To make sure your mount fits perfectly, you’ll need two key measurements — the external size (the size that fits inside your frame) and the aperture size (the window where your picture shows through).
Measure the opening at the back of your frame, not the visible area from the front.
The back opening gives you the true internal size your mount needs to fit.
The front view can be deceiving because of the frame’s rebate — that’s the small lip that overlaps the glass and mount to hold them in place.
For example, if the back opening of your frame measures 30 cm × 40 cm, your mount’s external size should also be 30 cm × 40 cm.
This is the size of the visible opening where your picture or artwork will show through.
To hold your image securely, the aperture should be slightly smaller than your actual picture — around 5–6 mm (¼ inch) smaller overall.
For example, if your picture is 20 cm × 30 cm, the mount aperture should be 19.5 cm × 29.5 cm.
This small reduction gives a neat overlap all the way around so your image doesn’t slip through.
The border (the area between the aperture and the outer edge) should ideally be at least 2.5 cm wide to look balanced and keep the mount strong and stable.
If your frame’s back opening measures 30 cm × 40 cm and your picture is 20 cm × 30 cm, and you allow a 5 mm overlap on each side:
So your mount will have a 5.25 cm border on each side — perfectly balanced and a snug fit inside your frame.
