How do I calculate my BMI?

BMI = weight (kg) ÷ height (m)². Example: 70 kg ÷ (1.75 m)² = 22.9 → Normal weight. Under 18.5 = Underweight. 18.5 – 24.9 = Healthy range. 25.0 – 29.9 = Overweight. 30.0+ = Obese (talk to a doctor).

BMI Calculator — Formula, Chart & How to Calculate (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.

BMI vs body fat %: which number should you actually track?

BMI is enough for casual screening, insurance or medical intake forms and population research — roughly 80% of use cases. You need body fat % if you are an athlete or strength trainer, post-pregnancy, elderly, or in a body-recomposition phase, because BMI actively misleads those groups. To estimate body fat without a lab: the US Navy tape method (neck, waist, height — plus hips for women), the YMCA formula (waist + weight), or 3-site skinfold calipers. Quick decision tree: Do you lift heavily or compete in sport? → body fat %. Are you over 65? → body fat % plus waist. Are you pregnant? → neither, follow your clinician. Otherwise → BMI is fine.

Age and BMI: why your healthy range shifts as you get older

The classic 18.5–24.9 band was validated mainly on adults aged 25–45. Adults 65+: multiple cohort studies associate BMI 25–27 with the lowest mortality, so 'overweight' by the standard chart may be protective in later life. Children and teens: never use adult charts — BMI-for-age percentiles are the correct tool. Pregnancy: BMI is not a meaningful health metric while pregnant; gestational weight-gain guidelines replace it. Age-adjusted reference: 20–35 → 18.5–24.9; 36–50 → 19–25.9; 51–65 → 20–26.9; 65+ → 22–27.9. Match your chart to your age before you judge your number.

How doctors actually use BMI in clinical settings

In practice BMI triggers a follow-up, never a diagnosis. A BMI of 31 prompts a clinician to check waist circumference, blood pressure, HbA1c, lipids and family history — the BMI alone decides nothing. Waist circumference is trusted more for metabolic risk: over 40 in (102 cm) for men or 35 in (88 cm) for women signals high risk at any BMI. The WHO itself calls BMI a 'crude population-level measure'; it was never designed as an individual diagnostic. Bring four things to your appointment: your BMI, your waist measurement, a recent blood-pressure reading and recent bloodwork. Together they form a real health picture.

Advanced: reading BMI as a trend, not a snapshot

The slope matters more than the number. Falling from BMI 28 to 24 in three months is medically concerning (possible illness or malnutrition); the same drop over 18 months is healthy fat loss. Track monthly under identical conditions — same time of day, same clothing, same scale — because weekly readings are mostly water noise. Beware the recomposition trap: gaining muscle while losing fat can hold BMI flat for 3–6 months while health improves dramatically, which is when most people quit; track body fat % instead during that phase. Override BMI tracking entirely after major surgery, during cancer treatment, during pregnancy, and after age 65.

How this guide is researched and fact-checked

This article is written by Dr. Priya Menon (MBBS, MD (Internal Medicine) · 11 yrs clinical practice) and reviewed by the ToolKit Pro editorial desk before publication. Every number, formula and threshold quoted here is traced back to a primary standards body or health authority rather than to another blog, and each figure is re-computed with our own calculators so the worked examples and the live tools on this site always agree. When a definition changes — as with the 2019 SI redefinitions — the affected articles are re-checked and the “updated” date above is bumped.

Written by: Dr. Priya Menon — Health & Nutrition Contributor
Reviewed by: ToolKit Pro Editorial Desk
Last reviewed: 2026-06-11
Corrections: spotted an error? Tell us and we will fix and re-date the page.

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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