BMI calculator

How to calculate BMI

BMI, short for body mass index, is weight in kilograms divided by height in meters squared. At 70 kg and 1.75 m: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, inside the healthy adult range of 18.5 to under 25. CDC applies these cut-offs to adults aged 20 and over.

BMI in pounds and inches

Multiply weight in pounds by 703, then divide by height in inches squared. At 140 lb and 5 ft 6 in (66 in): 703 × 140 ÷ (66 × 66) = 98,420 ÷ 4,356 = 22.6.

The 703 converts lb/in² to kg/m²: 0.45359237 ÷ 0.0254² = 703.07. Rounding it to 703 lowers BMI by about 0.01%, which matters only when a result sits right on a category boundary.

Example: BMI for 82 kg and 180 cm

1. Convert height to meters: 180 cm = 1.80 m.
2. Square it: 1.80 × 1.80 = 3.24.
3. Divide the weight by that number: 82 ÷ 3.24 = 25.31.
4. Round once, at the end: 25.3, just inside the overweight range.

BMI chart for adults

The boundaries are the WHO adult cut-offs, the same ones CDC uses. Each range includes its lower number and stops just below the next one.

Classify the unrounded number. At exactly 2 m tall, 99.84 kg gives a BMI of 24.96, which rounds to 25.0 but is still healthy. At 100 kg the BMI is exactly 25, the first overweight value.

Healthy weight for your height

Multiply 18.5 and 25 by your height in meters squared. At 1.75 m that is 18.5 × 3.0625 = 56.7 kg, up to just under 25 × 3.0625 = 76.6 kg.

Kilograms are rounded to one decimal and pounds to whole numbers, with exact halves rounded to the even digit: at 150 cm the upper boundary is exactly 56.25 kg, shown as 56.2. Feet and inches are rounded to the nearest inch.

At 175 cm the healthy range spans about 20 kg, and BMI cannot say where within it a person is healthiest. The ideal weight calculator sets the Devine, Robinson, Miller and Hamwi formulas side by side.

BMI for women and men

Adult women and men use the same formula and the same cut-offs, so the same height and weight give the same BMI. The body behind the number differs: women carry more body fat than men at the same BMI, and older adults lose muscle.

Children and teenagers are assessed differently. From age 2 to 19, CDC compares BMI with growth charts for the same age and sex, using percentiles instead of the fixed adult bands.

What BMI does not tell you

BMI is a screening number, and CDC recommends reading it alongside other health information. It misses three things:

• Muscle or fat. At 100 kg and 1.85 m, BMI is 29.2 whatever the mix of muscle and fat. The body fat calculator estimates composition from tape or skinfold measurements.
• Where fat sits. For adults with a BMI under 35, NICE recommends measuring waist-to-height ratio as well; 0.5 or more indicates increased central adiposity. The waist-to-hip ratio calculator works out both ratios.
• Ethnic background. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE starts overweight at 23 and obesity at 27.5, below the international cut-offs.

BMI is not a measure of health during pregnancy. The pregnancy weight gain calculator starts from pre-pregnancy BMI instead.

BMI prime and the ponderal index

BMI prime is BMI divided by 25, the top of the healthy range. A value of 1.00 sits on that limit, 0.92 is 8% below it and 1.20 is 20% above. For 70 kg at 1.75 m it is 0.91.

The ponderal index divides weight by height cubed (kg/m³). Body volume grows roughly with the cube of height, so it reads less high than BMI for very tall people and less low for very short ones.

The ponderal index is used mainly for newborns and in research, and has no widely adopted adult cut-offs.

Where BMI comes from

The Belgian statistician Adolphe Quetelet noticed in the 1830s that adult weight rises roughly with the square of height.

In 1972 Ancel Keys and colleagues compared several weight-for-height indices, found that this one correlated best with measured body fat and named it the body mass index. WHO adopted the current cut-offs in its 1995 and 2000 technical reports.

Common BMI mistakes

• Dividing by height once. 82 ÷ 1.80 = 45.6, which is not a BMI. Divide by 1.80 × 1.80.
• Typing 5.6 ft for 5 ft 6 in. 5.6 ft is 5 ft 7.2 in, and it lowers a 140 lb BMI from 22.6 to 21.8. Enter 5 ft 6 in or 5'6" instead.
• Reading stones as stones and pounds. Stones are decimal: 10 st 7 lb is 10.5 st, and typing 10.7 adds 2.8 lb.
• Rounding too early. Rounding 1.75² from 3.0625 to 3.1 turns a BMI of 22.9 into 22.6. Round only the final answer.

What to do with your result

A BMI outside 18.5 to under 25 is a reason to talk to a doctor, who can weigh it against your waist size, blood pressure, blood tests and family history.

To estimate the calories you burn each day, use the BMR and TDEE calculator. To turn a weight goal into daily calories and protein, use the calorie and macro calculator.