How to Calculate BMI by Hand: Formula, Categories and What It Really Means

Published on June 2, 2026 · 9 min read

Body Mass Index — BMI for short — is the number your doctor scribbles on your chart and the number every fitness app shows on its dashboard. It is a single value that tries to summarise whether your weight is healthy for your height. But what does the number actually mean, how do you compute it without an app, and where does it fall apart? This guide walks through the formula, both metric and imperial versions, and shows you how to interpret the result sensibly.

The BMI formula (metric)

The metric formula is the simplest:

Note the height is in metres, squared, not centimetres. A person who is 175 cm tall enters 1.75, then squares it to 3.0625. If they weigh 72 kg, their BMI is 72 ÷ 3.0625 = 23.5.

The BMI formula (imperial)

If you are working in pounds and inches, multiply by 703 to make the units cancel out:

A person 5'9" (69 inches) weighing 160 lb: 69² = 4761, then 160 ÷ 4761 = 0.0336, multiplied by 703 = 23.6. Almost identical to the metric example — that is the same body, just different units.

WHO BMI categories

The World Health Organization splits BMI into the following ranges for adults:

• Below 18.5 — Underweight
• 18.5 – 24.9 — Healthy weight
• 25.0 – 29.9 — Overweight
• 30.0 – 34.9 — Obesity class I
• 35.0 – 39.9 — Obesity class II
• 40.0 and above — Obesity class III

A BMI between 18.5 and 24.9 is generally considered the lowest statistical risk for chronic disease — but as we will see, this number is a starting point, not a verdict.

Worked examples

Example 1: average adult

Height 168 cm, weight 65 kg. Convert height: 1.68² = 2.8224. BMI = 65 ÷ 2.8224 = 23.0. Healthy range.

Example 2: tall and lean

Height 190 cm, weight 78 kg. 1.90² = 3.61. BMI = 78 ÷ 3.61 = 21.6. Healthy.

Example 3: imperial check

Height 6'0" (72 in), weight 195 lb. 72² = 5184. (195 ÷ 5184) × 703 = 26.4. Slightly above the healthy threshold.

Where BMI works well

BMI is cheap, fast, and reasonably accurate as a population-levelscreening tool. For the average sedentary adult between 20 and 65 years old, BMI correlates well with body fat percentage and with the risk of conditions like type 2 diabetes, cardiovascular disease, and sleep apnoea. Doctors use it as a triage signal: anything outside 18.5–24.9 prompts a closer look, not an immediate diagnosis.

Where BMI breaks down

BMI is calculated from just two numbers, so it cannot distinguish between fat and muscle. That makes it misleading in several situations:

• Athletes: A rugby player or bodybuilder may have a BMI of 28–32 with single-digit body fat. The formula calls them "overweight" while their cardiovascular health is exceptional.
• Elderly adults: After 60, people lose muscle mass (sarcopenia) but may retain visceral fat. A "healthy" BMI of 22 can hide unhealthy body composition.
• Pregnant or nursing women: BMI is not designed for them and should not be used during pregnancy.
• Children and teenagers: Use age-and-sex-specific BMI percentile charts, not the adult cutoffs.
• Different ethnic groups: Research suggests people of South Asian descent face higher cardiometabolic risk at lower BMI levels. Some health systems use 23 as the overweight threshold instead of 25 for these populations.

Better than BMI alone

For a fuller picture of health, combine BMI with one or two additional measurements:

• Waist circumference: Above 94 cm (men) or 80 cm (women) suggests elevated risk regardless of BMI.
• Waist-to-height ratio: Keep your waist below half your height. Easy to remember and harder to game.
• Body fat percentage: Measured with calipers, bioelectric scales, or DEXA scans. Far more informative for athletes.
• Resting heart rate and blood pressure: Cheap to measure and strongly tied to long-term outcomes.

Common questions

What is a "perfect" BMI?

There is no single perfect number. Large epidemiological studies consistently show the lowest all-cause mortality somewhere between BMI 22 and 25 for most adults — but the curve is shallow, and muscle, fitness, and waist size matter more than a single decimal point.

How quickly can BMI change?

A 1 kg change in weight shifts BMI by about 0.3–0.4 points for an average-height adult. Daily fluctuations of 1–2 kg from food, water, and glycogen are normal and meaningless. Look at weekly averages instead of single readings.

Is a BMI of 17 dangerous?

It is below the healthy range. For some naturally lean adults this is fine, but combined with fatigue, low energy, or amenorrhoea, it warrants a medical check.

Using ConvertProf to calculate BMI

You can run the whole calculation in under a minute on ConvertProf's scientific calculator. Type your weight in kg, divide by your height in metres, divide by your height in metres again — done. If your scale shows pounds, use the weight converter to switch to kilograms first, or use the length converter to translate feet and inches into metres.

Bottom line

BMI is a useful three-second screening tool, not a verdict on your health. Calculate it, note where you land in the table, and treat the number as a conversation starter rather than a final answer. Combine it with waist measurement and a basic fitness check for a far more honest picture of how your body is actually doing.