BMI Calculator (Body Mass Index)
Last Updated:
Calculate your BMI in metric (kg/cm) or imperial (lbs/ft) units. Includes BMI category, healthy weight range, and limitations of BMI as a health metric.
⚖️ BMI Calculator (Body Mass Index)
Results update instantly
How to Use the BMI Calculator
Enter your height
Input height in feet and inches or centimetres. Height is the primary driver of BMI — a 5cm difference changes BMI by approximately 1–2 points at the same weight.
Enter your weight
Input current body weight in kg or pounds. BMI is calculated from current weight, not goal weight.
Read your BMI score and category
The result shows your BMI number and which WHO category it falls in: underweight (<18.5), normal weight (18.5–24.9), overweight (25–29.9), or obese (30+).
Note the limitations
BMI does not distinguish between muscle and fat. An athletic person and a sedentary person of the same height and weight receive the same BMI. Use alongside waist circumference and body fat percentage for a complete picture.
What Is BMI and How Is It Calculated?
Body Mass Index (BMI) is a simple numerical measure of body weight relative to height. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the WHO as a population-level screening tool. BMI does not directly measure body fat — it is a proxy that works reasonably well at a population level but has well-documented limitations at the individual level.
📐 BMI Formula
Limitations of BMI
BMI overestimates adiposity in muscular individuals (athletes often register as "overweight" despite low body fat) and underestimates it in older adults who have lost muscle mass. It also does not account for fat distribution — abdominal (visceral) fat carries higher health risks than peripheral fat. Ethnicity also affects optimal BMI ranges; Asian populations tend to have higher cardiometabolic risk at lower BMI values.
Understanding Your BMI Score
Body Mass Index (BMI) is a numerical value calculated from your height and weight. It provides a simple screening tool to categorise body weight relative to height. While it doesn't directly measure body fat, it correlates well with more precise measures of body fat and is widely used in clinical and public health settings.
Imperial formula: BMI = (weight in lbs × 703) ÷ height in inches²
Metric formula: BMI = weight in kg ÷ height in metres²
BMI Categories — What Your Number Means
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, osteoporosis risk |
| 18.5 – 24.9 | Normal weight | Lowest risk for most adults |
| 25.0 – 29.9 | Overweight | Moderately increased risk |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| 40.0 and above | Obese Class III | Extremely high risk |
How to Calculate BMI Manually
Convert to metric (if using imperial)
Convert your weight to kilograms (lbs ÷ 2.205) and height to metres (inches × 0.0254).
Square your height in metres
Example: 5'9" = 1.753m. Squared: 1.753 × 1.753 = 3.073m².
Divide weight by height squared
Example: 165 lbs = 74.8 kg. BMI = 74.8 ÷ 3.073 = 24.3 (Normal weight).
Limitations of BMI — What it Doesn't Measure
BMI is a useful screening tool but has known limitations. It does not distinguish between muscle mass and fat mass — a heavily muscled athlete may have a "overweight" BMI despite having very low body fat. It also doesn't account for age-related muscle loss in older adults, or differences in fat distribution (abdominal fat is more harmful than fat in other areas).
For a more complete picture, consider pairing BMI with our Body Fat Calculator (US Navy method) and BMR Calculator.
BMI for Different Populations
The standard BMI thresholds were developed primarily from studies of European populations. For people of Asian descent, health risks begin at lower BMI values — some guidelines suggest an overweight threshold of 23.0 and obese at 27.5. For older adults (65+), a slightly higher BMI (23–29.9) may be associated with better outcomes than the standard "normal" range.
Healthy Weight Range by Height
The healthy BMI range of 18.5–24.9 corresponds to different weight ranges for different heights. For a 5'6" person, that's approximately 115–154 lbs (52–70 kg). For 5'10", it's 132–174 lbs (60–79 kg). Use the calculator above to find your exact healthy range.
How to Calculate BMI by Hand: Worked Example
BMI = weight (kg) ÷ height² (m²). Take a person weighing 70 kg at 1.75 m tall.
Step 1 — square the height. 1.75² = 3.0625 m².
Step 2 — divide weight by that figure. 70 ÷ 3.0625 = 22.86.
A BMI of 22.86 falls in the "normal weight" range (18.5–24.9) used by most public health guidelines. In imperial units, the equivalent calculation uses a different constant: BMI = 703 × weight (lb) ÷ height² (in). For 154 lb at 69 inches: 703 × 154 ÷ 69² = 703 × 154 ÷ 4,761 ≈ 22.74 — the small difference from the metric result (22.86 vs 22.74) comes from each measurement being rounded to whole units before conversion, not a flaw in either formula.
Why Does BMI Get Criticized as a Health Measure?
What does BMI fail to distinguish between?
BMI cannot tell the difference between muscle and fat, because it only uses total weight and height. A muscular athlete and a sedentary person of the same height and total weight will show an identical BMI despite very different body compositions and health profiles — which is why BMI is best treated as a fast population-level screening tool rather than an individual diagnostic measure.
Does BMI mean the same thing at every age and for every body type?
No. Standard BMI categories were developed primarily from adult population data and apply less precisely to children and teens (who use age- and sex-specific percentile charts instead), to adults over 65 (where a slightly higher BMI is sometimes associated with better outcomes), and to some ethnic groups whose health risks appear at different BMI thresholds than the standard cutoffs assume. A body composition measure like body fat percentage typically gives a more individually meaningful picture than BMI alone.
How much does a small height measurement error change the BMI result?
Because height is squared in the formula, small measurement errors compound. Recording 1.75 m as 1.73 m — a difference of just 2 cm — shifts the calculated BMI from 22.86 to about 23.39, enough to matter near a category boundary. Measuring height accurately (without shoes, standing fully upright) matters more for BMI precision than most people assume.
Frequently Asked Questions
Sources & Methodology
Calculations are based on the most current publicly available data from authoritative government and industry sources: