For a poster going into an existing frame, order the size of the paper the frame holds—not the frame’s outside dimensions. The moulding adds width and height that do not belong in your print measurement.
Where possible, remove the existing insert and measure it flat. Work in centimetres and record width first, then height. For an awkward dimension, 325 mm is 32.5 cm: divide millimetres by ten.
A frame without a mount
Check the insert against the frame’s stated print size. If the insert is missing, the backing or internal recess can help you establish the intended size. Measure carefully and do not force glass or dismantle a fragile frame unnecessarily.
Some frames overlap the print slightly at its edges. That means a print that fits physically can still have a small amount of its image hidden. Keep essential lettering and faces away from that boundary.
A frame with a mount
A mount creates two separate measurements: the outer paper space and the visible opening. You need enough paper for the print to sit securely behind the opening; ordering exactly the opening size may leave no overlap to hold it.
For example, a frame may hold a 30 × 40 cm sheet while its mount reveals a smaller area. If you order a 30 × 40 cm photograph printed across the sheet, the mount can hide substantial parts of the picture. Plan which part should be visible before ordering. A white border is not automatically included just because a frame has a mount.
Check the photograph’s proportions
Changing the paper dimensions can change the crop. After entering a custom size, inspect all four edges and confirm portrait or landscape orientation. Do not stretch the photograph to make it fit.
Write down the required paper size and, separately, any mount opening. If those measurements are uncertain, resolve them before placing the order. Explore custom-size poster printing and use the preview to check the photograph against the dimensions you need.