Backing Board Calculator — Size & Material

Backing board = Rebate size − Clearance (total)

The backing board sits behind the artwork and mount/mat at the very rear of the frame, secured with framer's points, brads or turn buttons. It is cut a little smaller than the frame's rear aperture (the rebate) so it drops in without jamming and leaves room for the materials to expand and contract — a total clearance of 1–3 mm is standard (UK framers typically work to ~3 mm; this defaults to 2 mm).

Weight is estimated from the cut area and the board's areal mass: MDF (3 mm) ≈ 2250 g/m², greyboard (2 mm) ≈ 1500 g/m², corrugated plastic (4 mm Correx) ≈ 700 g/m² and foam core (5 mm) ≈ 290 g/m². Sandwich boards are quoted by sheet weight, not bulk density, so this reflects real supplier figures.