Cornice & Coving Calculator — Lengths, Adhesive & Mitre Cuts

Measure the room perimeter, divide by the coving length, then add 10% for waste from mitre cuts, offcuts and breakage (15% for rooms with many corners or ornate cornice). This calculator adds that wastage for you. Coving normally runs continuously over standard doorways, so only deduct full-height openings or archways that reach the ceiling. A standard 4m x 5m room has an 18m perimeter. For a length-and-width based version, you can also use our Coving Calculator.

Each internal corner requires 2 mitre cuts (one 45° cut on each meeting piece). External corners also need 2 cuts. A standard rectangular room with 4 internal corners needs 8 mitre cuts.

Plasterers most often use a powder cove adhesive such as British Gypsum Gyproc Cove Adhesive, which fixes about 3m of coving per kilogram (a 5kg bag covers roughly 15m, a 12.5kg bag about 37.5m). A 310ml grab-adhesive cartridge covers roughly 3 to 5 metres instead. Apply to both the wall and ceiling contact surfaces. Set the adhesive coverage in this calculator's advanced options to match the product you buy.

Measure every wall and add them up, including the two walls of the notch. A useful check: for a room shaped like a rectangle with a corner cut off, the total is exactly 2 × (overall length + overall width) of the bounding rectangle, because the notch walls line up with the wall sections they replace. Set the corner counts to match: a plain L-shaped room has 5 internal corners and 1 external corner. A chimney breast adds run rather than removing it: include its two short return sides in your perimeter, and count 2 extra internal and 2 extra external corners for it.

Yes. Plain plaster cornice is ordered the same way as coving: divide the perimeter by the run length, add wastage and allow 2 mitre cuts per corner. Ornate or heavy plaster cornice is more fragile and is usually supplied in 2m runs, so consider the upper 15% wastage figure to cover extra breakage and pattern matching.