How to Calculate BMI
The BMI formula explained step by step, in metric and imperial units.
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BMI Calculator
Calculate your Body Mass Index from your height and weight. Choose your weight and height units separately.
No registration required
Secure & private
Works on any device
—Your BMI
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—Reference weight range
Underweight
below 18.5Healthy range
18.5–24.9Overweight
25–29.9Obesity
30+
below 18.5Healthy range
18.5–24.9Overweight
25–29.9Obesity
30+
100% Private
Your height and weight are calculated in your browser — nothing is sent anywhere.
Instant
Get your result the moment you press Calculate.
Free & Unlimited
No account and no limits on how often you use it.
How to calculate your BMI
Three quick steps. Nothing to install and nothing leaves your device.
1
Pick your units
Choose kilograms or pounds, and centimetres or feet and inches.
2
Enter weight and height
Type your measurements. Imperial values are converted automatically.
3
Calculate
See your BMI, its category and a reference weight range for your height.
About BMI
What the number means and what it does not.
How is BMI calculated?
For adults, BMI is calculated by dividing weight in kilograms by height in metres squared. This tool converts imperial measurements automatically.
Adult BMI categories
For adults aged 20 and older, commonly used screening categories are: underweight below 18.5, healthy weight 18.5–24.9, overweight 25–29.9, and obesity 30 or above.
Important note
BMI is a screening measure, not a diagnosis. It does not directly measure body fat and may not describe health accurately for every person. Adult categories are not intended for children or teens, whose BMI is interpreted using age- and sex-specific percentiles. If you have concerns about your weight or health, consult a qualified healthcare professional.
Learn about BMI
Helpful guides about BMI, units and healthy-weight ranges.
Frequently asked questions
Quick answers about the ConvertKit Lite BMI Calculator.
Is my data stored or sent anywhere?
No. The calculation runs entirely in your browser, so your measurements are not uploaded to a server.
Can I mix metric and imperial units?
Yes. Weight and height units are chosen separately, for example pounds with centimetres.
Is BMI accurate for everyone?
BMI is a screening measure and does not directly measure body fat. It may not reflect health accurately for every person, such as athletes or older adults.
Can children and teens use this calculator?
No. The categories here are for adults aged 20 and older. For children and teens, BMI is interpreted using age- and sex-specific percentiles.
What is the reference weight range?
It is the weight span that would give an adult BMI of 18.5–24.9 at the height you entered. It is a guide, not a personal target.