How to Work Out Your BMI and What Your Result Actually Means

Introduction

Try it now: use our free BMI Calculator to get the answer in seconds.

It usually happens at a clinic. The nurse weighs you, measures your height, writes something down, and says, “Your BMI is 26.” Then the conversation moves on — and you are left wondering what 26 actually means, and whether you should be worried.

So let’s slow that moment down. BMI stands for Body Mass Index. It is one number built from just two measurements, your height and your weight, and health workers use it the way you’d use a smoke alarm: a cheap, rough first signal, not a full inspection. It is not a diagnosis. It was never meant to be. Here’s how to work yours out, what the ranges really say, and — the part clinics often skip — what this number simply cannot know about you.

The BMI Formula

Metric version

If you know your weight in kilograms and your height in metres:

BMI = Weight (kg) ÷ [Height (m) × Height (m)]

In words: divide your height by itself (square it), then divide your weight by the result. Most adults’ height in metres is a number like 1.60, 1.70, or 1.80 — if you know your height in centimetres, just divide by 100 (170 cm = 1.70 m).

Imperial version

If you know your weight in pounds and your height in inches:

BMI = [Weight (lb) ÷ (Height (in) × Height (in))] × 703

The 703 is simply a conversion factor that makes the imperial answer land on the same scale as the metric one, so the ranges below apply to both.

A worked example, with a real person in mind

Meet Sara: 68 kg, 1.68 m tall. (If your numbers are in pounds and inches, jump to the 703 formula above — the steps are identical.)

Step 1: Square the height

1.68 × 1.68 = 2.8224

Step 2: Divide the weight by that number

68 ÷ 2.8224 = 24.09

Step 3: Round sensibly

BMI is usually reported to one decimal place: BMI ≈ 24.1

That’s it. Honestly, that’s the whole sum. And the imperial route tells the same story: someone of a similar build at 150 lb and 5 feet 6 inches (66 inches) gets (150 ÷ (66 × 66)) × 703 ≈ 24.2. Two systems, same answer — as there should be.

What the Adult BMI Ranges Mean

For adults aged 20 and over, the commonly used categories are:

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obesity

Sara’s 24.1 lands inside the healthy band, near the top of it. One thing we always say to people at this point: don’t treat these lines like cliff edges. A body at 24.9 and the same body at 25.1 have not changed in any real way overnight — the line was drawn for studying whole populations, not for judging the person standing on your bathroom scales. Any decent clinician will look at your waist measurement, blood pressure, how active you are, and your family history before saying anything firm.

A note for children and teenagers

Do not use the table above for under-20s. Children and teens are still growing, so their BMI is compared against other children of the same age and sex using percentile charts. A health professional or school nurse can interpret those correctly.

How to Measure Yourself Accurately

A small measuring error changes the result more than people expect, because height is squared in the formula. For a number worth trusting:

Weigh yourself in the morning, before eating, in light clothing, on a hard flat floor — not carpet.
Measure height against a wall, heels together, looking straight ahead, a flat book resting on your head. Ask someone to help.
Use the same scales and routine every time, so changes over time actually mean something.

What BMI Cannot Tell You

BMI is popular because it is cheap and quick — not because it sees the whole picture. Its main limits:

It cannot tell fat from muscle. A strength athlete can have an “overweight” BMI with very little body fat, because muscle is dense.
It cannot say where fat sits. Fat carried around the waist is linked with higher health risk than the same amount carried on hips and thighs; BMI treats them identically.
It does not fit every body equally. Older adults, very tall or short people, pregnant women, and some ethnic groups can be misread by the standard ranges — for example, health risk can rise at a lower BMI in some South and East Asian populations, which is why some guidelines use lower cut-offs there.
It says nothing about fitness, diet, or blood results. A person in the “healthy” range can still be unfit, and a person above it can have excellent blood pressure and stamina.

So use BMI as a starting point for a conversation, not as a verdict. If your result is well outside the healthy range and you are concerned, that is a good reason to talk to a qualified health professional — not a reason to crash-diet or panic.

Frequently Asked Questions

“Do men and women use different BMI charts?”

No — for adults, the formula and the ranges above are the same for everyone. Men’s and women’s bodies do differ in how much of their weight tends to be muscle versus fat, which is exactly why a good professional never reads BMI completely on its own.

“I weigh myself every morning. Bad idea?”

For tracking BMI, mostly, yes. Your weight can swing a kilo or more between morning and night depending on water, food, and salt. Weigh daily if you enjoy it, but judge the trend once a month, under the same conditions. Otherwise you will end up chasing noise.

“Mine came out at 25.4. Am I in trouble?”

You are 0.4 above a statistical line, not in trouble. Treat it as a nudge to look at the bigger picture — your waist, your energy, your blood pressure at your next check-up — rather than a reason to skip meals this week. If the number keeps climbing over several months, that’s the point where a proper conversation with a doctor is worth having.

Conclusion

Work yours out once, properly, with honest measurements. Write it down with the date. Then put the calculator away and get on with your life, checking again in a few months to see the direction of travel. A single BMI reading tells you very little; a slow trend, read alongside how you actually feel and function, tells you almost everything this simple number has to offer.