Envelope Size Conversion: mm to Inches Chart for Every Standard

Introduction

Envelope sizes are quoted in two unit systems, and neither one converts cleanly to the other. US commercial sizes were designed in inches, ISO sizes were designed in millimetres, and a conversion that looks trivial — one inch equals 25.4 millimetres — produces answers with long decimals that have to be rounded before they are useful.

That rounding is where ordering mistakes begin. A size quoted as 4.33 inches is really 110 millimetres, and treating the rounded figure as exact is how buyers end up with an envelope that does not quite fit.

This guide gives the conversion formula, the rounding convention, and a full conversion table covering the US commercial sizes and the ISO C, B, and A series in both units.

Why Conversions Cause Problems

Three properties of envelope sizing make conversion harder than it looks.

The practical result is that a converted figure should be treated as a description, not a specification.

The Conversion Formula

The conversion is a single multiplication or division.

Worked examples:

The US figures convert to short decimals and then to round millimetres; the ISO figures convert to long decimals that round to two places.

Rounding Conventions

Published conversions vary in how many decimal places they carry.

For ordering, the safe rule is to specify in the unit the envelope was designed in:

US Commercial Sizes

These sizes are inch-native; the millimetre figures are conversions.

Note that the US sizes convert to millimetre values with one decimal, not to round numbers — a #10 is 104.8 mm wide, not 105 mm.

ISO C-Series Sizes

These are millimetre-native; the inch figures are conversions.

ISO B-Series Sizes

The B-series sits between A sizes, and these are millimetre-native.

ISO A-Series Reference

The A-series defines the paper, not the envelope, but the figures are needed when checking whether contents will fit.

The DL format is the exception: it is sized to hold an A4 sheet folded in thirds rather than to hold a whole A size. For its dimensions, see the DL envelope size page.

Master Conversion Table

The full list in one place, sorted by approximate size.

For the complete size index, see our envelope sizes hub.

Three Ways Buyers Get the Size Wrong

Conversion errors tend to fall into three patterns.

1. Confusing the envelope’s outer size with its inner capacity.
An envelope listed at 110 × 220 mm is the outer dimension. The usable interior is smaller, and a 110 × 220 mm card will not fit inside it.

2. Treating a rounded conversion as exact.
4.33 inches and 110 millimetres differ by about a twentieth of a millimetre, which is harmless on its own — but a size rounded down by a full millimetre can be the difference between a card that slides in and one that catches.

3. Forgetting that postage class is judged on dimensions, not on the name.
A “6×9 envelope” and a “C5 envelope” are close in size, but postal class depends on actual measured dimensions and thickness, not on what the format is called.

Conclusion

Envelope size conversion is a single formula — inches × 25.4 for millimetres, or millimetres ÷ 25.4 for inches — but the answers need care, because US sizes convert to non-round millimetres and ISO sizes convert to long decimals. The reliable rule is to specify in the unit the envelope was designed in: inches for #10, #9, and #6¾; millimetres for DL and the C and B series.

Watch the three failure patterns: outer size is not inner capacity, a rounded figure is not exact, and postage class follows measured dimensions rather than format names. For the full size index, see our envelope sizes hub, or our guide to international envelope sizes.

Need a specific envelope size produced to an exact specification? Request a quote with the dimensions in the unit you want them made to.