BMI Calculator

What is body mass index (BMI)?

Body mass index is weight in kilograms divided by height in meters squared — a fast screening number that places adults into WHO weight categories. It flags weight-related risk; it does not measure body fat or diagnose anything.

BMI categories at a glance

The standard BMI chart for adults 20 and older:

The chart runs in reverse, too: multiply 18.5 and 24.9 by your height in meters squared and you have the healthy weight range in kilograms. At 175 cm that’s roughly 57–76 kg (125–168 lbs) — the entire job of a healthy weight calculator, done with one line of arithmetic.

How this calculator works

• Reads weight and height in imperial or metric, converting between the two automatically.
• Applies the standard BMI formula — kilograms divided by meters squared — and rounds to one decimal place.
• Matches the result against WHO cutoffs to assign a category, from underweight through obese class III.
• Plots your number on a band gauge so you can see how close it sits to the cutoffs on either side.
• Keeps recent results on your device only, so you can compare readings over time without creating an account.

How to use the result

1. Read the category, not just the score: 27.4 and 29.8 are both “overweight,” but the second sits right at the obesity cutoff.
2. Track the trend, not single readings. Recheck every 2–4 weeks; water, salt, and time of day move the scale without changing anything real.
3. If you want the number to move, anchor on your daily calorie needs first — a 300–500 kcal daily deficit works out to roughly 0.5–1 lb (0.2–0.5 kg) of loss per week.
4. Then decide how to split those calories: a higher-protein macro split protects muscle and appetite while you’re in a deficit.
5. If the result lands below 18.5 or in obese class II and above, treat it as a prompt to talk with a clinician rather than a DIY project.

How accurate is a BMI calculator?

As a screen for large groups, quite accurate; as a verdict on one person, limited. The formula can’t tell muscle from fat, so it routinely labels strength athletes “overweight,” and it can miss excess fat in older adults who have lost muscle. It isn’t valid during pregnancy or for anyone under 20, and several populations — including many of Asian descent — face elevated metabolic risk below the standard 25 cutoff. Clinicians therefore read BMI alongside waist circumference, bloodwork, and history rather than alone.

Common mistakes

• Treating BMI as a body-fat measurement. It’s a weight-for-height screen — it never measures composition directly.
• Applying adult cutoffs to kids. Under-20s are assessed with age- and sex-specific percentiles, not these bands.
• Overreacting to one reading. Day-to-day weight swings are mostly water; judge monthly trends.
• Worshipping the cutoff. The gap between 24.9 and 25.1 is statistical convenience, not biology.
• Assuming “normal” means healthy. Diet, cardio fitness, and metabolic markers matter independently of BMI.

If the goal is to move the number, start from your resting calorie burn and set the deficit above that floor — it’s the most defensible way to decide how low is too low.