How to Calculate BMI (Metric or Imperial) and What the Number Actually Tells You

I'm Sumit Kumar, founder of TooliFyra. BMI is one of those numbers everyone has heard of but few people calculate correctly, and even fewer know how to read sensibly. A doctor mentions it, an insurance form asks for it, a fitness app shows it in bright colours. Here is how it works, how to get it in ten seconds, and where it falls short.

What BMI is

Body Mass Index is a screening measure based on only two inputs, your weight and your height. The formula is:

BMI = weight (kg) ÷ height (m)²

So a person who weighs 70 kg and is 1.70 m tall has a BMI of 70 ÷ 2.89 = about 24.2. In imperial units the same idea is used with pounds and inches, with a conversion factor of 703.

The standard categories are:

How to use the BMI calculator

1. Open the BMI Calculator.
2. Pick a unit system: Metric (kg/cm) or Imperial (lbs/ft).
3. Enter your weight and height. In imperial mode, height is entered as feet and inches in separate boxes, so you don't have to convert 5'7" into decimals yourself.
4. Read your BMI value and category. A colour-coded gauge shows where you sit between Underweight, Normal, Overweight and Obese.
5. Check the ideal body weight range shown below the result. It is based on your height, so it changes as you adjust the height field.

The results update as you type, so there is no submit button to hunt for. Everything is calculated in your browser, which matters for a health measurement: your weight and height are not sent anywhere.

Three quick examples

• 55 kg, 160 cm: 55 ÷ 2.56 = 21.5, which is in the normal range.
• 70 kg, 170 cm: about 24.2, near the top of normal.
• 85 kg, 175 cm: 85 ÷ 3.0625 = 27.8, in the overweight range.

Try these in the calculator to confirm the numbers match what you calculate by hand.

Tips for getting a useful reading

• Measure the same way each time. Weigh yourself in the morning, before eating, with similar clothing. A 1 kg swing from one meal changes BMI by about 0.3 for an average adult.
• Measure height without shoes, standing straight against a wall. Being off by 2 cm changes the result more than people expect.
• Don't track daily. BMI is a slow-moving number. Checking monthly is enough to see a trend.
• Use the ideal range as a rough target, not a deadline. It tells you which weights fall in the normal band for your height.

What BMI doesn't tell you

BMI is a screening tool, not a diagnosis. It cannot tell muscle from fat, so a muscular person can land in the "overweight" band while having low body fat. It also says nothing about where fat is stored, and the same cut-offs may not suit every age group or body type. If your result surprises you, treat it as a prompt to talk to a doctor, not as a verdict. If you want a second angle, TooliFyra also has a Body Fat Calculator and a Calorie Calculator that look at different parts of the picture.

Frequently asked questions

What is a healthy BMI range?
18.5 to 24.9 is considered normal. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or more is obese.

Is BMI accurate for athletes?
Not always. Because it uses only weight and height, it can overstate risk for people with a lot of muscle.

Can I use pounds and feet?
Yes. Switch to the Imperial tab and enter weight in lbs and height in feet and inches.

Is my data stored anywhere?
No. The calculation happens locally in your browser, and nothing you type is uploaded.

Is it free?
Yes, no sign-up and no limits.

If you want to check your own number now, the free BMI Calculator is ready to use, and you can find the rest of the calculators and utilities on TooliFyra.