BMI Calculator

What the BMI Calculator Does

Body Mass Index (BMI) is a single number that relates your weight to your height, and it is the screening measure the World Health Organization uses to sort adults into weight categories. Enter your height and weight and this calculator returns your BMI to one decimal place together with the category it falls into — underweight, normal, overweight or obese. It is a screening tool, not a diagnosis: it cannot tell muscle from fat, and it says nothing about where fat sits on the body, which is why the reference tables below also give the waist thresholds that catch the cases BMI misses.

\[ BMI = \frac{\text{weight (kg)}}{\text{height (m)}^2} \]

In imperial units the same figure comes from \( BMI = 703 \times \frac{\text{weight (lb)}}{\text{height (in)}^2} \). The calculator accepts either system and converts between them, so you can enter feet and inches plus pounds, or centimetres and kilograms, without doing the arithmetic yourself. The result is rounded to one decimal place, which is the precision the WHO cut-offs are defined at.

BMI Formula and Reference Ranges

The category thresholds are not universal. The WHO cut-offs were derived from predominantly Caucasian populations, and because body composition differs across groups at the same BMI, several regions apply stricter numbers. The table below shows the three standards side by side; use the column that matches your background, and read the right-hand column for what the band means.

A related figure worth knowing is the healthy weight range for your height: the span between BMI 18.5 and 24.9, computed from the height you entered. For someone 175 cm tall that is 56.7 to 76.3 kg; at 180 cm it is 59.9 to 80.7 kg; at 162 cm it is 48.6 to 65.3 kg. If your weight sits inside the range for your height, the BMI is telling you what it was built to tell you; if it sits outside, the distance to the nearer boundary is the number to watch.

How to Use the BMI Calculator

1. Select your gender. Gender does not change the BMI arithmetic — the formula uses only height and weight — but it is saved with your history so a reading can be read back in context.
2. Optionally enter your age. Age is stored alongside the reading for reference. For anyone under 18 the adult thresholds do not apply, so see the children’s section below before reading the category.
3. Enter your height and weight. Use centimetres and kilograms, or switch to feet, inches and pounds; the tool converts between the two so nothing is left in the wrong unit.
4. Press Calculate. The result card shows your BMI, the category badge and a one-line reading of what it means, and the entry is added to your history on the page.
5. Export or share if you need to. The card can be downloaded as an image or a PDF, and the share link carries the numbers you typed so the same reading reopens from a QR code or a copied link.

Worked Examples

Example 1 — 175 cm and 70 kg: a normal-range reading

Divide 70 by 1.75 squared: 70 ÷ 3.0625 = 22.857, which rounds to 22.9. That is inside the WHO normal band of 18.5 to 24.9, comfortably below the overweight threshold of 25.0, and also inside the stricter Chinese/East Asian normal band of 18.5 to 23.9. The healthy weight range for 175 cm is 56.7 to 76.3 kg, so at 70 kg there is room on both sides before the category would change.

Example 2 — 180 cm and 95 kg: the overweight band

95 ÷ 1.8² = 95 ÷ 3.24 = 29.321, which rounds to 29.3. On the WHO scale that is the top of the overweight band, one tenth of a point below obesity at 30.0. On the Chinese/East Asian scale 29.3 is already past the obesity threshold of 28.0, and on the South Asian scale it is past 25.0 as well — a clear illustration of why the standard you pick changes the label without changing the body. The healthy range for 180 cm is 59.9 to 80.7 kg, so the reading sits about 14 kg above the upper boundary.

Example 3 — 162 cm and 58 kg: a healthy reading near the middle

58 ÷ 1.62² = 58 ÷ 2.6244 = 22.100, which rounds to 22.1. That is inside every one of the three normal bands, including the strictest, so the category badge reads Normal on all standards. The healthy range for 162 cm is 48.6 to 65.3 kg, and 58 kg sits close to the midpoint — the same BMI on a shorter frame corresponds to a smaller absolute weight, which is the whole point of indexing weight to height.

BMI Across Populations

BMI was never meant to be read the same way for everyone. The same number can mean different things at different ages, between men and women, and across ethnic groups, because what it measures is weight relative to height — not fat, and certainly not fat distribution.

Ethnic Differences

• Asians: stricter standards. Asians typically carry a higher body fat percentage than Caucasians at the same BMI and are more prone to visceral fat accumulation, so the thresholds for type 2 diabetes and cardiovascular risk are set lower. The Chinese/East Asian column above is the practical consequence.
• South Asians: the strictest standards. South Asians often show a “skinny fat” profile — lean in appearance while carrying high visceral fat. Studies find that South Asians at a BMI of just 22–23 face the same diabetes risk as Caucasians at BMI 30, which is why the obesity threshold for South Asians is typically set at 25.0.
• Black / African: BMI may overstate the risk. Research shows African individuals typically have greater muscle mass and higher bone density than Caucasians and Asians at the same BMI, so the number can overestimate obesity. Clinically, Black women may keep healthy metabolic indicators at a BMI of 32–33. If you are African American with a regular training routine, pair BMI with a waist circumference or a body fat percentage measurement.

Gender Differences

• Body fat difference. At the same BMI, women typically carry 8%–10% more body fat than men, because women need more essential fat for endocrine and reproductive function. Two people can share a BMI of 22 and differ by nearly ten points of body fat.
• Fat distribution. Men accumulate more abdominal fat (visceral, “apple shape”), which carries the higher health risk; women tend to store fat in the hips and thighs (subcutaneous, “pear shape”). This is why the waist thresholds in the table above differ by sex.

Children and Adolescents

For anyone aged 2–18, BMI must be read against age and sex, not against the adult table. Children’s body composition changes rapidly during growth and boys and girls mature at different rates, so the assessment uses percentiles rather than absolute values: a child is classified as obese only if their BMI exceeds the 95th percentile for their age and sex group.

The absolute numbers behind those percentiles move with age. As an illustration of how far, here are obesity thresholds for selected ages on one national (Chinese) standard:

Read across the rows and the point is plain: an adult BMI of 22 would be normal, but for a 10-year-old it is already at the obesity threshold. This calculator reports the adult category, so for anyone under 18 the number should be taken to a pediatric growth chart rather than read as a final label.

BMI Calculator FAQ

What is a healthy BMI?

For adults, a BMI of 18.5 to 24.9 is the WHO normal range; below 18.5 is underweight and 25.0 or above is overweight. Several populations use stricter upper limits — 23.9 for Chinese and East Asian adults and 22.9 for South Asian adults — so the same BMI can be normal on one standard and overweight on another. A single reading inside the normal band is a good sign, not a guarantee of health.

How is BMI calculated?

BMI is your weight in kilograms divided by your height in metres squared. In imperial units it is 703 times your weight in pounds divided by your height in inches squared. This calculator does that arithmetic for you and rounds the result to one decimal place, the precision the WHO cut-offs are written at.

Is BMI accurate for everyone?

No. BMI cannot distinguish muscle from fat, so a muscular athlete can read as overweight while carrying very little fat, and an inactive person with normal weight can carry a high fat percentage — the “skinny fat” profile. It also ignores where fat is stored, which is why the waist measurements above matter. Treat BMI as a fast first screen and confirm it with a waist circumference or a body fat measurement when the answer matters.

Can children or pregnant women use this calculator?

This tool reports adult categories and is intended for adults aged 18 and over. Children and adolescents should be assessed on age- and sex-specific BMI percentiles, as shown in the children’s section above. Pregnancy changes body composition and weight in ways the adult thresholds are not designed for, so BMI is not a meaningful measure during pregnancy — use the guidance of your healthcare provider instead.

Does BMI differ by ethnicity?

Yes, and materially. The WHO thresholds were derived largely from Caucasian populations, but the health risks linked to a given BMI appear at lower values in South and East Asian populations, which is why both apply stricter cut-offs. If your background is South Asian, East Asian or African, read the column that matches it rather than the international one alone.

How can I lower my BMI?

BMI falls when weight falls, so the levers are the usual ones: a modest calorie deficit, a diet built on whole foods, and at least 150 minutes of moderate aerobic activity a week, plus strength work to protect lean mass. Because BMI is weight divided by height, losing fat while keeping muscle moves the number the right way. For a plan that fits your circumstances, consult a physician or a registered dietitian — this calculator provides a reference figure, not a treatment plan.

Related Tools

This is the plain BMI index, and it sits at the centre of the health cluster on BunTool. When a single number is not enough, the enhanced BMI calculator adds BMI Prime, the ponderal index, body surface area, body fat, lean mass, BMR, TDEE and calorie targets from the same height and weight. For a measurement-based body fat figure rather than a height-and-weight estimate, the body fat calculator uses the US Navy tape method, and the lean body mass calculator compares three published regression formulas. If your concern is where the weight sits, the waist-to-height ratio and waist-to-hip ratio calculators read the central-fat risk that BMI leaves out.