Your BMI is more than a number — it's a health signal. Get a reading based on your age, gender, and height. See where you fall on the healthy weight range.
BMI = weight (kg) / height (m)²BMI is calculated by dividing body weight in kilograms by the square of height in meters. The result is a dimensionless number that categorizes weight status. For US units, the formula is: BMI = 703 × weight (lbs) / height (inches)². The calculation is arithmetically exact, but what it measures is narrower than its name suggests. BMI is a proxy for body mass relative to height — it makes no distinction between fat and lean tissue, water and bone. That limitation drives every caveat below, and it is why no major medical body now uses BMI alone as a screening verdict. Its accuracy also depends entirely on the inputs. Height is measured once and treated as fixed; a two-centimetre error in height shifts BMI by roughly 1 point, which is enough to move someone across a category boundary. Weight fluctuates daily by one to two kilograms from water, food, and sodium intake, so a reading taken in the evening after a meal is not comparable to one taken at dawn. The categories themselves come from the WHO: under 18.5,18.5 to 24.9, 25 to 29.9, and 30 or above. They were derived from population distributions, which is the second limitation — a threshold that describes a population does not describe an individual. For athletes and people with substantial muscle mass, BMI overstates adiposity and a muscular person at 27 may be lean. For older adults the same score carries a different risk profile than for a young adult, yet the cut-offs are identical. The error is directional rather than random: because muscle weighs more than fat, BMI tends to over-estimate body fat in people who train and under-estimate it in some older adults with low muscle mass. Use BMI as a screening signal that justifies a closer look, not as a diagnosis. If your BMI is elevated, the next question is whether it is fat, muscle, or water — and only a body composition assessment answers that. If you are pregnant, BMI is not a valid screening tool at all; gestational weight gain targets follow a separate guideline. Talk to a clinician before acting on any BMI-based advice, particularly if you have a chronic condition, are under 18, are over 65, or are pregnant.
| Symbol | Meaning |
|---|---|
| BMI | Body Mass Index (kg/m²) |
| weight | Body weight in kilograms |
| height | Height in meters |
Our calculations are based on established financial and scientific formulas. The following sources inform the default values and methodology used in this calculator:
BMI (Body Mass Index) is a screening tool that uses height and weight to estimate body fatness. It is calculated as weight (kg) divided by height (m) squared. While BMI does not directly measure body fat, it correlates moderately with body fat percentage and is widely used because it requires only height and weight.
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, osteoporosis risk |
| 18.5 - 24.9 | Normal weight | Lowest risk of chronic disease |
| 25.0 - 29.9 | Overweight | Increased risk of heart disease, diabetes |
| 30.0 - 34.9 | Obese (Class I) | High risk of cardiovascular disease |
| 35.0 - 39.9 | Obese (Class II) | Very high risk, sleep apnea likely |
| 40.0 and above | Obese (Class III) | Extremely high risk, medical intervention |
BMI = weight (kg) ÷ height (m)² Example: 70 kg, 1.75 m → 70 ÷ (1.75 × 1.75) = 22.9 (Normal)
BMI has important limitations: it does not distinguish between muscle and fat, so athletes with high muscle mass may be classified as overweight. It also does not account for fat distribution, age, or sex differences. For a more complete assessment, combine BMI with waist circumference and body fat percentage.
BMI is a quick screening tool, but body fat percentage directly measures the proportion of fat to total body weight. For muscular individuals, body fat percentage is significantly more accurate than BMI.
| Aspect | BMI | Body Fat Percentage |
|---|---|---|
| What it measures | Weight relative to height | Actual fat proportion of body |
| Accuracy for athletes | Poor (overestimates fat) | High (measures actual fat) |
| Accuracy for elderly | Poor (underestimates fat) | High (detects fat gain) |
| Method | Simple calculation | Requires measurement (calipers, BIA, DEXA) |
| Cost | Free | Varies ($10 for calipers to $100+ for DEXA) |
| Healthy ranges | 18.5-24.9 (universal) | Men: 10-20%, Women: 18-28% |
| Limitation | Cannot distinguish muscle from fat | Methods vary in accuracy |
Use BMI as a quick initial screening. If your BMI suggests overweight but you are muscular, or if your BMI is normal but you are sedentary, measure body fat percentage for a more accurate assessment.
The BMI formula (weight in kg / height in m²) is the same regardless of age or gender, but how you interpret the number should account for both. Muscle mass, bone density, hormonal changes, and fat distribution all shift over a lifetime and differ between men and women. A BMI of 23 may mean something very different for a 22-year-old athlete versus a 65-year-old retiree. Understanding these nuances helps you use BMI as a more meaningful health screening tool.
| Age Group | Healthy BMI Range | Notes |
|---|---|---|
| 19-24 | 19-24 | Lowest disease risk; metabolic rate at peak |
| 25-34 | 18.5-25 | Slight upward drift normal; muscle mass peaks |
| 35-44 | 18.5-26 | Metabolism slows; waist circumference matters more |
| 45-54 | 18.5-27 | Hormonal changes affect fat distribution (menopause) |
| 55-64 | 19-28 | Slightly higher BMI may be protective for seniors |
| 65+ | 20-29 | "Obesity paradox" - slightly higher BMI correlates with better outcomes |
| Gender | Average BMI | Body Fat at Same BMI | Health Risk Threshold |
|---|---|---|---|
| Men | ~28.5 (US avg) | Lower (more muscle) | BMI ≥ 30 |
| Women | ~28.5 (US avg) | Higher (more essential fat) | BMI ≥ 30 |
Sources: CDC — "About Adult BMI"; WHO — "Obesity: preventing and managing the global epidemic".
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