Measure paper and image separately
A print may have a white border, so its paper size can exceed its image area. Enter both in the frame size checker. The paper needs to fit the frame; the mat opening determines how much of the image remains visible.
Use the frame’s internal fitting dimensions, not the outer edge of its moulding. Confirm the actual usable space with the manufacturer when only a nominal size is listed.
What happens when the print is centred?
For the stated sizes, centring the paper leaves 0.4 cm on each side and 0.56 cm above and below. Those gaps are arithmetic differences, not known mounting tolerances.
A mat opening smaller than the paper can overlap its edges. If it is also smaller than the image, it covers some of the printed artwork. The checker assumes a centred image and opening and shows both relationships.
Check tolerances before buying
Frame depth, backing, glazing and manufacturing tolerances can affect the final fit. The tool cannot infer these from width and height, so it reports them as unknown.
For other sizes, the length converter can convert centimetres, millimetres, inches and feet. Keep the unrounded dimensions for your fit comparison.