How this instrument works
Body mass index is a screening ratio: weight in kilograms divided by height in metres squared. It was designed for comparing populations, not diagnosing individuals — but it remains the fastest honest signal of whether weight is in proportion to height, which is why clinicians still start with it.
The instrument computes your BMI, places the needle on the WHO bands (under 18.5, healthy to 25, overweight to 30, obese beyond), and — more usefully — inverts the formula to show the actual weight range that would put someone of your height in the healthy band. That range is the number worth remembering; the single BMI figure is just where you stand relative to it.
- Enter your height — switch the unit to inches or feet if you think imperial.
- Enter your weight in kilograms, pounds or stone.
- Read the index on the scale; the needle sits in your WHO band.
- Check the healthy range for your height beneath the readout — it is the same formula, solved backwards.
Worked example — 172 cm, 70 kg
The default sheet: 172 cm is 1.72 m, so height² = 2.9584 m². Dividing 70 kg by 2.9584 gives a BMI of 23.7 — inside the healthy band with room on either side.
Solving backwards for the same height: 18.5 × 2.9584 = 54.7 kg and 25 × 2.9584 = 74.0 kg. Anyone 172 cm tall sits in the WHO healthy band between roughly 55 and 74 kilograms — a 19-kilogram-wide corridor, which is a useful reminder of how coarse this screening figure really is.
Questions
Is BMI accurate for everyone?
No. It cannot tell muscle from fat, so muscular athletes read high and people who have lost muscle with age can read deceptively normal. It also shifts meaning across ethnic groups — some health agencies use lower cutoffs for South and East Asian populations. Treat it as a first look, not a verdict.
My BMI is just outside the healthy band — should I worry?
A figure like 25.4 is not categorically different from 24.9; the bands are administrative lines on a continuous scale. What matters is the trend over months and the picture a clinician assembles from waist measurement, blood pressure and history. Use the healthy-range output as a direction, not a deadline.
Why does the instrument use kilograms and metres internally?
BMI is defined metrically — the imperial shortcut (703 × lb ⁄ in²) is an approximation of the same ratio. This instrument converts your input to canonical units first and computes the definition directly, so entering 154 lb and 5 ft 8 in gives exactly the same figure as 69.9 kg and 172.7 cm.
What BMI should a child have?
Children need age-and-sex-specific percentile charts, not adult bands — a healthy 7-year-old's BMI would read 'underweight' on this adult scale. This instrument is calibrated for adults 20 and over; paediatric growth charts are a different sheet.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.