BMI Calculator

How to use the BMI calculator

Body mass index (BMI) is a quick number that compares your weight with your height. Enter both in the calculator above, in metric or US units, and the result appears instantly with its category and the healthy weight range for your height. Nothing is sent anywhere; the calculation runs in your browser.

1. Choose units: metric (cm and kg) or US (feet, inches and pounds).
2. Enter your height and weight.
3. Read your result: your BMI to one decimal place, the adult category it falls in, and where it sits on the scale.

The calculator is for adults aged 18 and over. Children and teenagers need a different method that takes age and sex into account (see below).

Reading your result

The calculator shows three things. First, your BMI to one decimal place. Second, the adult category it falls in, with a marker on a coloured scale from underweight to obesity so you can see how close you are to a boundary. Third, the healthy-weight range for your height, which turns the number into something practical: the weights at which your BMI would be 18.5 to 24.9. If you tick the option for the lower thresholds the NHS uses for some ethnic groups, the category is shown for both sets of cut-offs side by side.

The BMI formula

BMI is weight divided by height squared:

• Metric: BMI = weight in kilograms ÷ (height in metres × height in metres).
• US units: BMI = 703 × weight in pounds ÷ (height in inches × height in inches).

Worked example (metric): 70 kg and 1.75 m. 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.9.

Worked example (US): 154 lb and 5 ft 9 in (69 inches). 69 × 69 = 4,761, and 703 × 154 ÷ 4,761 = 22.7.

Common mistakes when working out BMI by hand

• Using centimetres instead of metres. 175 cm must become 1.75 m before squaring, or the answer is 10,000 times too small.
• Forgetting to square the height. Dividing by height once gives a meaningless number; divide by height × height.
• Mixing units. Pounds with metres, or kilograms with inches, give wrong results. The 703 factor is only for pounds and inches.
• Measuring height in shoes. A few centimetres of heel lowers your BMI noticeably.

Converting your measurements

The calculator converts automatically, but if you’re working on paper, these factors are exact by definition:

Example: 5 ft 9 in is 69 inches, and 69 × 2.54 = 175.26 cm.

BMI categories for adults

These are the adult cut-offs used by the World Health Organization (underweight below 18.5, overweight 25 or more, obesity 30 or more) and the US National Heart, Lung, and Blood Institute.

Lower thresholds for some groups: the UK’s NHS notes that people from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background are at risk of health problems at a slightly lower BMI, and uses 23 to 27.4 as overweight and 27.5 or higher as obese for these groups. The calculator shows both readings when you choose this option.

Healthy weight range by height

A BMI of 18.5 to 24.9 corresponds to these weights. The figures are arithmetic from the formula, rounded to one decimal place:

The calculator shows the exact range for your own height.

What BMI can’t tell you

BMI is a useful starting point, but it’s a simple ratio, not a measure of health. Keep these limits in mind:

• Muscle and fat weigh the same on a scale. As the NHS points out, BMI can’t tell the difference, so very muscular people can read as overweight without much body fat.
• Where fat sits matters. Fat around the waist carries more health risk; the NHS suggests a waist-to-height ratio check alongside BMI for people with a BMI under 35.
• Ethnic background changes the risk at a given BMI (see the lower thresholds above).
• It isn’t designed for pregnancy, and older adults and athletes may need other measures.

If your result worries you, or you’re planning to lose or gain weight, talk to a doctor or another qualified health professional. This calculator gives information, not medical advice.

Tracking your BMI over time

A single reading matters less than the direction it moves. If you’re tracking, weigh yourself under the same conditions each time, such as the same scale, the same time of day and similar clothing, and compare readings weeks apart rather than day to day, because body weight naturally varies from one day to the next. A change of a few tenths in BMI between weigh-ins is normal and rarely meaningful on its own.

Children and teenagers

For people under 18, the adult categories don’t apply. Bodies change as children grow, so their BMI is compared with other children of the same age and sex using growth charts (BMI-for-age percentiles). WHO defines overweight and obesity for children and adolescents differently from adults. Please use a children’s BMI tool or ask a health professional; this calculator only gives adult results.

Ask in plain words

• “I’m 5 ft 7 and 160 pounds. What’s my BMI?”
• “What weight range is healthy for 172 cm?”
• “Convert 68 kg to pounds and work out my BMI at 1.70 m.”

ToolsMotive runs the same calculation, shows the formula it used, and links to the sources above.

Privacy

Your height and weight are calculated in your browser and aren’t sent to our servers or stored. If you’re signed in and ask in the chat, the conversation follows your Library and privacy settings, and it is never used to train AI.

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