A BMI calculator divides weight by height squared and drops the result on a World Health Organization chart. The chart is a screening tool for population risk, built because height and weight are easy to collect. It is not a body-fat measurement, and it is a blunt description of any single person who lifts. A muscular 100 kg man at 1.80 m scores 30.9, which the chart calls obese, while a sedentary person at BMI 24 can carry more fat and less muscle. Use BMI to see which band the chart puts you in. Use a tape or a scan when the band and the mirror disagree.
The formula is the same in metric and in customary units if you convert properly. This page uses kilograms and meters. Categories follow the WHO adult cutoffs: under 18.5 underweight, 18.5 to 24.9 normal range, 25 to 29.9 overweight, 30 and above obese, with further obese classes above 35 and 40. Those cutoffs were not designed for children, pregnancy, or athletes. They are still the numbers clinicians and insurers quote, so it is worth knowing yours before you argue with it.
The BMI formula
BMI = weight(kg) / height(m)²
Quetelet’s index, adopted by the World Health Organization for adult classification. Categories used here: below 18.5 underweight, 18.5–24.9 normal range, 25.0–29.9 overweight, 30.0 and above obese. A “healthy weight range” for a height is the weight that lands between 18.5 and 24.9.
Height must be in meters, not centimeters. 180 cm is 1.80 m, and the square is 3.24, not 32,400. At 82 kg, BMI = 82 / 3.24 = 25.3, just inside the overweight band. The same person at 74 kg is 22.8, inside the normal range. The entire “normal” window for 1.80 m is 18.5 × 3.24 = 60.0 kg to 24.9 × 3.24 = 80.7 kg.
BMI prime is BMI divided by 25, a way of saying how far you sit from the top of the normal band. A BMI of 25 is a prime of 1.0. A BMI of 30 is 1.2. It does not add medical information the category does not already have. It is a second way to read the same ratio.
The chart does not know muscle, bone, age-related muscle loss, or ethnicity-specific risk. Some organizations use lower overweight cutoffs for some Asian populations because metabolic risk can appear at a lower BMI. This page prints the standard WHO adult cuts. If your clinician uses a different cutoff, theirs is the one attached to your care, not the gym version.
Worked examples
Example 1 — 82 kg, 180 cm
- Height = 1.80 m. Height² = 3.24.
- BMI = 82 / 3.24 = 25.3. WHO category: overweight.
- Normal-range weights at this height: 60.0 to 80.7 kg. This person is about 1.3 kg above that window.
- If a tape says 15% body fat, the chart is describing muscle as well as fat. Run the body fat calculator and FFMI before a deficit aimed only at the 25.3.
Example 2 — 68 kg, 165 cm
- Height = 1.65 m. Height² = 2.7225.
- BMI = 68 / 2.7225 = 25.0. The boundary of overweight, depending on rounding.
- Normal-range ceiling = 24.9 × 2.7225 = 67.8 kg. Floor = 18.5 × 2.7225 = 50.4 kg.
- A 200 kcal deficit and a waist measurement will tell you more than arguing about the 0.1 BMI point. Recalculate after 3 kg, not after a salty weekend.
Example 3 — 100 kg, 180 cm, lifter
- BMI = 100 / 3.24 = 30.9. WHO category: obese class I.
- If body fat is 12%, fat mass is 12 kg and fat-free mass is 88 kg. FFMI = 88 / 3.24 = 27.2, which is a very muscular score, not a sedentary obesity score.
- The chart is not “wrong.” It was not built to clear athletes. Insurance tables may still use 30.9. Training decisions should use waist, body fat, and FFMI.
- If body fat is 32%, the same BMI is a different health picture. That is the entire limitation, and the reason this site pairs BMI with a tape equation.
How to read the category without panicking or ignoring it
Under 18.5 is a reason to look at intake, illness, and whether the scale is real. It is not a suggestion to “eat clean” smaller. The normal band is wide on purpose: at 1.75 m it spans about 57 to 76 kg. Overweight is a prompt to check waist and fitness, not an order to crash-diet. Obese categories, especially with a large waist, line up with higher population risk of diabetes and heart disease. That risk is about the group. Your next step is still a measured waist, a calorie target you can keep, and a clinician if you have other risk markers.
Waist circumference is the add-on BMI needs. A waist above 102 cm in men or 88 cm in women is the threshold many guidelines flag, independent of the BMI decimal. Measure at the navel, morning, same tape as the body-fat page. If BMI is 27 and waist is falling, the plan is working. If BMI is 23 and waist is climbing, the chart is giving you false comfort.
What to do after you have the number
If you want the weight that sits at BMI 24, multiply 24 by height squared. That is an upper-normal target, not a mandatory weight. Subtract current weight, then use the TDEE calculator for a 10 to 20% deficit. Protein stays high so the kilograms you lose are not the muscle the BMI chart cannot see. Recalculate BMI after a 5% weight change. Daily BMI updates are noise.
Children, pregnancy, and some clinical edema states should not use this adult formula as a goal. Athletes in strength sports should report BMI when a form asks, then make training choices from body fat, FFMI, and performance. Both things can be true.
Frequently asked questions
How do I calculate BMI?
Divide weight in kilograms by height in meters squared. A person who is 180 cm and 82 kg has a BMI of 82 / 1.8² = 25.3. In US units the equivalent is 703 × pounds / inches². Do not mix centimeters into the denominator or the result is a hundred times too small. The calculator above expects the metric profile the rest of the site uses.
What is a healthy BMI?
The WHO adult normal range is 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese on that chart. “Healthy” for a person also depends on waist, blood pressure, labs, and muscle. A BMI of 26 with a small waist and a training habit is not the same risk as a BMI of 26 with a large waist and no activity. Use the category as a screen, then add a waist measurement.
Is BMI accurate for muscular people?
It is accurate as a ratio of weight to height and inaccurate as a body-fat test. Muscular people score higher because muscle weighs something. A BMI of 30 can be obesity or a heavy athlete; the chart cannot tell. If you lift and the category looks harsh, measure body fat with the Navy tape and compute FFMI. If body fat is also high, the BMI flag was fair. Ignore the chart only after you have the second number, not before.
What weight is a BMI of 25 for my height?
Multiply 25 by height in meters squared. At 1.70 m, height squared is 2.89, so BMI 25 is 72.3 kg. The bottom of the normal range, 18.5, is 53.5 kg at that height. Those two weights are the chart’s window, not a prescription to sit at the bottom of it. Pick a rate of loss with the TDEE calculator if you are above the window and your waist agrees.
Why does my BMI say overweight if I look lean?
Because the formula never asked how much of the weight is fat. Bone, muscle, and a last meal all count. Confirm with a waist measurement and a body-fat method before you start a large deficit. If the tape also says you are lean, train and eat for performance and let BMI be the number you report on forms. If the tape says otherwise, the mirror was the unreliable instrument.
Does BMI work for women and older adults?
The same formula is used. Interpretation changes. Women carry more essential fat, so a BMI in the normal band can still hide a low-muscle, higher-fat pattern after menopause, and a BMI just under 25 is not automatically “better” than 26 if muscle is vanishing. Older adults can also be harmed by a low BMI and a low protein intake. Pair the number with strength and waist, and do not chase the bottom of the chart in your 70s.
How often should I check BMI?
When weight has moved enough to matter, about every few weeks, not every morning. A 1 kg swing changes BMI by about 0.3 points at average height. That is sodium and food volume. The category boundaries are worth a recalculation after you have lost or gained 3 to 5 kg and held it for a week. Daily BMI apps mostly train you to react to noise.
Should I lose weight if my BMI is 25?
Not automatically. You are at the line. Check waist, how your clothes fit, blood pressure, and whether you want a performance goal or a health goal. A small deficit and more walking is a reasonable experiment if waist is up. A crash diet to reach 23 is not required by the chart. If you are muscular and the tape is lean, adding a deficit to satisfy a cutoff can cost muscle the BMI formula cannot see. Use FFMI and a protein target before you decide.
Related calculators
Use the number above with the tools that change how you eat and train next.