BMI Calculator
Calculate your Body Mass Index and see your healthy weight range.
Your Measurements
How to Use the BMI Calculator
This calculator gives you an instant BMI result the moment you finish entering your measurements — no button press needed. Here is how to get the most out of it:
- Select your preferred unit system — metric (centimetres and kilograms) or imperial (feet, inches, and pounds) — using the toggle at the top of the form.
- Enter your height. For imperial, fill in feet and inches in the two separate fields; for metric, enter your total height in centimetres.
- Enter your current weight in the weight field.
- Your BMI score, category badge, and the healthy weight range for your exact height appear instantly in the results card below the form.
- The colour-coded scale bar shows where your BMI falls relative to all four categories so you can see at a glance how far you are from each boundary.
- To compare different scenarios — say, a target weight — simply update the weight field. Results recalculate in real time.
No data is sent to any server. All calculations happen locally in your browser, so your measurements stay private.
How BMI Is Calculated
Body Mass Index was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a simple way to compare weight across populations of different heights. Despite being nearly two centuries old, it remains the most widely used screening tool for weight classification because it requires only two easily measured values.
The core formula divides weight by the square of height, which corrects for the fact that taller people naturally weigh more even at the same relative body composition:
Metric formula
BMI = weight (kg) ÷ height² (m²)
Imperial formula
BMI = 703 × weight (lb) ÷ height² (in²)
The constant 703 is a unit-conversion factor that makes the imperial result equal to the metric result.
This calculator converts imperial inputs to metric internally before computing, so both paths produce identical results for the same physical measurements. The healthy weight range shown in results is derived by solving the formula in reverse for the BMI boundaries of 18.5 and 24.9, using your entered height.
The World Health Organization and the CDC define four standard adult categories based on the resulting number:
| BMI Range | Category | Health Considerations |
|---|---|---|
| Below 18.5 | Underweight | May indicate nutritional deficiency; worth discussing with a doctor. |
| 18.5 – 24.9 | Normal weight | Associated with the lowest risk for most weight-related conditions. |
| 25.0 – 29.9 | Overweight | Modestly elevated risk for some conditions; lifestyle changes are often beneficial. |
| 30.0 and above | Obese | Higher risk for cardiovascular disease, type 2 diabetes, and other conditions. |
A Worked Example
Suppose someone is 5'10" (178 cm) and currently weighs 176 lb (80 kg). Here is how the calculation works step by step using both formulas.
Using the metric formula:
Height: 178 cm = 1.78 m
Height²: 1.78 × 1.78 = 3.1684 m²
BMI = 80 kg ÷ 3.1684 = 25.3
Using the imperial formula:
Height: 5'10" = 70 inches
Height²: 70 × 70 = 4,900 in²
BMI = 703 × 176 ÷ 4,900 = 123,728 ÷ 4,900 = 25.3
Both formulas agree: a BMI of 25.3, which falls just inside the Overweight range (25.0–29.9). The person is only 0.3 BMI points above the upper edge of the Normal category.
Healthy weight range for 5'10" (178 cm):
Min (BMI 18.5): 18.5 × 3.1684 = 58.6 kg ≈ 129 lb
Max (BMI 24.9): 24.9 × 3.1684 = 78.9 kg ≈ 174 lb
To reach the top of the Normal range (174 lb / 78.9 kg), the person would need to lose about 2 lb. To reach the midpoint of their healthy range (roughly 151 lb / 68.5 kg) would require losing around 25 lb. This kind of reference point is exactly what the calculator surfaces automatically — you can see your own healthy range in the green box that appears when you enter your measurements.
Common Misunderstandings About BMI
BMI is a useful first-pass screening number, but it is frequently misread. Understanding where it falls short helps you put your result in the right context.
| Misunderstanding | Reality |
|---|---|
| A high BMI always means excess fat | BMI cannot distinguish muscle from fat. A 200 lb athlete with very low body fat may have the same BMI as a sedentary person carrying excess fat. Body composition measurements (DEXA scan, hydrostatic weighing) give a clearer picture. |
| BMI thresholds are universal | Research shows that people of Asian descent can have elevated metabolic risk at BMIs below 25. Several health organisations recommend lower thresholds (e.g., 23 for overweight) for certain ethnic groups. |
| BMI works the same for children | Children and teenagers are assessed using age- and sex-specific growth chart percentiles, not fixed adult cut-offs. A BMI of 22 could be healthy or above average depending on the child's age and sex. |
| Older adults need higher BMIs | Adults over 65 naturally lose muscle mass (sarcopenia). A BMI in the range of 23–27 is often considered acceptable for older adults, as very low BMI in this group is associated with increased frailty and mortality risk. |
| BMI diagnoses obesity | BMI screens populations and flags individuals for further evaluation. A clinical diagnosis of obesity requires additional assessments, including waist circumference, blood markers, and in some cases body fat percentage testing. |
Frequently Asked Questions
Is BMI an accurate measure of health?
BMI is a useful population-level screening tool, but it has real limitations when applied to individuals. It does not distinguish between muscle mass and fat mass, so a very muscular person may register as overweight while carrying very little fat. It also does not account for age, sex, bone density, or how fat is distributed around the body — all of which matter for metabolic health. Use BMI as a starting point, not a final verdict.
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = weight (kg) ÷ height² (m²). In imperial units the equivalent formula is: BMI = 703 × weight (lb) ÷ height² (in²). The factor 703 is a unit-conversion constant that makes the two formulas produce the same number for the same person.
What BMI range is considered healthy for adults?
For most adults, a BMI between 18.5 and 24.9 is classified as Normal weight by the WHO and CDC. However, these thresholds are population averages. Older adults, people from certain ethnic backgrounds, and highly muscular individuals may have different optimal ranges. Always combine BMI with other health indicators and professional guidance.
Does BMI apply to children?
No — not in the same way. For children and teens (ages 2–19), BMI is plotted on age- and sex-specific growth charts and interpreted as a percentile rather than a fixed category. A BMI that falls at the 85th percentile for age and sex is considered overweight for that child, regardless of the absolute number. This calculator is designed for adults only. For children, consult a paediatrician or use CDC growth chart tools.
What's the difference between BMI and body fat percentage?
BMI is an indirect estimate calculated from height and weight alone. Body fat percentage is a direct measurement of how much of your total mass is fat tissue. Two people can have identical BMIs but very different body fat percentages. A 35-year-old male competitive cyclist at 25.5 BMI might carry 12% body fat; a sedentary person of the same height and weight might carry 28% fat. Methods for measuring body fat include DEXA scanning, hydrostatic (underwater) weighing, air displacement plethysmography, and bioelectrical impedance analysis. BMI is free and instant; body fat percentage testing is more expensive and less accessible, but far more informative about body composition.
Does BMI differ for men and women?
The standard BMI categories (18.5, 25, 30) are the same for adult men and women, but the biological reality differs. Women naturally carry more essential body fat than men — roughly 10–13% of body mass compared to 2–5% for men — due to hormonal and reproductive factors. This means a woman and a man with the same BMI may have meaningfully different actual body fat percentages. Some researchers advocate for sex-specific BMI adjustments, though no universal alternative thresholds have been officially adopted.
Why did my BMI category change with a small weight change?
BMI categories have hard boundaries at 18.5, 25.0, and 30.0. If your BMI sits near one of those lines — say 24.8 or 25.2 — a change of just a pound or two is enough to cross into the next category. This is not a meaningful health transition; it is an artifact of the discrete classification system. Research consistently shows that health risk changes gradually across the BMI range, not in sudden steps at the category boundaries. Focus on the trend over weeks and months rather than the exact category on any given day.
Disclaimer: BMI results are for informational and educational purposes only and do not constitute medical advice. Consult a qualified healthcare provider before making any changes to your diet or exercise habits.