Waist-to-Height Ratio Calculator: The 0.5 Rule

How to use this waist-to-height ratio calculator

Choose Metric (centimetres) or Imperial (inches, feet and inches).
Measure your waist at the midpoint between your lowest rib and the top of your hip bone, breathing out normally.
Enter your waist and your height, without shoes.
Read the large figure: your ratio, and how it sits against the 0.5 boundary.

Keep your waist below half your height

The rule behind this calculator is one sentence: keep your waist below half your height. It comes from decades of research, summarised by Professor Margaret Ashwell, showing that the fat carried around the middle — wrapped around the liver and other organs — predicts heart disease and type 2 diabetes better than weight alone.

Unlike BMI, the boundary is the same for men and women, and it self-adjusts for height: a taller frame is allowed a wider waist, in exact proportion.

How the ratio is worked out

The ratio

Waist ÷ height (same units)

One division. Centimetres over centimetres — the units cancel, which is why the number travels between unit systems untouched.

The bands

Under 0.4 low · 0.4–0.49 healthy · 0.5–0.59 increased risk · 0.6+ high risk

The screening bands used with Ashwell's boundary. They flag risk for a check-up conversation — they do not diagnose anything by themselves.

Your boundary waist

0.5 × your height

The largest waist that keeps you in the healthy band at your height. The page shows it, plus how far your waist sits from it, so the goal is a tape-measure number, not an abstraction.

Examples

Person	Waist	Height	Ratio	Band
Woman, waist 76 cm, 165 cm	76 cm	165 cm	0.46	Healthy
Man, waist 102 cm, 175 cm	102 cm	175 cm	0.58	Increased risk
Man, waist 110 cm, 170 cm	110 cm	170 cm	0.65	High risk

Four waist-measuring mistakes

Measuring over clothes

A jumper adds centimetres you do not own. Bare skin or one thin layer, tape flat all the way round.

Holding your breath in

Sucking in your stomach games the number and fools nobody but the chart. Breathe out gently and read the tape at the end of the breath.

A different spot every time

Navel one month, waistband the next — the readings cannot be compared. Same landmark (rib-to-hip midpoint), same time of day.

Judging by one reading

Bloating and big meals move a waist by centimetres. Monthly readings, trended, are the signal; a single morning is weather, not climate.

Frequently asked questions

What is a healthy waist-to-height ratio?

Below 0.5 — your waist less than half your height — is the widely used healthy boundary from Professor Margaret Ashwell's research. Between 0.5 and 0.6 risk rises, and 0.6 or above is the high-risk zone for heart disease and type 2 diabetes.

Why use this instead of BMI?

BMI cannot tell where fat sits. Waist-to-height targets the fat stored around the organs — the kind that drives metabolic risk — and large reviews have found it predicts heart and diabetes risk at least as well as BMI, with one boundary that works for men and women.

Where exactly do I measure my waist?

At the midpoint between the bottom of your lowest rib and the top of your hip bone, roughly at or just above the navel, on bare skin or thin clothing. Breathe out normally and keep the tape snug but not digging in — and measure the same way every time.

Does the 0.5 boundary work for children and older adults?

It was developed on adults. Children have their own growth-chart methods (the BMI percentile), and in older adults muscle loss changes the picture — over 70, a slightly higher ratio is common and less alarming. Treat 0.5 as a screening guide, not a verdict.

My BMI is normal but my ratio is over 0.5 — should I worry?

That combination (normal weight, central fat) is exactly what this measure exists to catch, and it does carry extra risk. It is a prompt for a check-up and lifestyle attention, not a panic — blood pressure, blood sugar and lipids complete the picture.

How often should I measure?

Every month or two is plenty — waist size moves slowly. Same time of day, same tape position, and write it down; the trend matters more than any single reading.

Privacy and important information

This calculator runs entirely in your browser: your numbers are never sent to a server, stored, or shared. A waist-to-height ratio is a screening signal, not a diagnosis — if yours is in the increased or high band, the right next step is a routine check-up, where blood pressure, blood sugar and cholesterol complete the picture.

Part of RahaKit's free health calculators.

Waist-to-Height Ratio Calculator