How it works
Body Mass Index (BMI) is weight in kilograms divided by height in metres squared — a single number that adjusts weight for stature so populations can be compared on one scale. The World Health Organization defines adult categories at fixed cutoffs: below 18.5 is underweight, 18.5–24.9 is normal weight, 25.0–29.9 is overweight, and 30.0 or above is obese. The engine converts centimetres to metres, computes BMI = kg ÷ m², assigns the WHO label, and back-solves the healthy weight band by multiplying 18.5 and 24.9 by height². Worked example on the defaults: 70 kg at 175 cm. Height in metres = 1.75 m. BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — squarely in the Normal range. Healthy weight for 175 cm: minimum = 18.5 × 3.0625 = 56.7 kg; maximum = 24.9 × 3.0625 = 76.3 kg. BMI is a screening tool, not a diagnosis. It cannot distinguish muscle from fat, ignores fat distribution (visceral fat carries higher cardiometabolic risk at the same BMI), and ethnic groups differ — WHO notes that some Asian populations face elevated risk at lower BMI cutoffs. Athletes with low body fat often land in overweight because muscle is dense. The CDC publishes the same adult categories for US clinical screening. Use BMI as a first filter, then waist circumference, body-fat percentage or clinical assessment for decisions about weight management.
Formula
height (m) = height (cm) ÷ 100
BMI = weight (kg) ÷ height (m)²
Healthy min (kg) = 18.5 × height (m)²
Healthy max (kg) = 24.9 × height (m)²
WHO adult categories
< 18.5 Underweight
18.5–24.9 Normal
25.0–29.9 Overweight
≥ 30.0 Obese
Worked example — defaults
70 kg · 175 cm (1.75 m)
BMI = 70 ÷ 1.75² = 22.9 → Normal
Healthy range = 56.7–76.3 kg
Tips
- Measure height without shoes and weight in light clothing for consistency with WHO reference data.
- BMI above 25 with a waist over 94 cm (men) or 80 cm (women) warrants closer clinical review per WHO guidance.
- Muscular builds can read overweight on BMI alone — add body fat calculator or waist measure.
- WHO adult categories apply from age 20; paediatric BMI uses age- and sex-specific percentiles instead.
- At 175 cm, each 5 kg above 76.3 moves BMI roughly 1.6 points — small weight changes shift category near the cutoffs.
Frequently asked questions
- What is a healthy BMI?
- WHO classifies **18.5–24.9** as normal weight for adults. At **175 cm** that corresponds to roughly **56.7–76.3 kg**. Below 18.5 is underweight; 25+ is overweight; 30+ is obese.
- How is BMI calculated for 70 kg and 175 cm?
- Divide weight in kilograms by height in metres squared: 70 ÷ 1.75² = **22.9**. The engine reports one decimal place and assigns the **Normal** WHO category.
- Is BMI 22.9 good?
- Yes — **22.9** falls in the Normal range (**18.5–24.9**). It sits near the middle of the healthy band for **175 cm**, suggesting weight is proportionate to height by the WHO standard.
- Why does BMI not work for athletes?
- BMI treats all weight equally. A lifter at 90 kg and 175 cm may have low body fat but a BMI of 29.4 (overweight). Body-fat percentage or waist-to-height ratio captures composition better for trained individuals.
- What weight should I target for 175 cm?
- The healthy band on the defaults is **56.7–76.3 kg** — the weight range that keeps BMI between 18.5 and 24.9 at that height. Target the middle of the band unless a clinician advises otherwise.