An A3 sheet made from two A4 sheets
A3 (297 x 420 mm) is exactly two A4 sheets. Turn the A4 landscape, stack two of them, and you have an A3 notice or menu without owning an A3 printer. There is a single seam, which makes this the easiest tiling job there is.
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One straight seam, nothing to line up twice
Rotating the A4 to landscape (297 x 210 mm) and stacking two of them wastes the least paper and leaves a single horizontal join. Align it once against a ruler and you are done. There is no cross-shaped junction of four corners, which is where four-sheet and nine-sheet posters visibly go wrong.
Before you go hunting for an A3 printer
An A3-capable home printer costs a few hundred dollars, and a copy shop charges roughly a dollar for a colour A3 sheet. For a notice you will use once, printing two A4 pages on the machine already on your desk is usually faster than the trip. On a noticeboard, viewed from a metre away, the seam is essentially invisible.
What the 5 mm flap is for
The default 5 mm overlap assumes you trim the upper sheet along the fold line and lay it 5 mm over the lower one. Use double-sided tape or spray adhesive rather than a glue stick, which makes thin paper cockle. Reinforce the back of the seam with wide tape so the poster does not tear along the join while it is up.
Frequently asked questions
What are A3 dimensions exactly?
297 by 420 millimetres. Two A4 sheets turned landscape and stacked come to precisely 297 by 420 mm, so nothing is wasted.
How do I hide the seam?
Arrange the artwork so the join falls across empty space rather than a face or a headline. Raising the overlap to 10 mm widens the shared strip so a small misalignment still leaves no white gap.
Does my file get uploaded?
No. Slicing and PDF writing happen inside the browser, so confidential internal artwork never leaves the machine you are working on.