Calculator
Complete Guide
BMI Calculator — Complete Guide to Body Mass Index
The BMI Calculator is the most widely used free tool for estimating whether a person's weight is in a healthy range for their height. BMI, or Body Mass Index, was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted as a global health standard by the World Health Organization (WHO). Our free BMI calculator delivers instant results using the internationally recognized formula, supports both metric and US units, and displays your result on an animated semicircle gauge.
What is BMI and How Does It Work?
Body Mass Index is a numerical value calculated from two measurements — your height and your weight. Despite its simplicity, it correlates well with body fat percentage in large population studies and is used as a first-line screening tool by healthcare providers worldwide. The formula is:
Metric Units (kg & cm):
BMI = weight(kg) ÷ height²(m²)US / Imperial Units (lbs & inches):
BMI = 703 × weight(lbs) ÷ height²(in²)Example: A person who weighs 73 kg and is 178 cm (1.78 m) tall has a BMI of: 73 ÷ (1.78 × 1.78) = 73 ÷ 3.1684 = 23.0 kg/m² — which falls in the Normal, healthy range.
BMI Classification Table — WHO Adults
| Classification | BMI Range (kg/m²) | Health Risk |
|---|---|---|
| Severe Thinness | < 16 | Very High |
| Moderate Thinness | 16 – 17 | High |
| Mild Thinness | 17 – 18.5 | Moderate |
| Normal Weight | 18.5 – 25 | Lowest ✓ |
| Overweight | 25 – 30 | Increased |
| Obese Class I | 30 – 35 | Moderate |
| Obese Class II | 35 – 40 | Severe |
| Obese Class III | > 40 | Very Severe |
BMI for Children and Teens (Ages 2–20)
For children and teenagers, the CDC uses BMI-for-age percentiles rather than fixed thresholds, because healthy body fat levels vary by age and sex during development.
| Category | Percentile Range |
|---|---|
| Underweight | Below 5th percentile |
| Healthy Weight | 5th – 85th percentile |
| At Risk of Overweight | 85th – 95th percentile |
| Overweight | Above 95th percentile |
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper limit of the normal range). A value below 1.0 means you are within or below the healthy weight range. A BMI of 23.0 gives a BMI Prime of 23 ÷ 25 = 0.92, meaning your BMI is 8% below the upper healthy limit. BMI Prime makes it very easy to tell at a glance how close you are to the healthy boundary.
What is the Ponderal Index?
The Ponderal Index (PI) is calculated as weight (kg) ÷ height³ (m³). It scales with height differently from BMI, making it more accurate for individuals at height extremes. The healthy Ponderal Index range for adults is typically 11–14 kg/m³. Our calculator shows this value automatically with every BMI result.
Limitations of BMI — What It Cannot Tell You
- Muscle mass: BMI cannot distinguish fat from muscle. Athletes and bodybuilders often have a high BMI despite very low body fat.
- Fat distribution: BMI doesn't reveal where body fat is located. Abdominal (visceral) fat carries greater cardiovascular risk than fat stored elsewhere.
- Age: Older adults may have a "normal" BMI while carrying excess body fat, as muscle mass naturally declines with age.
- Ethnicity: South Asian and East Asian populations face higher metabolic risks at lower BMI values. Many guidelines recommend an overweight threshold of 23 kg/m² for these groups.
- Sex: Women naturally carry more body fat than men at the same BMI value.
Medical Disclaimer: This calculator is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.
FAQ
Frequently Asked Questions — BMI Calculator
Calorie Calculator Guide
Calorie Calculator — How Many Calories Do You Need?
Our free Calorie Calculator estimates your Basal Metabolic Rate (BMR) — the number of calories your body burns at complete rest — and your Total Daily Energy Expenditure (TDEE) based on your activity level. This helps you understand exactly how much to eat to maintain, lose, or gain weight.
The Mifflin-St Jeor Equation (Most Accurate)
This calculator uses the Mifflin-St Jeor equation, considered the most accurate formula for estimating BMR in non-athletic adults by the American Council on Exercise:
For Men:
BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5For Women:
BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161Your TDEE is then calculated by multiplying your BMR by an activity factor ranging from 1.2 (sedentary) to 1.9 (very active/athlete).
How to Use Calorie Results for Weight Goals
- Maintain weight: Eat at your TDEE. Your weight will stay stable if calories in equals calories out.
- Lose weight: Create a calorie deficit of 500 kcal/day below TDEE to lose approximately 0.5 kg (1 lb) per week safely.
- Gain weight (muscle): Eat 250–500 kcal above TDEE combined with resistance training for lean muscle gain.
Activity Level Guide
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | ×1.2 | Desk job, no exercise |
| Lightly Active | ×1.375 | Light exercise 1–3 days/week |
| Moderately Active | ×1.55 | Moderate exercise 3–5 days/week |
| Very Active | ×1.725 | Hard exercise 6–7 days/week |
| Extra Active | ×1.9 | Physical job or twice-daily training |
Ideal Weight Guide
Ideal Weight Calculator — What Should You Weigh?
Our Ideal Weight Calculator computes your recommended body weight using four widely recognized medical formulas, plus the WHO BMI-based healthy weight range. There is no single "perfect" ideal weight — each formula provides a slightly different result, and the true healthy range for you depends on your body composition, muscle mass, and overall health.
The Four Ideal Weight Formulas
Devine Formula (Men):
50 kg + 2.3 kg per inch over 5 feetDevine Formula (Women):
45.5 kg + 2.3 kg per inch over 5 feetThe Robinson (1983), Miller (1983), and Hamwi (1964) formulas use similar structures but with slightly different base weights and per-inch increments. Our calculator shows all four results simultaneously for comparison.
WHO BMI-Based Healthy Weight Range
The most scientifically supported "ideal weight" concept is a range — the weight at which your BMI falls between 18.5 and 24.9 kg/m². For a person who is 178 cm (5'10") tall, this gives a healthy weight range of approximately 58.6 – 79.0 kg (129 – 174 lbs). This is broader than the single-number formula results and acknowledges natural variation in body composition.
Body Fat Guide
Body Fat Calculator — Estimate Your Body Fat Percentage
Our Body Fat Calculator uses the US Navy body fat estimation method, which was developed and validated by the US Navy for use without specialized equipment. It requires only simple circumference measurements (neck, waist, and hip for women) to estimate body fat percentage with reasonable accuracy.
The US Navy Body Fat Formula
Men:
% Fat = 495 ÷ (1.0324 − 0.19077×log10(waist − neck) + 0.15456×log10(height)) − 450Women:
% Fat = 495 ÷ (1.29579 − 0.35004×log10(waist + hip − neck) + 0.22100×log10(height)) − 450All measurements are in centimetres. To take accurate measurements, measure at the narrowest point (waist), the widest point (hips for women), and around the neck just below the Adam's apple.
Body Fat Percentage Categories
| Category | Men (%) | Women (%) |
|---|---|---|
| Essential Fat | 2–5% | 10–13% |
| Athletic | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Obese | ≥ 25% | ≥ 32% |
Unit Converter Guide
Unit Converter — Length, Weight & Temperature
Our free Unit Converter supports instant conversion between all major units of length, weight/mass, and temperature. Whether you need to convert centimetres to inches, kilograms to pounds, or Celsius to Fahrenheit, every result updates in real time as you type — no button click needed.
Common Unit Conversions
| From | To | Multiply by |
|---|---|---|
| Centimetres (cm) | Inches (in) | ÷ 2.54 |
| Kilograms (kg) | Pounds (lbs) | × 2.20462 |
| Kilometres (km) | Miles (mi) | × 0.621371 |
| Litres (L) | US Gallons | × 0.264172 |
| Celsius (°C) | Fahrenheit (°F) | × 9/5 + 32 |
| Fahrenheit (°F) | Celsius (°C) | (−32) × 5/9 |