BMI calculator — healthy ranges and what BMI means
For most adults, a healthy BMI sits between 18.5 and 24.9. You get the number by dividing your weight in kilograms by your height in metres squared — or by typing two values into the free BMI calculator, which shows your body mass index and weight category instantly, right in your browser.
This guide walks through the formula in metric and imperial with worked examples, the category table used by health authorities like the WHO and CDC, how sex and age change what the number means, and — just as important — what BMI cannot tell you about your health.
The BMI formula in metric and imperial, with examples
BMI (body mass index) is weight divided by height squared. In metric: BMI = kg ÷ m². Take someone who weighs 70 kg at 1.75 m tall: 1.75 × 1.75 = 3.06, and 70 ÷ 3.06 ≈ 22.9 — inside the healthy range. In imperial the formula adds a conversion factor: BMI = 703 × lb ÷ in². The same person is roughly 154 lb and 69 in tall: 703 × 154 ÷ 4,761 ≈ 22.7 (the small difference is rounding).
Two unit traps to avoid. If your height is in centimetres, divide by 100 before squaring — 175 cm becomes 1.75 m. If it is in feet and inches, convert to total inches first: 5 ft 9 in is 69 in. The unit converter switches between kg, lb, cm and inches in one tap, and the unit conversion cheat sheet keeps the common factors handy.
How to check your BMI in seconds
Working it out by hand once is instructive; doing it every week is a chore. The BMI calculator handles both unit systems and the category lookup for you:
- Open the BMI calculator and pick metric (kg, cm) or imperial (lb, ft/in).
- Enter your height and weight — the result updates as you type.
- Read your BMI to one decimal place and your category (underweight, healthy, overweight or obese) on the visual gauge.
- Adjust the values to see what weight would move you into a different category.
BMI categories — the healthy range table
Health authorities such as the WHO and CDC group adult BMI into the same standard bands, with 18.5 to 24.9 considered the healthy range for most adults:
- Underweight — BMI below 18.5.
- Healthy weight — 18.5 to 24.9.
- Overweight — 25 to 29.9.
- Obesity class 1 — 30 to 34.9.
- Obesity class 2 — 35 to 39.9.
- Obesity class 3 — 40 and above.
BMI for men vs women — and what changes with age
The formula is identical for everyone — there is no separate BMI calculator for women or for men, and the same 18.5–24.9 healthy range applies to both. What differs is what the number represents: at the same BMI, women typically carry more body fat and men more muscle, so two people with identical readings can have quite different body compositions.
Age shifts the picture too. Muscle mass naturally declines over the years, so an older adult can hold a "healthy" BMI while carrying more fat than the number suggests — and some research associates a slightly higher BMI with better outcomes after 65. For anyone under 20 the adult table does not apply at all: children and teens are assessed on BMI-for-age percentile charts, where the exact age in years and months matters. The age calculator gives you that figure precisely, and the age and date guide shows how. Some health bodies also apply lower cut-offs for people of Asian descent (overweight from 23, obesity from 27.5), because health risks appear at lower BMI values in those populations.
What BMI can't tell you
BMI is a screening number, not a diagnosis — and it is worth saying that plainly. The formula only knows two things about you: height and weight. It cannot tell muscle from fat, which is why muscular athletes routinely land in the overweight band while carrying very little fat. It says nothing about where fat is stored, even though abdominal fat carries more health risk than fat on the hips — one reason clinicians often pair BMI with a waist measurement. It also ignores bone density, ethnicity and fitness level.
What BMI does well is track trends. Measured the same way over months, a moving BMI is a meaningful signal — and if you are monitoring a change, the percentage calculator turns two weigh-ins into a clean "down 4.2%" figure. For decisions about your health, take the number to a doctor: this article is general information, not medical advice.
Your health data never leaves your device
Weight is personal data, and plenty of online calculators quietly send what you type to a server, wrapped in analytics. Carbide's BMI calculator works differently: the page loads once, then every calculation runs locally in your browser. Your height and weight are never uploaded, stored or shared — close the tab and they are gone.
There is no account to create, no email gate before the result and no usage cap — check once or track weekly, free either way. It works in any modern browser on desktop or phone, and the Carbide mobile apps — coming soon for Android and iOS — will run the same calculation fully offline. For the full routine, pair the calculator with the step-by-step BMI guide and measure the same way each time; consistency is what makes the trend meaningful.
Frequently asked questions
What is a normal BMI?
For adults aged 20 and over, the healthy range is 18.5 to 24.9 under the WHO and CDC bands. Below 18.5 counts as underweight, 25 to 29.9 as overweight and 30 or above as obesity. Some populations use lower cut-offs, and children are assessed on percentile charts instead of the adult table.
How do I calculate BMI with kg and cm?
Divide your height in cm by 100 to get metres, then divide your weight in kg by that height squared. Example: 70 kg at 175 cm → 70 ÷ (1.75 × 1.75) ≈ 22.9. Or enter kg and cm directly into the BMI calculator and read the result instantly.
Is my height and weight uploaded anywhere?
No. The calculator runs entirely in your browser — nothing you type is sent to or stored on a server, which matters when the numbers are health data. Close the page and the values are gone.
Is the BMI calculator free? Are there limits?
Yes — completely free, with no sign-up, no daily limit and no paywalled features. Use it as often as you like, on desktop or mobile.
Is BMI accurate for athletes and muscular people?
Often not. The formula counts muscle as extra weight, so a muscular athlete can be classed as overweight while carrying very little fat. Waist circumference or a body-fat measurement is more informative in that case — and remember BMI is a screening tool, not a medical diagnosis.
BMI is a quick, useful screening number: know your figure, where it sits against the 18.5–24.9 healthy range, and which way it is trending — while remembering it is not a medical diagnosis. Check yours in seconds with the free BMI calculator; your numbers stay on your device.