BMI Calculator
Calculate your Body Mass Index and understand what it means.
What Is the BMI Calculator?
A BMI calculator estimates body composition risk from just two inputs: weight and height. Body mass index divides your weight by the square of your height and maps the result onto standard ranges that clinicians use for population screening: underweight below 18.5, healthy between 18.5 and 24.9, overweight from 25 to 29.9, and obese at 30 or above. The measure is popular because it is fast, free, and surprisingly predictive at the population level. It also has well known blind spots, most famously mistaking muscular athletes for overweight people and ignoring where fat sits on the body. Treat the number as a screening conversation starter rather than a diagnosis, then bring waist measurement and other markers into the picture.
Key Statistics
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41.9%
US adult obesity rate
Source: CDC NHANES
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73.6%
US adults classified as overweight or having obesity combined
Source: CDC NHANES
-
$173 billion
Estimated annual US medical cost of obesity
Source: CDC
-
19.7%
Obesity rate among US children ages 2 to 19
Source: CDC NHANES
The Formula
BMI = weight(kg) / height(m)2. For imperial: BMI = (weight(lb) / height(in)2) x 703.
Worked Examples
A person standing 5 foot 10 inches (178 cm) and weighing 175 pounds (79 kg) has a BMI of 25.1, sitting at the overweight boundary. At 155 pounds (70 kg), that same person would have a BMI of 22.2, which falls within the healthy weight range.
Checking a reading in pounds and inches
- Weight is 170 pounds at 5 feet 9 inches tall
- Apply 703 times weight divided by height squared: 703 times 170 divided by 4,761
- The result rounds to 25.1
BMI lands just over 25, at the boundary between healthy and overweight.
Metric calculation
- Weight is 70 kilograms at 1.75 meters
- Square the height: 1.75 times 1.75 equals 3.0625
- Divide 70 by 3.0625
BMI is about 22.9, comfortably within the healthy range.
Real World Use Cases
Annual health checkups
Track your number across years so trends surface early instead of surprising you later.
Fitness program baselines
Record BMI alongside body measurements before starting training so progress has context.
Insurance and wellness screenings
Know where you stand ahead of employer wellness assessments.
Pediatric growth discussions
Understand how clinicians frame child growth percentiles before appointments.
Expert Tips
- ✓ Measure waist circumference too; under 35 inches for women or 40 for men lowers metabolic risk regardless of BMI.
- ✓ Athletes with high lean mass routinely read as overweight here, so pair BMI with body composition measures.
- ✓ For older adults slightly higher BMIs may be protective; guidelines differ after age 65.
- ✓ Asian populations face elevated cardiometabolic risk at lower BMIs, and several countries use adjusted cutoffs near 23.
- ✓ Direction matters more than one reading. A falling BMI inside the overweight band usually beats an unchanged perfect score.
Frequently Asked Questions
What is a healthy BMI? ▼
A BMI of 18.5 to 24.9 is considered a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 and above is obese. These categories come from the World Health Organization.
What is the BMI formula? ▼
BMI is weight in kilograms divided by height in meters squared (kg per m squared). In pounds and inches, the formula is 703 times weight divided by height squared. This calculator applies the correct formula for your units.
Is BMI accurate? ▼
BMI is a useful screening tool but not a direct measure of body fat. It can misclassify muscular athletes as overweight and miss hidden fat in older adults. Use it as a starting point, not a diagnosis.
Why is BMI not accurate for athletes? ▼
Muscle is denser than fat, so a muscular person can have a high BMI with very low body fat. BMI cannot tell muscle from fat, which is why athletes often get labeled overweight or obese despite being lean.
Is BMI a good measure of body fat? ▼
BMI tracks weight relative to height and correlates with body fat for most people, but it does not measure body fat directly. Muscle mass, age, sex, and where fat is stored all affect how well it applies, so treat the result as a screening estimate rather than a precise measurement.
How do I lower my BMI? ▼
Lowering BMI generally means reducing body fat through a modest calorie deficit and regular physical activity. Combine it with strength training to preserve muscle. Aim for slow, sustainable changes rather than crash dieting.
How is BMI calculated? ▼
Divide weight in kilograms by height in meters squared. In imperial units multiply pounds by 703 and divide by inches squared.
Is BMI accurate for muscular people? ▼
Not reliably. Muscle is denser than fat, so strength athletes often register as overweight despite low body fat. Waist measurement and body composition testing fill the gap.
Common Mistakes to Avoid
- ⚠ Assuming BMI measures body fat directly. A muscular 200 lb athlete at 5 foot 10 inches can have a BMI of 28.7 (overweight) but only 10% body fat. Use waist circumference or body fat percentage alongside BMI for a complete picture.
- ⚠ Weighing yourself at the wrong time of day. Weight can fluctuate 2-4 lb from morning to night due to food and water. Always weigh yourself first thing in the morning after using the bathroom for the most consistent BMI reading.
- ⚠ Using BMI for anyone outside the standard adult range. The formula is not validated for pregnant women, competitive athletes, elderly adults with muscle loss, or children under 18. These groups need alternative assessment methods.
- ⚠ Treating a single category boundary as a cliff. Risk rises continuously, so 24.9 versus 25.2 reflects nearly identical health status.
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Last updated: · by CalculatorPro Tools