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BMI (Body Mass Index) Calculator

✦ Health & Wellness

This BMI calculator works out your body mass index from height and weight in metric or US units and places it on the WHO and CDC adult bands. It also shows the healthy weight for your height and BMI prime.

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The formula

Body mass index (the Quetelet index), BMI prime and the healthy-weight range

Metric:     BMI = weight (kg) ÷ height (m)²
US units:   BMI = 703 × weight (lb) ÷ height (in)²
BMI prime = BMI ÷ 25
Healthy weight at your height = 18.5 × height² to 24.9 × height²   (height in m → kg)

The 703 converts pounds and inches to kg/m² (the exact factor is 703.07; the CDC and NHLBI use 703, and the difference never moves a one-decimal BMI by more than 0.1). The adult bands are the WHO, CDC and NHLBI 1998 classes; the WHO 2004 Asian action points are under Advanced. Adults 20 and over only.

Calculating it by hand

  1. Pick metric or US units and enter your height (cm, or feet and inches) and weight (kg or lb).
  2. Enter your age: the adult bands apply from 20; under 20 the CDC uses BMI-for-age percentiles instead.
  3. Metric: divide weight in kg by height in metres squared — 70 ÷ 1.75² = 22.9.
  4. US units: multiply weight in pounds by 703 and divide by height in inches squared — 703 × 180 ÷ 70² = 25.8.
  5. Read the band: below 18.5 underweight, 18.5–24.9 healthy, 25.0–29.9 overweight, 30.0 and above obesity (classes I, II and III at 30, 35 and 40).
  6. Divide the BMI by 25 for BMI prime, and multiply 18.5 and 24.9 by your height in metres squared for the healthy weight range.

Worked examples

175 cm and 70 kg, age 35 (metric, standard bands)

  1. Enter: Metric, height 175 cm, weight 70 kg, age 35.
  2. Height² = 1.75 × 1.75 = 3.0625 m²
  3. BMI = 70 ÷ 3.0625 = 22.86 → 22.9
  4. BMI prime = 22.86 ÷ 25 = 0.91
  5. Healthy weight = 18.5 × 3.0625 to 24.9 × 3.0625 = 56.7 – 76.3 kg

BMI 22.9 — Healthy weight (18.5–24.9), inside the 56.7–76.3 kg range for 175 cm

5 ft 10 in and 180 lb, age 40 (US units)

  1. Enter: US units, 5 ft, 10 in, 180 lb, age 40.
  2. Height = 70 in; 70² = 4,900
  3. BMI = 703 × 180 ÷ 4,900 = 126,540 ÷ 4,900 = 25.82 → 25.8
  4. BMI prime = 25.82 ÷ 25 = 1.03
  5. Healthy weight = 18.5 × 4,900 ÷ 703 = 129 lb to 24.9 × 4,900 ÷ 703 = 174 lb; 180 − 173.6 = 6 lb above

BMI 25.8 — Overweight (25.0–29.9), 6 lb above the 129–174 lb healthy range

165 cm and 65 kg, age 45, with the Asian cut-off points

  1. Enter: Metric, 165 cm, 65 kg, age 45; Advanced → Cut-off points → Asian populations: WHO 2004.
  2. Height² = 1.65 × 1.65 = 2.7225 m²
  3. BMI = 65 ÷ 2.7225 = 23.88 → 23.9
  4. Standard bands: 18.5–24.9, healthy weight. WHO 2004 action points: 23.0–27.4, increased risk
  5. Healthy weight on the Asian points = 18.5 × 2.7225 to 22.9 × 2.7225 = 50.4 – 62.3 kg; 65 − 62.3 = 2.7 kg above

BMI 23.9 — Increased risk on the WHO 2004 Asian action points (healthy weight on the standard bands)

Methodology

How BMI is worked out and read

BMI is weight scaled for height, nothing more. The bands it is read against were set for adults of European descent, which is why the cut-offs, not the arithmetic, are where most mistakes happen.

Read sources and limits

What is a healthy BMI for adults?

A BMI calculator divides your weight in kilograms by the square of your height in metres and compares the result with fixed adult bands. For adults aged 20 and over, the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) class 18.5 to 24.9 as a healthy weight.

The full adult classification is the same in the WHO tables, the CDC's adult BMI categories and the 1998 clinical guidelines of the National Heart, Lung, and Blood Institute (NHLBI): underweight below 18.5; healthy weight 18.5 to 24.9; overweight 25.0 to 29.9; obesity 30.0 and above, split into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0 and above), which the CDC also calls severe obesity. The same bands apply to men and women and do not change with age once you are an adult.

This page rounds BMI to one decimal and classifies the rounded figure, so a BMI of 24.96 shows as 25.0 and is read as overweight, never as a healthy 25.0.

How do you calculate BMI by hand?

In metric units, square your height in metres and divide your weight in kilograms by it. At 175 cm and 70 kg: 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.86, which rounds to 22.9 — inside the healthy band.

In US units, multiply your weight in pounds by 703 and divide by your height in inches squared. At 5 ft 10 in (70 in) and 180 lb: 70 × 70 = 4,900, 703 × 180 = 126,540, and 126,540 ÷ 4,900 = 25.8 — just into the overweight band. The 703 is only the conversion between the two unit systems; it is the same measure.

What is the healthy weight range for my height?

Because BMI is weight divided by height squared, the weights that land in the healthy band at your height are 18.5 × height² and 24.9 × height², with height in metres. At 175 cm that is 18.5 × 3.0625 = 56.7 kg to 24.9 × 3.0625 = 76.3 kg.

At 5 ft 10 in it is 18.5 × 4,900 ÷ 703 = 129 lb to 24.9 × 4,900 ÷ 703 = 174 lb.

The calculator shows that range beside your result, how far your weight sits outside it, and the weight each band boundary means at your height on the chart and in the table under it. These are reference figures for one height, not a target: the same BMI can describe very different bodies.

What is BMI prime?

BMI prime is your BMI divided by 25, the top of the healthy band, so 1.00 is the line between a healthy weight and overweight. A BMI of 22.9 is a BMI prime of 0.91; 25.8 is 1.03, three per cent over the line.

Readers in Asian populations sometimes divide by 23 instead; this page keeps 25 so the figure means the same thing everywhere.

The ponderal (Rohrer) index divides weight by height cubed instead of squared, in kg/m³. It drifts less with height, so very tall and very short people sometimes find it a fairer comparison; 175 cm and 70 kg is 13.1 kg/m³. Neither figure has official cut-offs for adults.

Who should not read BMI on these bands?

Asian adults: a WHO expert consultation (The Lancet, 2004) found that the BMI at which health risk rises is lower in many Asian populations — between 22 and 25 for increased risk and 26 to 31 for high risk — and set public-health action points at 23.0, 27.5, 32.5 and 37.5.

Choose those cut-offs under Advanced: 165 cm and 65 kg is BMI 23.9, a healthy weight on the standard bands but increased risk on the Asian ones.

Athletes and other very muscular people score high because muscle is heavy; older adults who have lost muscle can carry more fat than their BMI suggests; and BMI is not classified during pregnancy. For anyone aged 2 to 19 the CDC reads BMI against age- and sex-specific percentiles (healthy weight from the 5th to below the 85th percentile), which this page does not compute. In each case the arithmetic still runs — the bands simply do not mean what they say. The NHLBI guidelines add waist circumference (over 102 cm for men, 88 cm for women) as a second signal for adults with a BMI of 25 to 34.9.

BMI (Body Mass Index) questions

What is a healthy BMI?

18.5 to 24.9 for adults aged 20 and over, in the WHO, CDC and NHLBI classifications alike. Below 18.5 is underweight, 25.0 to 29.9 overweight and 30.0 or more obesity. At 175 cm, a healthy BMI corresponds to 56.7 to 76.3 kg. For many Asian populations the WHO 2004 expert consultation put the first action point lower, at 23.0.

Is BMI different for men and women?

No. Adults of both sexes use the same formula and the same bands: 18.5 to 24.9 is a healthy weight for a woman and for a man. Women carry more fat than men at the same BMI on average, which is why body-fat ranges differ by sex while BMI bands do not. Sex only enters BMI for children, whose percentiles are sex-specific.

What BMI is considered obese?

30.0 or higher. Obesity is split into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0 and above), which the CDC also calls severe obesity and the NHLBI 1998 guidelines extreme obesity. At 175 cm, a BMI of 30 corresponds to 91.9 kg and a BMI of 40 to 122.5 kg.

How do I calculate BMI in pounds and inches?

Multiply your weight in pounds by 703, then divide by your height in inches squared. At 5 ft 10 in (70 in) and 180 lb: 703 × 180 = 126,540, and 126,540 ÷ 4,900 = 25.8. The 703 converts the units to kilograms per square metre, so the result is the same BMI the metric formula gives.

What is BMI prime?

BMI prime is BMI divided by 25, the upper limit of the healthy band, so 1.00 marks the start of overweight. A BMI of 22.9 is 0.91 and a BMI of 25.8 is 1.03, meaning 3% above the line. It is a convenience ratio with no separate guideline; the bands behind it are the same 18.5, 25 and 30.

What is the healthy weight range for my height?

Multiply 18.5 and 24.9 by your height in metres squared. At 175 cm (3.0625 m²) that is 56.7 to 76.3 kg; at 5 ft 10 in it is 129 to 174 lb; at 160 cm it is 47.4 to 63.7 kg. The calculator shows the range for the height you enter and how far your weight sits outside it, as information rather than a goal.

What is the BMI cut-off for Asians?

23.0 for increased risk and 27.5 for high risk, the action points from the WHO expert consultation published in The Lancet in 2004, which also proposed 32.5 and 37.5. The consultation found risk rising from BMI 22–25 in different Asian populations. Choose these cut-offs under Advanced; 165 cm and 65 kg (BMI 23.9) then reads as increased risk.

Is BMI accurate for athletes?

Often not. BMI cannot tell muscle from fat, so a muscular athlete can score 27 or more while carrying little body fat. For anyone with a lot of muscle, a body fat percentage estimate or a waist measurement says more about health risk than BMI does.

Does BMI change with age?

The adult bands do not: 18.5 to 24.9 is a healthy weight at 20 and at 80. What changes is body composition — older adults tend to lose muscle, so the same BMI can mean more body fat at 75 than at 30. Under 20, BMI is read against age- and sex-specific percentiles instead of fixed bands.

Can I use this BMI calculator for a child or teenager?

The BMI number is the same arithmetic, but the adult bands do not apply under 20. For ages 2 to 19 the CDC compares BMI with growth charts for the child's age and sex: below the 5th percentile is underweight, the 5th to below the 85th healthy, the 85th to below the 95th overweight, and the 95th or above obesity. This page does not compute percentiles.

Is BMI used during pregnancy?

Not on the adult bands. Weight in pregnancy includes the baby, placenta and extra fluid, so a BMI measured then does not describe body fat. Guidance on weight gain in pregnancy is based on the BMI before pregnancy, which is the figure to bring to a clinician.

Source notes

Method, sources & limitations

Every figure on this page comes from the formula above applied to the values you enter. Nothing is estimated from market data, and no result is a forecast. These are the sources behind the method and any fixed figures it uses:

Written and maintained by CalculatorManager Editorial, Editorial team. Figures last verified .

Educational information only. Results are estimates produced from the assumptions you enter, and are not financial, tax, legal or medical advice. Check anything that matters against a qualified professional and the cited sources.

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