BMI & Healthy Weight — Complete Guide to Body Mass Index

BMI, or body mass index, is your weight in kilograms divided by your height in metres squared (kg/m2). A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 / 1.75^2 = 22.86, which rounds to 22.9 and falls in the healthy weight range. In US units the same calculation is 703 x weight in pounds / height in inches squared. This guide is a how-to: how to calculate BMI by hand, how to read the result, how much weight moves you between categories, and what to do next. For a deeper look at where BMI misleads, see our separate post on BMI limitations.

How to calculate BMI by hand

1. Metric: divide weight in kg by height in metres, then divide again by height in metres. Example: 70 kg and 1.75 m gives 70 / 3.0625 = 22.86.
2. Imperial: multiply weight in pounds by 703, then divide by height in inches squared. Example: 140 lb and 5 ft 4 in (64 in) gives 703 x 140 / 4,096 = 24.03.
3. Cross-check: 140 lb is 63.5 kg and 64 in is 1.6256 m, so 63.5 / 1.6256^2 = 24.03, the same answer.

The CDC and WHO both describe BMI as weight divided by height squared, with weight in kilograms and height in metres.

Read the result: the categories

The CDC applies these categories to adults 20 and older regardless of age, sex or race. The WHO uses 25 or more for overweight and 30 or more for obesity. Our BMI calculator uses the 18.5, 25 and 30 boundaries, and it does not adjust for ethnicity.

What the healthy range looks like at different heights

Because BMI is weight over height squared, the same category covers a different weight at each height. This table was computed from the boundaries 18.5 and 25 (a BMI of exactly 25 is the first overweight value, so the upper figure is the boundary and not a target). Values are rounded to the nearest 0.1 kg and whole pound.

How close are you to a boundary?

Small weight changes move BMI by small amounts, and that matters near a category line. A person 5 ft 5 in (65 in) weighing 150 lb has a BMI of 703 x 150 / 4,225 = 24.96, just inside the healthy range. At 153 lb the same person is at 25.46, in the overweight category, although nothing meaningful about their health changed in 3 pounds. The weight at BMI 25 for 65 in is 150.2 lb. Likewise, someone 1.75 m tall who weighs 82 kg has a BMI of 26.78; the weight at BMI 25 for that height is 76.56 kg, which is 5.44 kg lower. That gap describes where the boundary sits, not a goal you should set without advice.

One kilogram changes BMI by about 0.44 at 1.50 m, 0.39 at 1.60 m, 0.35 at 1.70 m, 0.31 at 1.80 m and 0.28 at 1.90 m. Shorter people move through BMI points faster per kilogram.

Using BMI for children, teens and adults over 20

For ages 2 to 19 the CDC uses BMI-for-age percentiles that depend on sex and age. Underweight is less than the 5th percentile, healthy weight is the 5th to less than the 85th, overweight is the 85th to less than the 95th, and obesity is the 95th percentile or greater. WHO defines overweight and obesity in ages 5 to 19 using standard deviations from a growth reference median. Our calculator is built on adult cutoffs and is not calibrated for children, so for anyone under 20 use a pediatric growth chart with a clinician.

Where BMI helps

The CDC calls BMI a quick, safe and reliable screening measure of weight relative to height, and says it is useful for screening, for tracking health risks over time and as a population-level measure of obesity trends. It is a good first check and a convenient way to track your own trend with the same scale and method.

Where it falls short, in brief

• It does not distinguish fat, muscle and bone mass, and says nothing about where fat is stored (CDC).
• A 2010 meta-analysis of 31,968 people found that common BMI cutoffs failed to identify about half of those with excess body fat percentage.
• Some populations have higher risk at lower BMI values; the WHO 2004 consultation highlighted 23.0 and 27.5 as potential public health action points for Asian populations.
• The WHO calls BMI a surrogate marker of fatness and notes that waist circumference can help the diagnosis of obesity.

Common mistakes

• Entering height in centimetres into the metric formula. 70 / 175^2 gives 0.0023, which is nonsense. Convert 175 cm to 1.75 m first.
• Forgetting the 703 factor in the imperial formula. 140 / 64^2 gives 0.034, not 24.03.
• Mixing units, such as pounds with metres.
• Rounding too early. Keep two decimals while calculating, then round at the end.
• Reading 25.0 versus 24.9 as a big change. It is a convention, not a cliff.

What to do with your result

The CDC says BMI is one potential health indicator and should be considered with other factors, and recommends discussing your result with a healthcare provider. Useful companions are waist measurement, blood pressure and blood tests, which a clinician can interpret in your context.

Frequently asked questions

Is my BMI the same at any age?

The CDC uses the same adult categories for everyone 20 and older. It does not provide separate age-based adult ranges, and we have not found a primary source that does, so we do not publish one.

Can I use BMI if I lift weights?

You can calculate it, but be careful reading it. Because BMI cannot separate muscle from fat, a body fat measurement and your waist size give you information BMI does not.

Should I rely on the healthy weight range in the calculator?

Treat it as a reference range for your height computed from BMI 18.5 and 25, not as a personal goal. Your clinician can tell you what is appropriate for you.