The 4 × 6 shipping label
The 100 × 150 mm label — sold nearly everywhere as 4 × 6 inches — is the format every parcel carrier and every desktop thermal printer agrees on. It is also the one format where the usual advice about page margins does not apply, because a roll printer does not have a page. Here is what actually matters.
100 × 150 mm and 4 × 6 inches are not identical
Four by six inches is 101.6 × 152.4 mm, so the metric label is about 1.6 mm narrower and 2.4 mm shorter. In practice this rarely matters: thermal printers detect the media with a gap or black-mark sensor and feed one label at a time, so the artwork is placed against the label the printer found rather than against a fixed page. What does matter is the aspect ratio and that nothing important sits within about 3 mm of any edge, because feed alignment varies slightly label to label.
What to send the printer
Export the label at its real size — a single-label PDF, one label per page, no page margins. In the printer settings choose the label media, not A4 or Letter, and set scaling to none. On desktop thermal printers the two settings that cause most bad prints are a paper size left on A4, which shrinks the label to a corner, and darkness set too low, which produces bars that scan on the desk under good light and fail on a handheld scanner. If your printer offers a media calibration routine, run it once after loading a new roll size.
Barcodes on a shipping label
Carrier tracking barcodes are normally Code 128, and they need to survive being scanned at speed, at an angle, through tape. Give the barcode the full width you can, keep the clear space either side, and never stretch it vertically to fill a gap — a barcode's height is about giving the scanner a target, but its width is data, and a horizontally squashed barcode stops being readable. If you also print a QR code, it needs its own clear border on all four sides.
When not to make your own
If a carrier issues you a label, print theirs. It carries their routing data, their tracking number and often a machine-readable block encoding the depot, and a lookalike you produce yourself will not be accepted at the counter. Use this tool for the labels that are yours to produce: return addresses, box content and handling labels, internal routing, and shipping labels for deliveries you handle yourself.
Frequently asked questions
Is a 4 × 6 label the same as 100 × 150 mm?
Not exactly — 4 × 6 inches is 101.6 × 152.4 mm — but thermal printers feed to the media they detect, so both work in practice as long as you keep content a few millimetres away from the edges.
Can I print a shipping label on A4 paper?
Yes, and it is common: print at actual size and cut it out, or use an A4 sheet layout with two large labels. It is not thermal, so use tape or a document pouch to protect it.
Why does my thermal label print tiny in one corner?
The printer's paper size is still set to A4 or Letter. Select the label media in the driver and set scaling to none.
My barcode scans on the desk but not at the depot.
Usually print darkness or a barcode squeezed too narrow. Increase darkness one step, give the barcode more width, and keep the clear space either side of it empty.
Should I use a thermal printer or a laser?
A thermal printer if you ship regularly: no ink, one label per feed, and the right stock. For occasional parcels an A4 sheet and a laser is perfectly acceptable.
Can I make my own version of a carrier's label?
No. Print the label the carrier issued you. Produce your own only for labels that are genuinely yours, such as return addresses and content labels.
Labels that line up with the sheet you actually bought. Label Sheet lays out address labels, SKU barcodes and shipping labels at the exact millimetres of your sheet, then exports a PDF that prints at 100 per cent. Your data stays in the browser.
Make a label sheet in the browser