How to Calculate BMI: Formula, Chart and Normal Range

Body Mass Index is the most widely used weight screening tool in the world. It requires only your height and weight, takes seconds to calculate, and gives you a single number that maps onto a category.

That simplicity is both its strength and its weakness. BMI is genuinely useful for spotting trends across large populations and for giving an individual a rough starting point. It is far less reliable when applied to a single person without context, because it cannot distinguish muscle from fat.

This guide covers both formulas, works through examples in metric and imperial units, gives the full category chart and healthy weight ranges by height, and is honest about where the measure breaks down.

The BMI Formula

BMI compares your weight to your height, on the reasoning that taller people are expected to weigh more. Dividing by height squared produces a number that is broadly comparable across different heights.

There are two versions of the formula depending on which units you use.

Metric

BMI = weight (kg) / height (m)^2

• Weight in kilograms
• Height in metres, not centimetres – 175 cm becomes 1.75

Imperial

BMI = 703 x weight (lb) / height (in)^2

• Weight in pounds
• Height in inches – 5 feet 9 inches becomes 69 inches
• The 703 factor converts the result to match the metric scale

Worked Examples

The same person, calculated both ways.

The small difference between 22.9 and 22.7 is just rounding in the unit conversion. Both formulas describe the same person and both land in the same category, which is all BMI is really designed to tell you.

Metric: 70 kg, 1.75 m

• Height squared = 1.75 x 1.75 = 3.0625
• BMI = 70 / 3.0625 = 22.9
• Category: healthy weight

Imperial: 154 lb, 5 feet 9 inches

• Height in inches = (5 x 12) + 9 = 69
• Height squared = 69 x 69 = 4,761
• BMI = 703 x 154 / 4,761 = 108,262 / 4,761 = 22.7
• Category: healthy weight

The BMI Category Chart

The World Health Organization classification for adults is the most widely used standard:

Two important caveats. These categories apply to adults aged 20 and over – children and teenagers are assessed against age and sex percentile charts instead, because their body composition changes as they grow.

Second, some health authorities apply lower thresholds for people of South Asian, Chinese, and certain other ancestries, where cardiometabolic risk rises at a lower BMI. In several countries the overweight threshold is set at 23 rather than 25 for these groups.

BMI Range	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obesity class I
35.0 – 39.9	Obesity class II
40.0 and above	Obesity class III

Healthy Weight Ranges by Height

Rather than calculating your BMI and checking the category, you can work backwards: multiply the BMI thresholds by your height squared to get your healthy weight range directly.

Notice how wide each range is – around 16 to 22 kg. That width exists because BMI is a coarse instrument. Two people at opposite ends of the same range can look and function very differently.

Height	Healthy Range (kg)	Approximate (lb)
1.60 m (5’3″)	47.4 – 63.7	104 – 140
1.65 m (5’5″)	50.4 – 67.8	111 – 149
1.70 m (5’7″)	53.5 – 72.0	118 – 159
1.75 m (5’9″)	56.7 – 76.3	125 – 168
1.80 m (5’11”)	59.9 – 80.7	132 – 178
1.85 m (6’1″)	63.3 – 85.2	140 – 188

What BMI Gets Wrong

BMI uses two inputs, and neither of them says anything about what your weight is made of. That single limitation explains almost every case where the number misleads.

• Muscle weighs more than fat. A trained athlete can register as overweight or obese while carrying very little body fat. This is the best-known failure of the measure.
• Fat distribution is invisible to it. Fat stored around the abdomen carries considerably more health risk than fat on the hips and thighs, but BMI treats them identically.
• It ignores age. Body composition shifts with age – muscle mass declines while fat increases – so an unchanged BMI can conceal a real change.
• Ancestry affects risk thresholds. Health risk begins at different BMI levels across populations, which the single standard chart does not reflect.
• It does not apply to everyone. Pregnant people, children, and those with certain medical conditions cannot be meaningfully assessed with standard BMI.

Better Measurements to Use Alongside It

BMI is most useful when it is not the only thing you look at. These add the context it lacks:

A quick and surprisingly good check is the waist-to-height ratio: your waist should measure less than half your height. It needs only a tape measure, works across different body types, and correlates better with cardiometabolic risk than BMI does.

Used together, BMI and a waist measurement give a far more reliable picture than either alone.

Measurement	What It Adds	How to Get It
Waist circumference	Abdominal fat, the highest-risk kind	Tape measure at the navel
Waist-to-height ratio	Risk adjusted for your size	Waist divided by height; aim below 0.5
Body fat percentage	Distinguishes muscle from fat	Calipers, smart scales, or a body fat calculator
Waist-to-hip ratio	Fat distribution pattern	Waist measurement divided by hip measurement

Conclusion

BMI is easy to calculate and easy to over-interpret. Divide your weight in kilograms by your height in metres squared, or multiply pounds by 703 and divide by inches squared, and you have a number that places you on a standard chart in seconds.

Treat that number as a starting point rather than a verdict. If your BMI sits well inside the healthy range and your waist is under half your height, that is a reassuring pair of signals. If either is outside the expected range, it is worth a conversation with a doctor who can look at the things a two-input formula never could.

Frequently Asked Questions

What is the formula for calculating BMI?

In metric units, BMI equals weight in kilograms divided by height in metres squared. In imperial units, it is 703 multiplied by weight in pounds, divided by height in inches squared. Both produce the same scale, so a BMI of 22 means the same thing either way.

What is a healthy BMI range?

For most adults aged 20 and over, 18.5 to 24.9 is classified as healthy. Below 18.5 is underweight and 25 or above is overweight. Some health authorities use a lower threshold of 23 for people of South Asian and certain other ancestries.

Is BMI accurate for muscular people?

No. BMI cannot distinguish muscle from fat, so people with high muscle mass frequently register as overweight or obese despite having low body fat. Athletes and regular strength trainers should use body fat percentage or waist measurements instead.

Does BMI work for children?

Not in the same way. Children and teenagers are assessed using age- and sex-specific percentile charts rather than the adult categories, because their body composition changes substantially as they grow. Use a paediatric BMI calculator or consult a doctor.

What is a better alternative to BMI?

No single measure replaces it, but waist-to-height ratio is a strong and simple option – your waist should measure less than half your height. Combining BMI with a waist measurement gives a much more reliable picture than either on its own.