BMI Calculator

What BMI measures

Body mass index is a screening number built from weight and height. In metric units it is weight in kilograms divided by height in meters squared: BMI = kg ÷ m². This calculator's metric mode asks for centimeters and converts them, dividing height by 100 before it squares the meters. In imperial units the equivalent is BMI = (weight in pounds ÷ height in inches squared) × 703. The 703 factor exists so pounds and inches land on the same scale as kilograms and meters. The result is rounded to one decimal place.

The category labels follow the adult bands commonly used on US clinical charts. Under 18.5 is labeled underweight. From 18.5 up to but not including 25 is labeled normal weight. From 25 up to but not including 30 is overweight. From 30 up to but not including 35 is obese class I. From 35 up to but not including 40 is obese class II. 40 and above is obese class III. These words are the chart's labels for an adult number. They are not a diagnosis, and they are not a plan.

Informational only. This calculator is not medical advice and does not diagnose, treat, or prevent any condition. Always consult a qualified healthcare provider for medical decisions.

Worked examples

Example 1: The imperial example on the form

Weight 160 pounds, height 70 inches. Five feet 10 inches is 5 × 12 + 10 = 70 inches, not 5.10. BMI = (160 ÷ 70²) × 703 = (160 ÷ 4,900) × 703 ≈ 23.0. The category label is normal weight. If the height is typed as 5.10, the formula treats that as barely five inches tall and the result is nonsense. Convert feet and inches to total inches first. The Length Converter and the Weight Converter are there when the source numbers are in the other system.

Example 2: The same person in metric

About 72.6 kilograms and 178 centimeters. Height in meters is 1.78. BMI = 72.6 ÷ (1.78 × 1.78) ≈ 22.9. A small difference from 23.0 is rounding in the conversion of pounds and inches, not a second definition of BMI. Use one unit system for a series of measurements so the trend is real.

Example 3: Why height has to be measured

At 220 pounds and 70 inches, BMI = (220 ÷ 4,900) × 703 ≈ 31.6, labeled obese class I. At the same 220 pounds and 74 inches (6 feet 2 inches), BMI = (220 ÷ 5,476) × 703 ≈ 28.2, labeled overweight. The weight did not change. A wrong height moves the category. Measure height without shoes rather than using a remembered number from years ago.

Example 4: A high-muscle body on the same chart

An adult at 70 inches and 200 pounds has BMI ≈ 28.7, inside the overweight band. The formula cannot see muscle, bone, or fat. A strength athlete and a person with a similar weight and a higher fat share can share a BMI and have different health pictures. That limit is why BMI is a screening start and not a verdict. The article BMI vs body composition walks through what the index misses, and the body fat percentage tool estimates composition with the US Navy circumference method, which is also a screening estimate rather than a medical test.

When to use this

Use it when a form asks for BMI, when you want the adult category the standard chart will print, or when you are comparing your own weight and height over time with the same method. Population research uses BMI because it is cheap to calculate and, across large groups of adults, it tracks with health risk well enough to be a sorting tool. Your own next step, if a label surprises you, is a conversation with a qualified clinician who can look at waist, labs, history, and goals. It is not a reason to start a restrictive diet from the category word alone.

Who the adult chart does not fit

The bands above are for adults, generally ages 20 and older in the way US growth charts are split. Children and teens need BMI-for-age percentiles that account for age and sex. A raw adult category on a 12-year-old is the wrong chart. Pregnancy changes the meaning of a weight-for-height number; prenatal care uses its own guidance. Older adults can lose muscle and keep a "normal" BMI while body fat rises. Athletes can sit in the overweight band with low fat. None of these situations is fixed by recalculating the same formula more carefully. They need a different interpretation from a clinician.

Common mistakes

• Feet typed as a decimal. 5 feet 10 inches is 70 inches. 5 feet 6 inches is 66 inches. The number 5.5 feet is 66 inches only if you meant five and a half feet, and 5.10 is not a height this form understands.
• Mixed units. Pounds with centimeters, or kilograms with inches, produce a meaningless BMI. Match the unit menu to both fields.
• Using the adult labels for a child. Ask a pediatric clinician about BMI-for-age instead of reading class I, II, or III.
• Treating the label as a diagnosis. The calculator prints the band the number falls in. It does not examine you.
• Comparing across a change in method. A home scale with shoes on, then a clinic scale before breakfast, is two measurements. Track one method.

Limitations

BMI does not measure body fat, muscle, bone, fitness, or blood work. The same adult formula is applied regardless of sex, frame, or age; how a clinician reads the number can differ, and that reading is not something this page can do. There is no waist measurement and no age field. The display rounds to one decimal, so a value that sits on a boundary can show as 25.0 or 24.9 depending on the underlying hundredths. The tool does not store entries. Daily water targets and heart-rate zones are separate topics; they do not change the BMI formula. See the daily water intake and target heart rate tools if those are the questions you actually have.

Nothing here diagnoses, treats, or prevents any condition. It is not a substitute for professional medical advice, a physical exam, or emergency care. If you have symptoms that worry you, contact a qualified health professional.

FAQ

Is this medical advice?

No. It is arithmetic plus the standard adult category labels. It does not diagnose, treat, or prevent any condition. Decisions about weight, diet, and health belong with a qualified healthcare provider who knows your history.

Which formula is the real BMI?

Kilograms divided by meters squared is the definition. The 703 version is the same definition after pounds are converted to kilograms and inches to meters. Pick the unit menu that matches the numbers you have so you do not convert twice.

Why do two calculators disagree by a tenth?

Usually rounding, or one of them received height in the wrong form. 69 inches and 70 inches are a full category apart at some weights. Recheck total inches and the unit menu before you compare tools.

Should I look at body fat instead?

If the question is composition, BMI will not answer it. Read the body-composition article, and treat circumference estimates and the Navy method described in the Navy body fat formula as estimates. A clinician can talk through DEXA or other methods if a precise measurement is actually needed.

Do water intake or exercise zones change BMI?

They change other parts of a health picture. They are not inputs to kg ÷ m². Use those calculators for what they measure, and keep BMI as weight and height only.

Practical tips

Measure height without shoes, heels against a wall, and use that total-inch or centimeter figure until the next real measurement. Weigh under similar conditions if you are watching a trend: similar clothes, similar time of day, the same scale. Write the units next to the numbers so a later entry does not mix pounds with kilograms.

If the category changes because of a one-time heavy meal or a different scale, do not rebuild a plan around a single decimal. Bring a surprising result to a clinician, especially if you are pregnant, under 20, an older adult losing strength, or training in a way that adds muscle. The number is a starting point for that conversation.

For a form that wants BMI to one decimal, the figure on this page matches the usual rounding. For a form that wants it to two decimals, compute (pounds ÷ inches²) × 703 and round in the last step only.

Related calculators

• Body Fat Percentage
• TDEE Calculator for a calorie estimate from age, weight, and height
• Daily Water Intake
• Target Heart Rate Zones
• Weight Converter
• Length Converter
• BMI vs body composition