside the healthy range |
Population screening bands; your category is printed in the result table above.
What body mass index measures
Body mass index is weight divided by height squared, expressed in kilograms per square metre. It was devised in the 1830s by a Belgian statistician to describe populations, not individuals, and it survives because it needs only two numbers that anyone can measure at home.
Across large groups BMI tracks body fat reasonably well, which is why public health bodies still use it for screening. For a single person it is a first filter rather than a verdict: it cannot tell muscle from fat, it ignores where fat is stored, and it does not adjust for frame size or age.
| Category | BMI range | What guidance usually says |
|---|---|---|
| Underweight | below 18.5 | check for unintended weight loss and underlying causes |
| Normal weight | 18.5 to 24.9 | lowest population risk band |
| Overweight | 25 to 29.9 | check waist and blood markers before assuming risk |
| Obesity class I | 30 to 34.9 | weight loss of 5% to 10% improves markers |
| Obesity class II and III | 35 and above | clinical assessment recommended |
How BMI is calculated
In metric units, BMI is weight in kilograms divided by height in metres squared: 75 kg at 1.78 m is 75 / 3.1684, or 23.7. In imperial units the same calculation is weight in pounds multiplied by 703 divided by height in inches squared. Both produce the same number, and the calculator handles the conversion for you.
BMI Prime, shown in the output, is your BMI divided by 25. It answers a slightly different question - how far you are from the top of the healthy range as a proportion - and a value of 1.0 means you are exactly at that boundary. The distance-to-range row gives the same information in kilograms and pounds, which is usually more actionable.
Reading your BMI result
The first two rows give the value and its WHO category. The third converts the healthy BMI range into a weight range for your height, and the fourth tells you how far outside it you are. That last row is the useful one: 'lose 4.2 kg to reach the healthy range' is a concrete goal in a way that 'BMI 26.4' is not.
BMI Prime puts the same information on a scale where 1.0 is the boundary. It is used in some clinical settings because the numbers are small and easy to compare over time: 1.08 means 8% above the top of the healthy range.
What BMI cannot see
BMI has three well-documented blind spots. It cannot distinguish muscle from fat, so trained people are frequently classified as overweight. It ignores fat distribution, so two people at BMI 29 - one with most fat around the waist, one with it around the hips and legs - can have very different cardiometabolic risk. And it does not adjust for age or ethnicity, despite risk thresholds differing between populations.
| Better measure | What it adds | When to use it |
|---|---|---|
| Waist circumference | where the fat is stored | always, alongside BMI |
| Waist-to-height ratio | a single simple threshold: under 0.5 | quick screening at home |
| Body fat percentage | separates muscle from fat | athletes, people who lift |
| Waist plus blood markers | actual metabolic risk | clinical assessment |
BMI, risk and what the category really means
The relationship between BMI and mortality is U-shaped: risk is lowest in the 20 to 25 band, rises gradually above it, and also rises below about 18.5. The rise above 25 is driven mostly by visceral fat and its links to blood pressure, blood sugar and cholesterol rather than by weight itself.
That distinction matters for what to do about it. A BMI of 27 with a waist under half your height, normal blood pressure and good fitness is a very different situation from a BMI of 27 with a large waist and rising fasting glucose. BMI tells you to look; the other measures tell you what you are looking at.
Using BMI sensibly
Treat BMI as a screening number that decides whether to look further, not as a diagnosis or a target. If it falls outside the healthy band, check your waist, your blood pressure and your fasting glucose before concluding anything about your health; if it is inside the band but your waist is large, that deserves attention too.
- Measure BMI and waist together, never BMI alone.
- Ignore single-day weight swings; BMI moves with hydration.
- Do not use BMI to set a goal weight for a muscular person.
- Recheck after meaningful weight change rather than weekly.
Frequently asked questions
What is a healthy BMI?
The World Health Organization healthy band is 18.5 to 24.9. Population risk is lowest across that range, although risk also depends on where your fat sits and on your blood markers, not only on the number.
Is BMI accurate?
For populations, reasonably. For individuals, it misreads muscular people as overweight and can miss high-risk fat distribution in people with a normal BMI. Pair it with a waist measurement.
Why does BMI not adjust for muscle?
Because the formula only uses weight and height. Muscle is denser than fat, so a lean athlete can weigh more than a sedentary person of the same height and score worse despite having less fat.
Does BMI work for children?
Not with these thresholds. Children and teenagers are assessed with age- and sex-specific BMI percentile charts rather than fixed cut-offs, so use a paediatric chart instead of this page.
What is BMI Prime?
Your BMI divided by 25. Below 1.0 is inside the healthy band, and values above 1.0 tell you the proportion by which you exceed the top of the range.
Should I aim for the middle of the healthy range?
There is no evidence that the exact middle is better than the edges. Staying inside the band with a waist under half your height is the practical goal.
What BMI counts as obese?
For adults a BMI of 30 or above is classed as obese, 25 to 29.9 as overweight and 18.5 to 24.9 as healthy. The thresholds come from the WHO and are identical for men and women.
Is BMI different for men and women?
No. The adult thresholds are the same, even though women carry more body fat at the same BMI. That mismatch is one reason waist measurement is a useful companion to BMI.
Sources
Formulas and thresholds are taken from the sources above. They are a starting point for adults, not a diagnosis.
Last reviewed September 12, 2026. How we calculate · Editorial policy