How this instrument works
Suit and sport-coat sizes in the US and UK are conventionally sold in even numbers only — 38, 40, 42, and so on — anchored directly to chest circumference in inches. A 40-inch chest is a size 40 jacket; there's no size 39 or 41 on the rack, so a chest measurement that falls on an odd number gets rounded to the nearest even size. This calculator applies that same even-number rounding automatically.
The EU size shown alongside it uses a constant offset from the US/UK figure — EU size equals US size plus 10 — a menswear-retail rule of thumb rather than a formula derived independently from the chest measurement itself. It's a widely used approximation for translating between the two labeling conventions, close enough for most off-the-rack shopping, though individual brands can and do run their own house sizing that shifts slightly from this offset.
This calculator covers chest-to-size only. A full jacket fit also depends on length (short, regular, long, sized separately by height in many brands' systems) and drop — the difference between chest and waist measurement that determines cut — neither of which a single chest number can capture. Treat the size here as the right chest measurement to start trying on, not a guarantee of overall fit.
- Measure chest circumference in inches — around the fullest part of the chest, with the tape measure snug but not compressed, worn over a typical dress shirt.
- Enter that figure into Chest circumference (in).
- Read US/UK jacket size — your chest measurement rounded to the nearest even number, the standard suit-sizing convention.
- Read EU jacket size alongside it, calculated as US size plus 10.
- Try on the suggested size and one size on either side where possible — length and cut vary enough by brand that the chest-based size alone won't guarantee a perfect fit.
Worked example — a 40-inch chest for an off-the-rack suit
A customer's chest measures 40 inches around the fullest part. Since 40 is already an even number, US/UK jacket size reads 40 directly — no rounding needed. EU jacket size follows as 40 + 10 = 50.
A slightly different customer measuring 41 inches doesn't get a size 41 — there isn't one — so the calculator rounds up to the nearest even size, 42, and the EU figure follows as 42 + 10 = 52. That rounding step is the reason two people just an inch apart in chest measurement can end up two full jacket sizes apart on the size tag.
Questions
Why are suit and jacket sizes always even numbers?
It's a long-standing menswear manufacturing convention — suit jackets are cut and sold in even-numbered chest sizes (38, 40, 42...) rather than every whole number, so a chest measurement that lands on an odd inch has to round to the nearest even size to match what's actually available on the rack. This calculator applies that same rounding automatically rather than returning a size that doesn't exist.
Is the EU size an exact conversion or an approximation?
An approximation — EU size equals US size plus 10 is a widely used menswear-retail rule of thumb, not a measurement independently derived from the chest circumference. It's a reliable starting point for most mainstream brands, but individual manufacturers sometimes run their own house sizing that shifts a size or so from this offset, so treat the EU figure as a strong estimate to try on rather than a guaranteed match.
Does this account for jacket length (short, regular, long)?
No — this calculator converts chest circumference to a size number only. Many brands separately offer a short, regular, or long length option for the same chest size, typically tied to the wearer's height, which this tool doesn't account for. Use the chest-based size here as your starting point, then check the brand's own length guidance.
What if my chest measurement is exactly between two even numbers, like 39?
The calculator rounds to the nearest even size, so 39 inches rounds up to size 40. If you're genuinely between two even sizes and can try both on, many people in that position size up rather than down for a suit jacket, since a slightly roomier chest is usually easier for a tailor to take in than a too-tight one is to let out.
Should I measure my chest over a shirt or bare-chested?
Over a typical dress shirt, which is closer to how the jacket will actually be worn, rather than bare-chested, which can measure slightly smaller. Keep the tape measure level around the fullest part of the chest and snug against the body without pulling it tight or compressing the shirt underneath.