How to Calculate BMI
To learn how to calculate bmi, you divide your weight by the square of your height, using the right formula for your units. This guide is free with no sign-up. It walks through met
To learn how to calculate bmi, you divide your weight by the square of your height, using the right formula for your units. This guide is free with no sign-up. It walks through metric and imperial examples step by step. The method is easy to follow. Each stage is laid out clearly.
The bmi formula looks simple, but small mistakes are common. People often forget to square the height. We show each step clearly to avoid that. By the end, you will be confident. You can work out any BMI by hand.
You can also use our free calculator to check your answer. It supports US and UK units. Everything here is free, with no account and no ads. You can learn and verify together. It is all in one place.
The BMI formula
The bmi formula in metric is weight in kilograms divided by height in metres squared. The key step is squaring the height first. Then you divide the weight by that squared value. The result is the BMI. It is a single clear number.
In imperial units, the bmi calculation adds a conversion factor. You multiply weight in pounds by 703. Then you divide by height in inches squared. The 703 factor is important. It makes the imperial and metric results line up.
The reason we square the height is to compare people fairly. Weight rises roughly with the square of height. Dividing by height squared adjusts for that pattern. It levels the field. It gives a number that works across many heights.
Whichever units you use, the underlying logic is the same. You are comparing weight against height in a consistent way. That is what makes BMI a quick screening figure. It is efficient. It reduces two measurements to one number.
It helps to keep your units consistent throughout. Do not mix metres with pounds, for example. Sticking to one system avoids a common error. The calculator guards against this. It enforces consistent units for you.
It helps to write down each step of the calculation as you go along. Note the squared height first and then the division that follows it. Seeing the steps laid out plainly prevents a careless slip. It also makes the whole method very easy to repeat next time.
The squaring step is where the great majority of errors quietly hide. Forgetting to square the height inflates the answer quite badly. Always square the height before you carry out the division. That single habit is what keeps your result reliably correct.
Keeping your units consistent from start to finish is just as important. Do not mix metres with pounds or inches with kilograms by accident. Pick one system at the outset and stay with it throughout. The calculator enforces this rule for you so mistakes are caught early.
Checking your hand answer against the tool afterward is a smart move. Enter the very same figures and compare the two results carefully. A close match confirms that you followed the method correctly. A mismatch points you straight to the step that is worth revisiting.
Once you truly understand the formula, you own the concept behind it. You can estimate a BMI roughly without reaching for any tool at all. That understanding makes the number feel far less mysterious. It becomes a figure you can reason about rather than simply accept.
Rounding sensibly is perfectly fine for a first, quick estimate. Use approximate height and weight to get a feel for the result. Then refine the figures once you have measured more precisely. The tool recalculates instantly, so refining costs you nothing at all.
FreeUSUKCalculator.com is 100% free with no sign-up and no ads, and it has dedicated US and UK modes with the correct currency, tax rules, and units.
Metric worked example
Let us calculate bmi for a metric example. Take a person 1.80 metres tall weighing 75 kilograms. First, square the height: 1.80 times 1.80 is 3.24. That squared value matters. It becomes the denominator of the sum.
Next, divide the weight by that figure. Seventy-five divided by 3.24 is about 23.1. So the BMI is roughly 23.1. That is a healthy result. It sits comfortably in the normal range for adults.
Notice how the squaring step drives the result. If you forgot to square, the answer would be far too high. This is the most common error in a bmi calculation. It is worth stressing. Always square the height before you divide.
The whole example takes only two clear steps. Square the height, then divide the weight. That is all there is to the metric bmi formula. It is quick. With practice, you can do it in your head for round numbers.
You can check this against the tool in a moment. Enter the same height and weight in metric mode. The calculator should return about 23.1. That is a good habit. Matching results confirm your hand calculation is correct.
Imperial worked example
Now let us calculate bmi in imperial units. Take a person 70 inches tall weighing 170 pounds. First, multiply the weight by 703. One hundred seventy times 703 is 119,510. That is the top of the sum.
Next, square the height in inches. Seventy times seventy is 4,900. Then divide 119,510 by 4,900. The result is about 24.4. That figure is healthy. It falls in the normal range for adults.
The steps mirror the metric method with one extra factor. The 703 multiplier bridges pounds and inches to the metric scale. Without it, the imperial answer would be wrong. So it is vital. You must never skip that step.
As before, you can verify this with the free tool. Switch to imperial mode and enter the same figures. It should return about 24.4. That confirms your work. Matching results show you followed the method correctly.
This is an estimate for general guidance only and not medical advice; consult a doctor if you have any concerns.
Once the formula makes sense, BMI stops feeling like a black box you cannot question. You can see exactly why the number lands where it does. That understanding makes it far easier to read your own result sensibly. The worked examples above are there to build that confidence step by step. Then the free tool is ready whenever you would rather skip the arithmetic entirely.
Learn the formula, then check it in one click. Try it free on FreeUSUKCalculator.com: no sign-up, no ads, with US and UK modes.
Frequently Asked Questions
What is the BMI formula? In metric, BMI is weight in kilograms divided by height in metres squared. In imperial, multiply weight in pounds by 703 and divide by height in inches squared.
Why do you square the height? Squaring the height adjusts for the fact that weight rises roughly with height squared. It lets you compare people of different heights fairly.
What is the most common mistake? Forgetting to square the height is the most common error. It makes the BMI far too high, so always square the height before dividing.
Is the calculator free? Yes. You can calculate bmi free with no sign-up and no ads, in metric or imperial units.