Body Mass Index is the quick, evidence-based number that tells you whether your weight is healthy for your height. This BMI calculator takes your height and weight in either metric or imperial units, runs the standard Quetelet index, and returns both your BMI number and the category that public-health bodies such as the CDC, NIH, and WHO use to group adults. Slide the inputs and the result updates instantly.
BMI was developed about two hundred years ago by the Belgian mathematician Adolphe Quetelet, and in 1972 the physiologist Ancel Keys gave it the name we use today. It became the standard screening tool because it is cheap, repeatable, and tracks population-level weight trends surprisingly well. Roughly 73% of US adults fall into the overweight or obese range, which is exactly why a simple, free calculator that anyone can run is still useful.
How to Use
- Type your Height into the first box. The default is 175, a typical adult value, but overwrite it with your own measurement.
- Pick the Height Unit — centimeters if you use the metric system, or inches if you use the imperial system.
- Type your Weight into the next box (default 70).
- Pick the Weight Unit — kilograms or pounds.
- Read the result card: your BMI number, your Weight Category, your BMI Prime, and the healthy weight range for your height.
For the most accurate result, measure height without shoes and weight at the same time of day you would normally weigh yourself. If you switch units, make sure both height and weight are consistent — the engine converts for you, but the numbers you type must match the unit you selected.
The BMI Formula
BMI is just weight divided by height squared:
BMI = weight(kg) / height(m)²
If you work in pounds and inches, the same result is reached with a conversion constant:
BMI = weight(lbs) / height(in)² × 703
The 703 constant converts inches-and-pounds into the same kilograms-and-meters scale, so a 150 lb, 68 in person lands near 22.8 — the same answer you get from 68 kg and 1.75 m. That is the beauty of the index: it collapses height and weight into a single number that works across the whole adult population.
NIH and WHO Weight Categories
Adults aged 20 and older are grouped into four standard bands:
- Underweight — BMI below 18.5.
- Normal (healthy) — BMI from 18.5 to 24.9.
- Overweight — BMI from 25.0 to 29.9.
- Obese — BMI of 30.0 or higher.
These cutoffs come from the World Health Organization and are used by the US National Institutes of Health and the CDC. They are screening thresholds, not diagnoses: a BMI of 24.9 is healthy, and 25.0 is overweight, even though the underlying difference is tiny. The calculator also reports BMI Prime, which is your BMI divided by 25 — under 1.0 means you are within the healthy upper limit, and over 1.0 means you are above it.
What BMI Does and Does Not Tell You
BMI is a useful population tool, but it is not a measure of body fat, fitness, or health on its own. Very muscular athletes can carry a high BMI while being very lean, because muscle is denser than fat. Likewise, older adults can lose muscle and appear "normal" while carrying more body fat than the number suggests. That is why clinicians also look at waist circumference, blood pressure, cholesterol, and family history. For pregnant people, children, and teens, BMI uses age- and sex-specific percentiles rather than the adult bands on this page.
Examples
Example 1 — Standard metric. A person who is 175 cm tall and weighs 70 kg: height in meters is 1.75, squared is 3.0625, and 70 ÷ 3.0625 = 22.9. Category: Normal. BMI Prime is 22.9 ÷ 25 = 0.92. The healthy weight range for this height is about 56.6 kg to 76.2 kg.
Example 2 — Imperial units. A person who is 68 in (5 ft 8 in) tall and weighs 150 lb: 150 ÷ (68 × 68) × 703 = 22.8. Category: Normal. Switch the height unit to inches and the weight unit to pounds to reproduce this directly.
Example 3 — Overweight range. A 175 cm person weighing 90 kg: 90 ÷ 3.0625 = 29.4, which lands them in the Overweight band, just below the 30 threshold for obesity. Losing roughly 8 kg would move them back into the Normal range.
Example 4 — Underweight. A 1.60 m person weighing 45 kg: 45 ÷ (1.60 × 1.60) = 17.6, category Underweight. The healthy floor for this height is about 47.4 kg (18.5 × 1.60²).
FAQ
How do I calculate my BMI manually?
Divide your weight in kilograms by your height in meters squared. For example, 70 kg ÷ (1.75 m × 1.75 m) = 22.9. If you use pounds and inches, multiply 703 by your weight in pounds and divide by your height in inches squared.
What is a good BMI for an adult?
For adults aged 20 and older, a BMI between 18.5 and 24.9 is considered normal or healthy. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 and above is obese, per WHO and NIH cutoffs.
Does BMI work for muscle-bound athletes?
Not perfectly. Because BMI cannot tell muscle from fat, a lean, heavily muscled athlete can read as overweight. If your body fat percentage is low and your waist is narrow, a high BMI is usually not a health concern — use waist circumference and body-fat measures alongside it.
Why is the cutoff between normal and overweight exactly 25?
The 25 and 30 thresholds come from large population studies linking those BMI levels to rising risk of type 2 diabetes, hypertension, and cardiovascular disease. They are epidemiological boundaries chosen for public-health screening, not precise medical diagnoses.
Should children and teens use this calculator?
No. Adults aged 20+ use the fixed bands above. For children and teens aged 2 to 19, BMI must be plotted on age- and sex-specific growth percentiles, because body composition changes as they grow. Use the CDC BMI-for-age calculator for younger people.
Is BMI accurate for older adults?
BMI is less precise in older adults because it does not capture muscle loss (sarcopenia). Some research suggests a slightly higher BMI range is associated with better outcomes in older age, so treat the adult bands as a starting point rather than a hard rule.
Disclaimer
This BMI calculator is provided for informational purposes only and does not constitute medical advice, diagnosis, or treatment. BMI is a screening tool and not a substitute for professional assessment by a qualified healthcare provider. The categories used follow standard NIH/WHO adult cutoffs and do not apply to children, pregnant people, or athletes in the same way. Always consult a doctor or registered dietitian about your individual health, weight, and fitness goals before making changes to diet or exercise.