Drywall calculator

Area first, sheets second

Start with the wall area: the perimeter multiplied by the height, which is 2 × (length + width) × height. Add the ceiling if you are boarding it. Then subtract the openings, and only then divide by the sheet size.

Dividing area by sheet area gives a theoretical minimum that no real job achieves, because sheets have to be cut to fit and offcuts are rarely reusable. That is what the waste allowance is for.

How much waste to allow

• 10% — a simple rectangular room with few openings, worked by someone competent.
• 15% — several openings, or a room with awkward corners.
• 20% — sloped ceilings, dormers, curved walls, or a first attempt.

Buying one sheet too many costs a few pounds. Running one short means a second trip, and if the batch has changed you may get a slightly different board thickness. Round up.

Whether to deduct openings at all

Many professionals do not subtract doors and windows, and board straight over the opening before cutting it out. That gives a cleaner result — the joint does not land at the corner of the opening, which is exactly where cracks start — at the cost of wasting the cut-out.

This calculator deducts them, which gives the lower number. If you intend to board over openings, set the waste allowance higher, around 15-20%, to compensate.

Sheet size and orientation

Larger sheets mean fewer joints, and joints are where the work is. On a wall over 2.4 m long, a 3 m or 3.6 m board can remove a butt joint entirely. Against that, big sheets are heavy — a 4 × 12 ft board weighs around 40 kg — and are genuinely difficult to handle alone.

Hanging horizontally is standard on walls in most of the world, because it puts the long tapered edges together in one continuous horizontal joint at a comfortable height, and it crosses more studs for rigidity. Ceilings are usually run perpendicular to the joists for the same reason.

The consumables

The screw estimate assumes roughly 32 per sheet, which suits fixings at 300-400 mm centres on studs and closer spacing on ceilings. Use 32 mm screws for 12.5 mm board on timber.

Tape and compound are estimated from net area rather than sheet count, since they follow joint length. Allow more compound than you think — three coats is normal, each wider than the last, and the third is mostly sanded off. Setting compound for the first coat and lightweight for the finish is the usual approach.

Common questions