4x6 label size in mm, and how it differs from A6 and 100 x 150

A 4 x 6 inch shipping label is 101.6 x 152.4 mm. An inch is defined as exactly 25.4 mm, so that is an exact conversion, not a rounded one.

The confusion is rarely about that number. It is about the two sizes that sit next to it on shopping listings — 100 x 150 mm and A6 — where one is effectively the same label and the other is not.

The sizes side by side

The last column is the one that matters, and it is the one nobody prints on the packaging.

100 x 150 mm: the same label, 1.6% smaller

It is 1.6 mm narrower and 2.4 mm shorter. Those look like two unrelated differences and they are not — both are the same 1.6% reduction, because 100 x 150 has exactly the 1 : 1.5 proportions of a 4 x 6.

Same shape, marginally smaller. That is why every thermal printer in this class treats them as one stock, why rolls are sold under either name, and why you can buy whichever is cheaper where you are and select whichever size your print dialog offers.

A6 is a different shape, not just a different size

A6 is 105 x 148 mm: wider than a 4x6 label and shorter. It belongs to the ISO A series, whose pages all share the 1 : 1.41 proportion — the same shape as A4, halved twice.

So a 4x6 design does not simply sit on A6 slightly cropped. It has to shrink about 3% to fit the shorter height, and then leaves a band of unused width down the side. On a die-cut A6 roll that band is wasted label, and on a 4x6 roll A6 stock does not track properly at all.

This is worth checking before ordering, because listings use the terms loosely. “A6 shipping labels” is frequently used to describe 100 x 150 mm rolls that are not A6 at all — read the millimetres rather than the name.

Why a label on an A4 page looks so small

A 4x6 label occupies 24.8% of an A4 page by area, and 25.7% of a Letter page. Roughly a quarter, either way.

That single figure explains the most common complaint in label printing. When a carrier hands you the label on a full page and you send that page to a 4x6 roll, the printer fits the whole page onto the label — so everything on it lands at about 48% of its intended size on A4, or 47% from Letter.

Text at 48% is readable if you squint. A barcode at 48% has bars and gaps at half their designed width, which is where scanners start to fail — and you discover that at the drop-off counter rather than at the printer.

This is why the label has to be cut out of the page rather than scaled down. Cropping keeps it at 100%; fitting halves it.

If you need a label at these exact dimensions out of a bigger page, our free browser tool crops it without rescaling and tells you the resulting size in millimetres, so you can check it against the table above before printing.

Setting a custom size on macOS

When macOS does not offer 4x6 for your printer you can define it yourself — and the dimensions are the easy half.

The margins matter more than the numbers. A custom size with the default non-printable margins leaves a printable area smaller than the label, and macOS then shrinks your content to fit that reduced area. You get a small label from a correctly-sized paper definition, which is a genuinely confusing place to end up.

Set all four margins to zero when you create the size. The full walkthrough is in how to print 4x6 labels on a Mac.

Where the exact millimetres are worth using

For most printing, 100 x 150 and 101.6 x 152.4 are interchangeable. Two cases where they are not:

• Die-cut rolls. The label boundary is fixed by the roll, so the printer needs the real dimensions to place content within it. Where a driver names the roll — Brother's DK sizes, for instance — pick it by name rather than typing a measurement.
• Long batches. A 1.6% error per label is invisible on one and accumulates over a hundred, which is how labels start drifting down the roll partway through a run.