How to Calculate BMI: A Complete Guide to Body Mass Index
Body Mass Index, or BMI, is the quickest widely used way to check whether your weight is in a healthy range for your height. Doctors, insurers and fitness programmes all use it as a first screening step, and the calculation itself takes less than a minute once you know the formula. This guide explains the formula in both metric and imperial units, walks through worked examples, and shows what your number actually means — and where BMI falls short.
What BMI is, in one paragraph
BMI is your weight divided by the square of your height. That is the whole formula. Squaring the height adjusts for the fact that taller people naturally weigh more, so the result is comparable across different body sizes. The output is a single number, and standard cut-offs place that number into categories: underweight, healthy, overweight or obese. It was designed as a population-level screening tool, not a personal diagnosis, which is why it is fast and rough rather than precise.
The BMI formula
There are two versions of the same formula, depending on your units:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
The 703 in the imperial version is just a unit-conversion constant. If you measure in centimetres, convert to metres first by dividing by 100. If you know your height in feet and inches, convert the whole thing to inches (feet × 12 + inches) before squaring.
How to calculate BMI, step by step
- Weigh yourself in kilograms or pounds, ideally in the morning before eating.
- Measure your height in metres (or centimetres, then divide by 100) or in inches.
- Square your height — multiply it by itself.
- Divide weight by height squared. In imperial units, multiply the result by 703.
- Round to one decimal place and compare with the categories below.
Worked examples
Metric example: a person weighing 70 kg at 1.75 m tall. Height squared is 1.75 × 1.75 = 3.0625. Then 70 ÷ 3.0625 = 22.9 — comfortably in the healthy range.
Imperial example: a person weighing 160 lb at 5 ft 9 in (69 inches). Height squared is 69 × 69 = 4761. Then 703 × 160 ÷ 4761 = 23.6 — also healthy.
Notice how the two examples describe roughly the same body and land in the same zone. That is the point of the formula: unit-independent comparison.
BMI categories for adults
| BMI range | Category | What it suggests |
|---|---|---|
| Below 18.5 | Underweight | May need to gain weight; worth discussing with a doctor |
| 18.5 – 24.9 | Healthy weight | Associated with the lowest health risks for most people |
| 25.0 – 29.9 | Overweight | Elevated risk for some conditions; lifestyle review helps |
| 30.0 and above | Obese | Higher health risks; medical guidance recommended |
These cut-offs apply to adults aged 20 and over, and they are the same for men and women. Children and teenagers use BMI percentiles adjusted for age and sex, which is a different calculation.
Finding your healthy weight range
The formula works in reverse too. Multiply 18.5 and 24.9 each by your height squared to get the weight range that keeps you in the healthy zone. For the 1.75 m person above: 18.5 × 3.0625 = 56.7 kg, and 24.9 × 3.0625 = 76.3 kg. So roughly 57–76 kg is the healthy window at that height. A target range is more useful than a single target number, because healthy bodies naturally vary.
What BMI does not tell you
BMI cannot distinguish muscle from fat. A rugby player and a sedentary office worker can share the same BMI with very different health profiles, because muscle is denser than fat. It also says nothing about fat distribution: fat carried around the waist is linked to higher risk than fat carried on the hips and thighs, and two people with identical BMIs can differ greatly there.
Age and ethnicity matter as well. Older adults naturally carry more fat at the same BMI, and some populations face elevated health risks at lower BMI cut-offs. For these reasons, doctors treat BMI as a starting flag. If your BMI is outside the healthy range, the sensible next step is a conversation and possibly a waist measurement or body-fat estimate — not panic, and not complacency if the number looks fine but other risk factors exist.
Five practical tips for a healthy BMI
- Track the trend, not the day. Weigh yourself under consistent conditions and watch the direction over weeks, not the noise of a single morning.
- Pair BMI with a waist check. A tape measure around the waist adds the distribution information BMI misses.
- Move the number with habits, not crashes. Small, sustained changes in food and activity move BMI gradually and keep it moved.
- Keep muscle in the picture. Strength training preserves the muscle that BMI cannot see, which matters more than the number alone.
- Re-check every few months. That is often enough to catch drift early without obsessing.
How to use the calculator
Open the BMI calculator, choose metric or imperial units, and enter your weight and height. The result appears instantly with your category colour-coded, plus the healthy weight range for your height — the reverse calculation done for you. Because everything runs in your browser, no data is sent anywhere or stored.
Bottom line: weight divided by height squared, compared against 18.5–24.9. It is a screening number, not a verdict — useful for spotting drift, best read alongside waist size, activity and how you actually feel.
Frequently Asked Questions
What is a good BMI for adults?
For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese. These cut-offs are the same for men and women.
How accurate is BMI as a health measure?
BMI is a useful screening tool at population level, but it has limits for individuals: it cannot tell muscle from fat, so very muscular people can score as overweight, and it says nothing about where fat is carried. Treat it as a first check, not a diagnosis.
Why are there two BMI formulas?
There is really one formula: weight divided by height squared. The metric version uses kilograms and metres directly, while the imperial version uses pounds and inches and multiplies by 703 to correct the units. Both give the same result.
Does BMI work for athletes and muscular people?
Not reliably. Muscle is denser than fat, so athletes and people who lift weights often land in the overweight or even obese range despite low body fat. For them, waist measurement or body-fat percentage are more meaningful.
How often should I check my BMI?
Checking every few months is plenty for most people. Weight changes slowly, and daily fluctuations from food, water and exercise can make frequent checks noisy. What matters is the trend over time, not a single reading.