How to Calculate BMI and Find Your Healthy Weight Range
Learn the BMI formula in metric and imperial units, see the WHO categories, find a healthy weight range for your height, and understand where BMI falls short.
Body mass index, or BMI, is one of the most common numbers you’ll see on health forms, in fitness apps and at check-ups. It’s a quick way to compare your weight with your height, and it’s often the first thing people look up when they start thinking about their weight.
The good news is that BMI is simple to work out yourself. This guide walks through the formula in both metric and imperial units, explains what the categories mean, shows how to turn BMI into a healthy weight range for your height, and covers the important limits of this number. It doesn’t replace advice from a doctor, but it will help you understand what the number on the screen actually means.
What BMI measures
BMI is a ratio of your weight to your height. It doesn’t measure body fat directly. It’s a screening number: a fast, rough way to sort people into broad weight categories across large groups. Doctors and researchers use it because it needs nothing more than a scale and a tape measure.
So BMI is a useful starting point, not a verdict. Two people with the same BMI can have very different body compositions and health.
The BMI formula
Metric version (kilograms and meters)
The standard formula is:
BMI = weight (kg) ÷ height (m)²
“Height squared” means your height multiplied by itself. For example, take someone who weighs 70 kg and is 1.75 m tall:
- Square the height: 1.75 × 1.75 = 3.0625
- Divide the weight by that number: 70 ÷ 3.0625 ≈ 22.9
Their BMI is about 22.9.
If you measure height in centimeters, divide by 100 first (175 cm = 1.75 m).
Imperial version (pounds and inches)
If you use pounds and inches, the formula needs a conversion factor of 703:
BMI = weight (lb) × 703 ÷ height (in)²
Take someone who weighs 154 lb and is 5 ft 9 in tall:
- Convert height to inches: 5 × 12 + 9 = 69 inches
- Square it: 69 × 69 = 4,761
- Multiply the weight by 703: 154 × 703 = 108,262
- Divide: 108,262 ÷ 4,761 ≈ 22.7
Their BMI is about 22.7. The two examples come out slightly differently because 154 lb and 5 ft 9 in are close to, but not exactly, 70 kg and 1.75 m.
WHO BMI categories for adults
The World Health Organization (WHO) uses these categories for adults:
| BMI range | WHO category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Pre-obesity (often called overweight) |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
Some countries and health organizations use slightly different cut-off points for certain populations, so you may see other thresholds in local guidance. Children and teens are assessed differently, using growth charts that compare them with others of the same age and sex.
How to find a healthy weight range for your height
You can flip the formula around to find the weight range that falls in the “normal” category for your height. Just multiply your height squared by the lower and upper limits. The normal range starts at 18.5 and runs up to just under 25.
Metric: weight (kg) = BMI × height (m)²
For someone 1.75 m tall (height squared = 3.0625):
- Lower end: 18.5 × 3.0625 ≈ 56.7 kg
- Upper end: 25 × 3.0625 ≈ 76.6 kg (the range stops just below this)
Imperial: weight (lb) = BMI × height (in)² ÷ 703
For someone 5 ft 9 in tall (69 in, squared = 4,761):
- Lower end: 18.5 × 4,761 ÷ 703 ≈ 125 lb
- Upper end: 25 × 4,761 ÷ 703 ≈ 169 lb (the range stops just below this)
The calculator works the same way, so for 1.75 m it shows about 56.7 – 76.5 kg. Notice how wide that range is, roughly 20 kg or 44 lb. That’s a reminder that “healthy weight” isn’t a single number.
How to calculate your BMI with Deftivo
If you’d rather skip the arithmetic, the Deftivo BMI calculator does it for you. It runs in your browser, so the numbers you type aren’t sent anywhere.
- Open the BMI calculator.
- Under Standard, keep WHO selected. A Korea (KSSO) option is also available if you want the Korean cut-offs.
- Under Units, choose cm · kg or ft · in · lb.
- Enter your height and weight. The result updates as you type.
- Read your BMI, the category it falls into and the healthy weight range for your height.
Try a few weights to see how the result changes. That makes it easy to see where the edges of each category are for your height.
Metric vs. imperial: quick comparison
| Metric | Imperial | |
|---|---|---|
| Units | Kilograms, meters (or cm) | Pounds, feet and inches |
| Formula | kg ÷ m² | lb × 703 ÷ in² |
| Extra step | Convert cm to m | Convert feet to inches |
| Result | Same BMI scale | Same BMI scale |
Whichever units you use, the BMI number means the same thing.
The limits of BMI
BMI is handy, but it has well-known blind spots:
- Muscle vs. fat. Muscle is denser than fat. Athletes and people who do a lot of strength training can have a high BMI without carrying much body fat.
- Where fat is stored. BMI doesn’t show whether weight sits around the waist or elsewhere, which can matter for health. Some professionals also look at waist size for this reason.
- Age. Body composition tends to change with age, so the same BMI can mean different things for a younger and an older adult. For children and teens, a plain adult BMI category doesn’t apply. If you need an exact age for a growth chart, the age calculator can work it out.
- Pregnancy. BMI isn’t meant to judge weight during pregnancy. Pregnancy weight guidance is usually based on BMI from before pregnancy, and is best discussed with a midwife or doctor.
- Different populations. Health risks at a given BMI can differ between ethnic groups, which is why some regions use different cut-offs.
Most importantly, BMI is not a diagnosis. It can’t tell you whether you’re healthy or unhealthy. If your result concerns you, or you’re planning a big change to your diet or exercise, talk to a doctor or another qualified health professional.
Tips and pitfalls
- Measure accurately. Weigh yourself at a similar time of day, ideally without shoes or heavy clothing. Measure height standing straight against a wall, without shoes.
- Don’t mix units. A common mistake is using centimeters in the metric formula without converting to meters. That gives a tiny, meaningless number.
- Watch the trend, not one reading. Weight changes from day to day. Looking at changes over weeks says more than one weigh-in.
- Use the range, not a single target. The healthy range for your height is broad, and where you sit within it depends on your build and other factors.
- Remember the context. A BMI just outside a category line isn’t a sudden change in health. The categories are guides, not hard limits.
FAQ
Is BMI the same for men and women?
For adults, the same formula and the same WHO categories apply to everyone. Men and women often have different body composition at the same BMI, which is one reason BMI is treated as a rough screen rather than a full assessment.
Can I use BMI for my child?
Not with the adult categories. For children and teens, BMI is compared against growth charts for the same age and sex, so the result is usually given as a percentile. A pediatrician or health visitor can help you interpret it.
Why is my BMI “overweight” when I’m very fit?
BMI can’t tell muscle from fat, so people with a lot of muscle often get a higher number. If you’re active and strong, measures like waist size or a body composition check by a professional may give a clearer picture.
How often should I check my BMI?
There’s no set rule. Many people check it occasionally, for example when tracking a long-term goal or before a check-up. Because daily weight naturally goes up and down, checking now and then and looking at the overall trend is more useful than checking every day.