Most size charts ask for your chest, your height and your weight, then guess. The problem is that two brands can put wildly different garments behind the same letter, and "medium" has never meant anything consistent.
There is a better method, and it takes about four minutes. Measure a shirt you already own and like the fit of, then compare those numbers against the garment measurements a brand publishes. You are no longer guessing what fits your body — you are matching one object against another.
What you need
A flat surface, a tape measure, and a shirt you actually like wearing. A hard ruler works if the tape is soft and keeps sagging. Do not measure a shirt that is damp or stretched out from the wash.
The four measurements that matter
Lay the shirt flat, front up. Smooth it out but do not stretch it. The seams should sit naturally.
1. Chest. Measure straight across the shirt from one armpit seam to the other. This is the single most important number. It is a flat measurement, meaning it covers half the shirt — double it to get the full circumference.
2. Length. From the highest point of the shoulder, next to the collar, straight down to the bottom hem. Some brands measure from the centre back of the collar instead, which reads about an inch longer. If a chart seems off by an inch, this is usually why.
3. Shoulder. Across the back, from the point where one sleeve seam meets the shoulder to the same point on the other side. This is the measurement that decides whether a shirt reads as fitted or dropped. On a boxy cut the seam is meant to sit down your upper arm, so this number will be larger than your actual shoulder width — that is the design, not an error.
4. Sleeve. From the shoulder seam to the sleeve opening, along the outside of the sleeve.
Why chest is doubled, and why it trips people up
This is the most common mistake, and it is an expensive one because it makes people order two sizes too big.
A flat chest measurement of 20 inches means the shirt measures 20 inches across when laid down. Wrapped around a body that is 40 inches of circumference. If your own chest measures 40 inches and the shirt is 20 inches flat, that is a close fit with no ease at all. For a regular fit you want roughly 2 to 4 inches of total ease — so a flat measurement of 21 to 22 inches. For a genuinely boxy fit, more.
Any brand publishing a flat measurement should say so explicitly. If a chart just says "chest: 20" with no indication of method, measure their shirt against yours rather than against your body.
Reading fit names against the numbers
Fit vocabulary is unregulated, so treat the words as a hint and the numbers as the truth.
- Slim — little to no ease at the chest, shoulder seam sits on the shoulder bone.
- Regular — 2 to 4 inches of total ease, seam on or just past the shoulder.
- Relaxed — 4 to 6 inches of ease, seam slightly dropped.
- Boxy or oversized — 6 inches or more, seam clearly down the arm, and length often shortened so the shirt does not look like a tunic.
The length-to-chest ratio is what separates a deliberate boxy cut from a shirt that is simply too big. A true boxy tee gets wider without getting proportionally longer. If both numbers scale up together, you are just looking at a larger size.
Two things that shift the numbers
Fabric weight. A heavyweight knit at 260 gsm holds its measured shape. A light 140 gsm jersey drapes and clings, so it will read smaller on the body than the tape suggests. The same flat measurement fits differently in different weights.
Stretch. Anything with elastane gives well beyond its flat figure. On a four-way stretch fabric the flat measurement is a starting point, not a limit — which is why for stretch garments, sizing down usually means more compression rather than a shirt that does not fit.
When you are between sizes
Decide what you want the shirt to do. For a boxy silhouette that sits off the shoulder, take the larger. For a close fit under a layer, take the smaller. For a stretch garment where you want compression, take the smaller.
We publish flat garment measurements in both inches and centimetres on every product page, and we say plainly that chest is measured flat so it needs doubling. Size exchanges are free within 30 days regardless — but the point of publishing real numbers is that you should not need one.