BMI Calculator
Calculate your Body Mass Index instantly. Free, private, browser-local — no signup required.
Open the BMI Calculator →Body Mass Index (BMI) is a simple measure that uses your height and weight to estimate whether your weight is in a healthy range for your height. It is calculated by dividing your weight in kilograms by the square of your height in meters.
The TinyToolbox BMI Calculator works entirely in your browser. Enter your height and weight in metric or imperial units, and you get your BMI number plus its weight category (underweight, normal, overweight, or obese) instantly. Nothing is uploaded — your measurements never leave your device.
It’s completely free — no signup, no credit card, no hidden limits, no ads interrupting your result. The calculator works on any device with a modern browser, including phones and tablets.
Want the full picture of your health?
Pair your BMI with related health tools — calorie needs, heart rate zones, sleep cycles, and water intake — all free and browser-local. Or try TinyToolbox Premium for advanced features and no limits.
Use the Free BMI CalculatorHow to Calculate BMI
The standard BMI formula is weight (kg) ÷ height (m)². For example, a person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the normal range. If you prefer imperial units, the calculator handles the conversion for you automatically — just choose your preferred unit system.
Once you have your BMI, here is how the World Health Organization classifies the result for adults: below 18.5 is underweight, 18.5 to 24.9 is normal weight, 25.0 to 29.9 is overweight, and 30.0 or higher is obese. These thresholds are population-level screening guides; they are not a diagnosis.
Limitations to Keep in Mind
BMI does not distinguish between muscle mass and fat mass, so it can overestimate body fat in highly muscular people and underestimate it in older adults or those with low muscle mass. For a more complete picture, combine BMI with other indicators such as waist circumference, activity level, and how you feel day to day. When in doubt, consult a healthcare professional.
Because everything runs locally in your browser, you can check your BMI as often as you like while tracking progress toward your wellness goals — your data stays private, and there is no account to create or subscription to manage.
BMI Calculator FAQ
Is the BMI Calculator really free?
Yes, completely free. There are no hidden fees, no signup, and no usage limits. The tool runs entirely in your browser, so there are no server costs passed on to you — and no ads interrupting your results.
Is my height and weight data stored or sent anywhere?
No. The calculator processes everything locally in your browser. Your measurements are never transmitted, logged, or stored on any server. Close the page and the data is gone.
What is the difference between the regular, men's, and women's BMI calculators?
The calculation formula is the same. The dedicated pages simply present the standard BMI categories with additional context tailored to each audience. Use whichever page you find clearest — the result will be identical.
Can I use the BMI Calculator on my phone or tablet?
Yes. TinyToolbox tools are fully responsive and work on smartphones, tablets, and desktop browsers. Since everything runs locally, results appear instantly even on slower connections.
Does BMI work the same for children and teenagers?
No. For people under 20, BMI is interpreted using age- and sex-specific percentiles rather than adult categories. This calculator is intended for adults; for children and teens, consult a pediatrician or growth chart specialist.