Why do two brands' size charts give different measurements for the same size?

Because each brand uses its own base size and block. A UK 8 and a US 4 are different labels, not equivalent measurements.

Should I size up if I fall between sizes?

It depends on the fabric, not the number. A fitted woven bodice typically has 3-4 inches of ease, while stretch knit can have negative ease of minus 2 inches.

How to Read Clothing Size Charts When They Disagree

Most size charts describe the body a garment is intended to fit, not the garment itself. That single distinction explains most of the confusion.

Step one: understand what you are looking at

Most size charts describe the body a garment is intended to fit, not the garment itself. That single distinction explains most of the confusion.

A chart reading 36 inch bust means the garment suits a 36 inch bust person, and the garment will be physically larger than 36 inches.

Step two: add ease into your head

New Mexico State University Extension specifies 3 to 4 inches of ease on the bust, 0.5 to 1 inch on the neckline, 0.5 inch on shoulders, 1 to 1.5 inches across the back, 1 to 2 inches from nape to waist, 1 to 3 inches on thighs and 0.5 to 1 inch on inseam.

For stretch knit, ease goes negative, typically minus 2 to 0 inches. A jersey garment is deliberately cut smaller than you.

If a brand charts a fitted stretch garment at your bust measurement, it will be skin-tight. Charts rarely publish ease, which is the core missing number.

Step three: discount the chart

Research measuring over 1,000 garments across 50 UK brands found only 64% matched their own chart within 1 cm, with 4.2 cm of average within-brand variance. 68% of garments labelled UK 12 fell outside industry standard parameters.

Accuracy fell with size, from 71% at UK 8-10 to 54% at UK 20-22.

Step four: calibrate per brand

After a purchase, note how that brand actually ran. Two brands with identical charts can fit completely differently, and only your own experience reveals that.

This is the most reliable fit data you will ever have, because it is measured against your body.

A reading order that works

Find the brand's chart and note your position relative to it.
Determine the garment type and its likely ease.
Read fit reviews for how it actually runs.
Check whether finished garment dimensions are published.
If between sizes, consider which you prefer rather than which is larger.

When to size up

Between two sizes, size up for woven fabrics and fitted shirts. Size down for stretch knit. Between sizes is not a decision to default to larger.

The larger truth

Size charts are inherited from an era before body measurement existed. The last comprehensive US anthropometric survey for apparel was conducted in 1941. You are reading a historical document and treating it as a specification.

More in using measurements online and the chart research.