Free BMI Calculator
Calculate your Body Mass Index and healthy weight range.
<18.5Normal
18.5–25Overweight
25–30Obese
30+
...Athlete
...Fitness
...Acceptable
...Obese
...
What is a BMI calculator?
A BMI (Body Mass Index) calculator is a free tool that computes your BMI from your height and weight using the WHO formula: BMI = weight(kg) / height(m)². BMI is then mapped to a category — Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), or Obese (≥30). It is the most widely used screening tool for identifying weight-related health risks in clinical settings.
BMI is one of those numbers that's useful to know but annoying to calculate — especially if you're working in pounds and inches and don't remember the 703 conversion factor. Plug in your height and weight, and you get your BMI, your weight category, and a realistic weight range for your height. Takes about two seconds.
Most BMI calculators stop at the basic number and category. This one goes further: it estimates your BMR (how many calories you burn at rest), gives you a body fat estimate using the US Navy method, shows you what weight you'd need to hit for a normal BMI, and even calculates child percentiles if you're checking for someone under 20. All the extras are there if you want them, but if you just want your BMI number, that's front and center.
How BMI is calculated
The formula is straightforward: weight in kilograms divided by height in meters squared (kg/m²). If you're using pounds and inches, multiply your weight by 703 and then divide by your height squared. A result of 18.5 to 24.9 is the healthy range for most adults. The calculator shows you where you land on a color-coded scale — blue for underweight, green for normal, yellow for overweight, red for obese — so you can see at a glance whether you're in the green zone or somewhere else.
Our calculator does more than just compute BMI. It also estimates your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation, which tells you how many calories your body burns at rest. For children and teenagers under 20, it provides age- and gender-adjusted BMI percentiles based on CDC growth chart data, which is more accurate than the adult classification for growing bodies. The body fat estimate feature uses the US Navy Method, taking waist, neck, and hip measurements to give you a more complete picture of your body composition beyond what BMI alone can provide.
When to use a BMI calculator
Most people encounter BMI calculators at their doctor's office, but checking it yourself at home gives you a heads-up before that annual physical. If you're starting a new workout program, tracking weight loss progress, or just curious where you stand, a quick BMI check puts a number on it. I find it useful as a baseline metric — not because BMI tells the whole story, but because it's the one number your doctor will ask about and insurance forms still require.
Parents use it to check whether their teenager's BMI percentile is trending in a healthy direction. Fitness coaches use it alongside body fat measurements to give clients a fuller picture. Insurance companies use BMI as one factor in underwriting health policies. Anyone applying for a life insurance policy, enrolling in a workplace wellness program, or setting a weight-loss goal will benefit from knowing their BMI before those conversations happen.
Limitations of BMI
BMI is a population-level screening tool, not a diagnostic measure. It does not directly measure body fat percentage and can be misleading for certain groups. Athletes and bodybuilders often have a high BMI due to muscle mass rather than excess fat, which can incorrectly classify them as overweight or obese. Older adults tend to lose muscle mass as they age, so they may have a normal BMI while carrying a higher percentage of body fat. People of Asian descent may face increased health risks at lower BMI thresholds — some health organizations recommend a lower cutoff of 23 for overweight in Asian populations. Pregnant women and growing children also require specialized interpretation that the standard adult BMI scale does not provide.
This is why our calculator includes additional metrics like body fat estimation and BMR. The body fat percentage gives you a more direct measure of adiposity, while the weight-to-reach-normal-BMI feature shows you exactly how many pounds or kilograms you would need to gain or lose to fall into the healthy BMI range. Use BMI as one data point alongside waist circumference, blood pressure, cholesterol levels, and your doctor's guidance — not as the sole measure of your health. Regular checkups and professional medical advice remain essential for a complete health assessment.
BMI categories for adults
The World Health Organization defines four BMI categories: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), and Obese (30.0 and above). Obesity is further divided into Class I (30–34.9), Class II (35–39.9), and Class III (40 and above). This calculator uses the standard WHO thresholds. For most adults, a BMI in the normal range is associated with the lowest risk of cardiovascular disease, type 2 diabetes, and certain cancers. Being underweight carries its own health risks including weakened immune function and decreased bone density. If your BMI falls outside the normal range, consider consulting a healthcare provider for personalized advice tailored to your specific situation, family history, and lifestyle factors.
How to use this calculator
- Choose your unit system — Select Metric (kilograms and centimeters) or Imperial (pounds and feet/inches) using the toggle at the top of the calculator.
- Enter your height and weight — Type your height and weight in the fields provided. The result updates instantly as you type.
- Read your BMI and category — Your BMI number and weight category (underweight, normal, overweight, or obese) appear immediately, along with your healthy weight range.
Frequently asked questions
What is a healthy BMI range for adults?
The WHO defines healthy BMI as 18.5–24.9. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. These ranges apply to adults aged 20 and older regardless of gender. For children and teens, BMI is age- and gender-specific — use a pediatric BMI chart rather than adult cutoffs.
How do I calculate BMI manually without a calculator?
BMI = weight (kg) ÷ height² (m²). For imperial units: BMI = (weight in pounds × 703) ÷ height in inches². For a 150-pound person who is 5'6" (66 inches): (150 × 703) ÷ 66² = 105,450 ÷ 4,356 = 24.2 BMI. The formula is the same worldwide — only the unit conversion differs.
What BMI is considered overweight or obese?
A BMI of 25.0–29.9 is classified as overweight. A BMI of 30.0 or higher is classified as obese, with sub-categories: Class 1 obesity (30–34.9), Class 2 obesity (35–39.9), and Class 3 severe obesity (40+). These WHO thresholds have been standard since 1998 and are used by most healthcare providers globally.
Is BMI an accurate measure of body fat and health?
BMI is a useful screening tool but has significant limitations. It cannot distinguish between muscle and fat — a muscular athlete may have an 'overweight' BMI while carrying very little body fat. It also doesn't account for fat distribution (belly fat is more dangerous than hip fat). BMI is best used alongside waist circumference, body fat percentage, and clinical assessment.
Does BMI calculation differ for men and women?
The standard BMI formula is the same for men and women, but the healthy body fat percentage differs by sex. Women naturally carry more body fat than men at the same BMI — typically 10–15% more. A BMI of 22 in a woman may represent 25% body fat, while the same BMI in a man may represent only 15% body fat.
What is BMI for children and teenagers?
For anyone under 20, BMI is plotted on age- and sex-specific growth charts rather than using adult thresholds. The same formula calculates BMI, but the result is expressed as a percentile. Below the 5th percentile is underweight, 5th–84th is healthy weight, 85th–94th is overweight, and 95th percentile or above is obese for children and teens.