BMI Calculator

The BMI Formula (Metric and Imperial)

The BMI formula is the same everywhere — weight divided by height squared. Only the units and a constant change.

Metric formula

BMI = weight (kg) / [height (m)]²

Imperial formula

BMI = 703 × weight (lb) / [height (in)]²

The 703 factor simply converts pounds and inches into the metric result. Both formulas give an identical number for the same person.

Worked example 1 — metric

A person weighs 70 kg and is 1.75 m tall.

BMI = 70 / (1.75 × 1.75)
BMI = 70 / 3.0625
BMI = 22.9  →  Normal weight

Worked example 2 — imperial

Someone weighs 180 lb and is 5 ft 9 in (69 inches) tall.

BMI = 703 × 180 / (69 × 69)
BMI = 126,540 / 4,761
BMI = 26.6  →  Overweight

Worked example 3 — metric, higher end

A person weighs 95 kg at 1.68 m.

BMI = 95 / (1.68 × 1.68)
BMI = 95 / 2.8224
BMI = 33.7  →  Obese (Class I)

Notice that BMI rises steeply as weight climbs at a fixed height — each 3 kg here moves the number by roughly one point.

BMI Categories and Chart for Adults

For adults aged 20 and over, the World Health Organization (WHO) and CDC use fixed thresholds that do not depend on age or sex. This BMI chart shows where you land:

The healthy weight band of 18.5–24.9 is where risk of weight-related disease is statistically lowest for most adults.

Finding your healthy weight range

You can flip the formula around to find the weight range that keeps you in the normal band at your height. Multiply the boundary BMI by your height squared:

Healthy weight (kg) = BMI × height² (m²)

At 1.75 m:
Lower = 18.5 × 3.0625 = 56.7 kg
Upper = 24.9 × 3.0625 = 76.3 kg

So a person 1.75 m tall is in the healthy range between roughly 57 kg and 76 kg.

BMI for Children, Teens, and Different Populations

BMI is interpreted differently depending on who you are — this is the most misunderstood part of the measure.

Children and teens (ages 2–19)

For young people, the same weight can be healthy or unhealthy depending on age and sex, because bodies change rapidly during growth. Instead of fixed cutoffs, pediatricians plot BMI on age-and-sex percentile charts:

A calculator that asks for age and gender uses these percentiles for anyone under 20, not the adult table.

Ethnicity adjustments

Risk does not appear at the same BMI for every population. For people of South, Southeast, and East Asian descent, health risk rises at lower numbers, so many guidelines lower the overweight threshold to 23.0 and obesity to 27.5.

Older adults

After about age 65, a slightly higher BMI (in the 23–27 range) is often protective, providing reserve during illness. Aggressively targeting 18.5 in an older person can do more harm than good.

How to Interpret Your BMI Result

Getting the number is easy; reading it wisely takes a little more care. Follow these steps:

1. •Locate your category. Match your BMI to the adult chart (or percentile for under-20s). This is your screening result.
2. •Add context. BMI ignores muscle, bone density, and where fat sits on your body. A muscular athlete can score "overweight" while being very healthy; a sedentary person with a "normal" BMI can still carry unhealthy visceral fat.
3. •Check your waist too. A waist circumference over 40 in (102 cm) for men or 35 in (88 cm) for women signals higher risk regardless of BMI, because abdominal fat is the dangerous kind.
4. •Look at the trend, not one snapshot. A BMI moving from 27 to 25 over six months matters more than the exact value on any single day.
5. •Talk to a professional. If you're outside the healthy band, a doctor can order body-composition tests, blood work, and blood pressure checks that BMI can't provide.

Rule of thumb: use BMI to decide whether to look closer, never as the final word on your health.

How to Use This BMI Calculator

This BMI calculator is built to give you an accurate result in seconds while handling the special cases above automatically.

Inputs

• •Units — switch between metric (kg, cm/m) and imperial (lb, ft/in). The tool converts internally, so you never do the math yourself.
• •Height — enter it in your chosen units. Be as precise as you can; a half-inch changes the result slightly.
• •Weight — your current weight; measure in the morning for consistency.
• •Age and gender (optional but recommended) — these let the calculator apply child/teen percentiles when appropriate and add age-aware context to your result.

Outputs

• •Your BMI value, rounded to one decimal place.
• •Your category (underweight, normal, overweight, or obese class).
• •Your personal healthy weight range in your own units — the low and high weights that keep you in the 18.5–24.9 band at your height.
• •For young users, a percentile instead of a fixed category.

Enter your numbers, read the category, and note the target weight range. Everything updates instantly as you adjust the inputs, so you can see how much a change in weight moves your BMI.

Tips for Using BMI to Reach a Healthy Weight

BMI is most useful as a nudge toward action. Here's how to put it to work:

• •Set a realistic target inside the range. You don't need to hit the exact middle. Landing anywhere in 18.5–24.9 dramatically lowers weight-related risk.
• •Aim for slow change. A safe, sustainable rate is about 0.5–1 kg (1–2 lb) per week. Faster loss usually costs you muscle, which is exactly what you want to keep.
• •Combine the scale with the tape measure. If your BMI barely moves but your waist shrinks and clothes fit better, you're likely trading fat for muscle — a win the calculator can't see.
• •Recheck monthly, not daily. Water weight makes day-to-day BMI noisy. Monthly readings show the real trend.
• •Pair BMI with habits, not obsession. Use it to confirm that better sleep, more movement, and improved nutrition are working — then keep going.

When to use BMI: annual check-ins, tracking a fitness goal, or as a quick self-screen. When not to rely on it: judging elite athletes, pregnant people, or anyone with very high muscle mass.

Common BMI Mistakes and Myths to Avoid

Even though the body mass index formula is simple, people misread it constantly. Correct these:

• •Myth: BMI measures body fat. It doesn't — it measures weight relative to height. Two people with identical BMIs can have very different body-fat percentages.
• •Mistake: using adult thresholds for a 12-year-old. Always use age-and-sex percentiles for anyone under 20; the adult 25/30 cutoffs don't apply.
• •Mistake: mixing units. Entering weight in pounds but forgetting the 703 factor (or squaring height in centimeters instead of meters) produces wildly wrong numbers. Let the calculator handle unit conversion.
• •Myth: a 'normal' BMI means you're healthy. So-called normal-weight obesity — a healthy BMI hiding high visceral fat and poor fitness — is real. Blood pressure and blood sugar still matter.
• •Mistake: chasing the lowest possible number. A BMI under 18.5 carries its own risks: weakened immunity, bone loss, and nutrient deficiencies. Lower is not always better.
• •Myth: BMI is useless. For screening large groups and tracking your own trend over time, it remains cheap, fast, and genuinely informative — as long as you respect its limits.