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BMI is a screening number, not a diagnosis. It does not measure body fat, muscle, or health on its own — see the guide below for who it can mislead.
What Is BMI?
BMI stands for Body Mass Index. It is a single number that relates your weight to your height, and it has been the world’s most widely used weight screening tool for decades — not because it is perfect, but because it is simple, fast, and surprisingly useful as a first check.
Doctors, insurers, and public health researchers use it to sort adults into broad weight categories. If your BMI lands in the healthy range, your risk of weight-related conditions like type 2 diabetes and heart disease is statistically lower. If it lands well outside that range, it is a prompt to look closer — not a diagnosis, but a reason to pay attention.
Our calculator above gives you that number in seconds. Enter your weight and height, in pounds and feet or kilograms and centimeters, and it works out your BMI, its classification, and the weight range that would put you in the healthy band for your height. Nothing is stored or sent anywhere — the math runs entirely in your browser.
The Formula, in Both Unit Systems
The metric formula is the cleanest version: divide your weight in kilograms by your height in meters, squared.
BMI = weight (kg) ÷ height (m)²
For the imperial system, the same idea uses a conversion constant of 703:
BMI = 703 × weight (lb) ÷ height (in)²
A worked example makes it concrete. Take a person who weighs 176 pounds and stands 5 feet 11 inches tall. That height is 71 inches, and 71 squared is 5,041. Multiply 703 by 176 to get 123,728, divide by 5,041, and you get about 24.5. The same person in metric — 79.8 kilograms and 1.80 meters — gives 79.8 ÷ 3.24, which is about 24.6. The tiny difference is just rounding between the two unit systems.
The calculator shows you exactly which version of the formula it used for your result, so you can check the arithmetic yourself.
The Classification Table
The World Health Organization’s adult cutoffs are the standard almost everyone uses:
| BMI | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These bands apply to most adults aged 20 and over. They are the same for men and women, which is why the calculator’s gender question never changes your number — it only shapes the note shown with your result.
Alongside your classification, the calculator shows your healthy weight range: the lowest and highest weights that would keep your BMI between 18.5 and 24.9 at your height. For someone 5 feet 11 inches tall, that range is roughly 132 to 178 pounds. A range is more useful than a single “ideal weight” because healthy bodies genuinely vary.
Where BMI Misleads
BMI only sees weight and height. It cannot tell muscle from fat, and that blind spot matters for several groups:
Very muscular adults. A trained athlete can carry a lot of muscle, weigh a lot for their height, and land in the “overweight” or even “obese” band while carrying very little body fat. For them, BMI overstates the risk.
Pregnancy. Weight gain during pregnancy is expected and healthy, so BMI categories simply do not apply in the usual way.
Children and teenagers. Growing bodies are judged against age-and-sex percentile charts, not adult cutoffs. If you entered an age under 20, the calculator flags this in your result.
Older adults. Muscle loss with age can make BMI look healthier than it is — a “normal” BMI can hide low muscle and higher body fat.
Very short or very tall people. The formula’s squaring of height slightly favors taller people and penalizes shorter ones at the extremes.
None of this makes BMI useless. It makes it a starting point. If your BMI and your waist measurement, your fitness, and how you feel all point the same way, the picture is clear. If they disagree, trust the fuller picture and talk to a doctor.
What to Do With Your Number
If your BMI is in the healthy range, the goal is maintenance: keep eating roughly what you eat now, stay active, and recheck every few months. Small drifts are normal; large ones are worth noticing early.
If it is above the healthy range, you do not need a dramatic overhaul. A gradual loss of 5 to 10 percent of body weight — for a 200-pound person, that is 10 to 20 pounds — measurably improves blood pressure, blood sugar, and joint comfort. Our TDEE calculator can turn that goal into a daily calorie target.
If it is below the healthy range and you have been losing weight without trying, or you feel tired or unwell, that deserves a conversation with a doctor rather than a do-it-yourself eating plan.
Whatever your number, recheck it every couple of months, or after a noticeable change in weight or routine. Weight is a moving target, and the value of BMI is in the trend, not any single reading.
Frequently Asked Questions
1. What is a good BMI?
For most adults, 18.5 to 24.9 is considered the healthy range. But “good” also depends on muscle mass, age, and overall health — a muscular athlete at 26 and a sedentary adult at 26 are not in the same situation, even though the number is identical.
2. Is BMI accurate?
It is accurate as a population-level screening tool and a reasonable first check for individuals, but it is not a body fat measurement. It can mislead muscular people, pregnant women, children, and older adults. Pair it with waist circumference and general fitness for a fuller picture.
3. How is BMI different from body fat percentage?
BMI uses only weight and height. Body fat percentage measures how much of your weight is fat versus lean tissue, which needs calipers, a smart scale, a DEXA scan, or a similar method. Two people can share a BMI of 27 and have very different body fat percentages.
4. Why does the calculator show a healthy weight range instead of one ideal weight?
Because there is no single ideal weight for a height. The range from BMI 18.5 to 24.9 spans real, healthy variation in build, bone structure, and muscle. Aiming for anywhere inside the range is a sensible goal.
5. Should children use this calculator?
Not with these cutoffs. Children and teens are assessed with BMI-for-age percentile charts that account for growth. The calculator warns you if you enter an age under 20, and a pediatrician can plot the proper chart.
6. How often should I check my BMI?
Every couple of months is plenty, or after a noticeable weight change. Daily checks add noise — weight fluctuates with water, meals, and clothing — while the months-long trend is what actually matters.
Try these calculators too: TDEE Calculator, Maintenance Calories, Walking Calorie, Body Fat Calculator.
Disclaimer: This calculator is for general information only and is not medical advice. BMI is a screening tool, not a diagnosis. Talk to a doctor or registered dietitian before starting a new diet or exercise program, especially if you have a health condition, are pregnant, or are under 20.