How to Calculate BMI: A Complete Guide (2026)

Body Mass Index (BMI) is a simple numerical measure of body weight relative to height, widely used as a quick health-screening tool. This guide gives you the exact formula, a full chart, worked examples for both metric and imperial units, and answers to the most common BMI questions.

The BMI formula (metric and imperial)

Metric: BMI = weight (kg) ÷ height² (m²). Imperial: BMI = (weight in lbs ÷ height in inches²) × 703. Both formulas give the same result — pick whichever matches your units.

Worked example: 70 kg adult at 1.75 m

Step 1: square the height → 1.75 × 1.75 = 3.0625. Step 2: divide weight by that → 70 ÷ 3.0625 = 22.86. Result: BMI = 22.9, which falls in the normal range.

Worked example: 165 lb adult at 5'9" (69 in)

Step 1: square height → 69 × 69 = 4761. Step 2: weight ÷ height² → 165 ÷ 4761 = 0.03466. Step 3: multiply by 703 → 24.4. Result: BMI = 24.4, still inside the normal range.

Healthy BMI ranges (adults)

Under 18.5 = underweight. 18.5–24.9 = normal. 25.0–29.9 = overweight. 30.0–34.9 = obesity class I. 35.0–39.9 = class II. 40+ = class III (severe obesity).

BMI by age and sex

The adult ranges above apply to people 20+ regardless of sex. Children and teens use age-and-sex-specific percentiles instead of fixed ranges. Older adults (65+) may have slightly higher healthy ceilings (up to ~27).

Limitations of BMI

BMI does not measure body fat directly and cannot tell muscle from fat. Athletes, bodybuilders, and people with dense bones often score 'overweight' while being very lean. Always combine BMI with waist circumference, body-fat percentage and a doctor's assessment.

How to use a BMI calculator

Enter your height (cm or feet/inches) and weight (kg or lbs) into our free BMI calculator. It returns your BMI to one decimal place, your category, and a healthy-weight target range for your height.

FAQ: Is BMI accurate for women?

Yes — the same formula and adult ranges are used for both sexes. Women typically carry more essential body fat than men, so a healthy female may sit slightly higher within the normal range.

FAQ: What is the ideal BMI?

Most research points to a BMI of 20–25 as the lowest-risk range for adults under 65. Above 25, risks of heart disease, type 2 diabetes and hypertension begin to rise gradually.

FAQ: How do I lower my BMI?

Lower body weight, or both lose fat and add height (only realistic in growing children). A modest 500 kcal/day deficit yields ~0.5 kg/week of weight loss, dropping BMI roughly 0.16 per week for a 1.7 m adult.

FAQ: Why is my BMI high but I look lean?

You likely carry more muscle than average. Skeletal muscle is denser than fat, so muscular athletes routinely score 26–29 BMI while having sub-15% body fat. Use DEXA, calipers or a body-fat scale for a clearer picture.

FAQ: Should children use the adult BMI chart?

No. Use the CDC or WHO BMI-for-age percentile charts, which compare a child's BMI to peers of the same sex and age in months.

Sources & further reading

BIPM — The International System of Units (SI Brochure)
NIST — Guide for the Use of the International System of Units
WHO — Body weight & health guidance

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