BMI Calculator Formula

Understand the math behind the bmi calculator. Each variable explained with a worked example.

Formulas Used

BMI

bmi = (weight_lbs * 703) / (total_inches * total_inches)

Variables

Variable	Description	Default
`weight_lbs`	Weight(lbs)	170
`height_feet`	Height (feet)(ft)	5
`height_inches`	Height (inches)(in)	10
`total_inches`	Derived value`= height_feet * 12 + height_inches`	calculated

How It Works

How BMI Works

BMI divides your weight by your height squared. That's it. The number it produces places you into one of four categories: underweight (below 18.5), normal (18.5 to 24.9), overweight (25 to 29.9), or obese (30+). Doctors use it as a quick screening tool because it takes two seconds and needs no equipment.

The Formula

BMI = weight (kg) / height (m)^2

Or in imperial units: BMI = weight (lbs) * 703 / height (inches)^2

When to Use This

BMI is a rough population-level indicator. It works well for identifying weight-related health risk in sedentary adults who don't have unusual muscle mass. If your doctor mentions BMI at a checkup, this is the formula they're using.

What BMI Doesn't Tell You

BMI can't distinguish muscle from fat. A 5'10" person weighing 200 lbs gets a BMI of 28.7 (overweight) whether they're a competitive athlete or sedentary. It also doesn't account for where you carry weight. Belly fat around internal organs carries more health risk than fat on your hips or thighs, and BMI ignores that entirely.

For a more complete picture, consider waist-to-hip ratio, body fat percentage, or a DEXA scan.

How to Read Your Result

BMI RangeCategory Below 18.5Underweight 18.5 to 24.9Normal weight 25.0 to 29.9Overweight 30.0+Obese

These ranges were set by the WHO and apply to adults over 20. They don't apply to children, pregnant women, or highly muscular individuals.

Common Mistakes

Mixing up units. Using pounds with the metric formula (or kg with imperial) gives a meaningless number.

Applying BMI categories to children. Pediatric BMI uses age-specific percentile charts, not the adult cutoffs.

Treating BMI as a diagnosis. A high BMI isn't a disease. It's a screening number that might prompt further evaluation.

Worked Example

A person weighing 170 lbs at 5 feet 10 inches.

weight_lbs = 170height_feet = 5height_inches = 10

01Height in inches = 5 x 12 + 10 = 70 inches
02BMI = (170 x 703) / (70 x 70)
03= 119,510 / 4,900
04= 24.4 (Normal weight)

When to Use This Formula

Getting a quick screening number during a routine health checkup to flag whether weight may be a health risk worth discussing with a doctor.
Tracking your own BMI over time as one data point alongside waist circumference and body composition when managing a weight loss or fitness plan.
Completing health insurance or life insurance applications that require a BMI figure as part of their underwriting process.
Comparing population-level health data in public health research, where BMI is the standard metric for categorizing weight status across large groups.
Setting a baseline measurement before starting a new diet or exercise program, so you have a concrete number to measure progress against.

Common Mistakes to Avoid

Mixing up units — the metric formula uses kilograms and meters, but many people enter pounds and inches without converting, which produces a wildly incorrect number. The imperial formula requires a correction factor of 703.
Forgetting to square the height — dividing weight by height (instead of height squared) gives a meaningless result that does not scale correctly with body size.
Treating BMI as a direct measure of body fat — it is a ratio of weight to height and cannot distinguish muscle from fat, which is why muscular athletes often register as "overweight" despite having low body fat.
Applying adult BMI categories to children — pediatric BMI uses age- and sex-specific percentile charts, not the standard 18.5-24.9 healthy range for adults.
Using BMI as the sole indicator of health — it does not account for fat distribution, bone density, age, sex, or ethnicity, all of which significantly affect health risk at any given BMI.

Frequently Asked Questions

What is a healthy BMI?

A BMI between 18.5 and 24.9 is considered normal weight. Below 18.5 is underweight, 25-29.9 is overweight, and 30 or above is classified as obese. However, BMI does not account for muscle mass, bone density, or body composition.

Is BMI accurate?

BMI is a useful screening tool but has limitations. It may overestimate body fat in athletes with high muscle mass and underestimate it in older adults who have lost muscle. It does not distinguish between fat and lean tissue.

What is a good BMI for my age?

For adults, the standard BMI categories apply regardless of age (18.5-24.9 is normal). However, older adults (65+) may have healthier outcomes at a slightly higher BMI (23-27). For children and teens, BMI is interpreted using age- and sex-specific growth charts.

Can I have a healthy BMI but still have too much body fat?

Yes. This is sometimes called "normal weight obesity" or "skinny fat." A person with low muscle mass and high body fat can fall in the normal BMI range while still carrying unhealthy levels of visceral fat. Body fat percentage tests give a more accurate picture.

Learn More

Guide

Understanding BMI: What It Means and How to Calculate It

Learn what BMI is, how to calculate it, what the ranges mean, and the limitations of body mass index as a health metric. Comprehensive guide with formulas and context.

Ready to run the numbers?

Open BMI Calculator