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Work out how many plasterboards you need for a ceiling. Choose the board size and thickness and the joist spacing, enter the ceiling size, and press Calculate to see the boards to order, a layout with the ends on joists, the screws, the weight and the tape and plaster to finish.
| Board | Usable Length | Boards | Weight Each |
|---|
The same ceiling, thickness and joist spacing with each board size.
You get the boards to order, how much of each board can be used with its ends on joists, the layout and waste, the weight, the drywall screws, the scrim tape and the finish plaster to skim it. A plan drawing shows the joists and the staggered boards, a table compares the three board sizes, and you can copy the result or save it as a PDF. For walls as well as ceilings, use our plasterboard calculator.
Dividing the ceiling area by the board area is a rough start, but a ceiling has a rule that walls do not: every board end must finish on a joist. So the calculator works in rows:
Boards = rows × length across the joists ÷ usable board length, rounded up
The rows run across the joists, one board width wide. The usable length is the longest part of a board that starts and ends on a joist. Offcuts start the next row, which also staggers the end joints as they should be. A thin last row, half a board wide or less, can be cut from the same boards as the row before it.
| Board Length | 400 mm Joists | 450 mm Joists | 600 mm Joists |
|---|---|---|---|
| 1800 mm | 1600 mm usable | 1800 mm, no waste | 1800 mm, no waste |
| 2400 mm | 2400 mm, no waste | 2250 mm usable | 2400 mm, no waste |
| 3000 mm | 2800 mm usable | 2700 mm usable | 3000 mm, no waste |
Most UK homes have ceiling joists at 400 mm or 600 mm centres, and some older or loft conversions have 450 mm. Measure from the centre of one joist to the next in several places before you buy. If a board does not divide evenly by the joist spacing, every board has to be trimmed so its end lands on a joist, which adds waste; the calculator shows this and the table lets you compare sizes. Where an end cannot land on a joist, fix a timber noggin between the joists to support it.
| Thickness | Weight | 2400 × 1200 Board | Maximum Joist Centres |
|---|---|---|---|
| 9.5 mm | About 6.3 kg/m² | About 18 kg | 450 mm |
| 12.5 mm | About 8.0 kg/m² | About 23 kg | 600 mm |
| 15 mm | About 9.8 kg/m² | About 28 kg | 600 mm |
12.5 mm is the usual choice for domestic ceilings and is fine on joists up to 600 mm apart. 9.5 mm boards are lighter but should only be used on joists up to about 450 mm apart. 15 mm boards are stiffer and give better fire and sound performance, at the cost of more weight. Weights are from the British Gypsum data for Gyproc WallBoard; other makes are similar.
Screw spacings and penetration follow British Gypsum's guidance, such as its timber joist ceiling specification. Sink the screw heads just below the paper without tearing it.
Use moisture-resistant board in bathrooms and other damp rooms, and treat it with a bonding agent before plastering. Ceilings that must resist fire, such as under a loft conversion or between a garage and a room above, may need special fire-rated boards, two layers and noggins under every board edge, so follow the specification and Building Regulations. Sound-rated boards and resilient bars help between floors. In all these cases the board count from the calculator still applies, but check the board type and number of layers.
Once the boards are up, either tape and fill the joints with jointing compound for a dry-lined finish, or tape the joints with scrim and skim the whole ceiling with finish plaster. The calculator allows the length of every joint for the scrim, in 90 m rolls, and about one 25 kg bag of finish plaster for every 10 m², plus 10%. For a more detailed plaster estimate, use our plaster skim coat calculator. If you are building new ceiling joists, our timber calculator works out the lengths.
Bedroom ceiling: 4.2 m across the joists and 3.6 m along them, joists at 400 mm, 12.5 mm board in 2400 × 1200 mm.
The same ceiling on 450 mm joists:
The usable length of a board is its length rounded down to a whole number of joist spaces. The number of rows is the width along the joists divided by the board width; a last row of half a board or less counts as half a row. The boards are the rows times the length across the joists divided by the usable length, rounded up. Waste compares the board area bought with the ceiling area. Screws are based on a full board with screws at 230 mm along each joist and 150 mm along the two ends, scaled to the ceiling area, plus 5%, rounded up to 50. Scrim covers the joints between rows and between board ends, and finish plaster is one bag for every 10 m² plus 10%.
Work in rows across the joists: rows × ceiling length ÷ usable board length. A 4.2 × 3.6 m ceiling on 400 mm joists needs 6 boards of 2400 × 1200 mm.
With the long edges across the joists and the ends on joists, staggered from row to row.
12.5 mm is usual on joists up to 600 mm apart. 9.5 mm should only be used up to about 450 mm centres.
About 230 mm along each joist and 150 mm along board ends, following British Gypsum guidance.
Long enough to go 25 mm into the timber: 38 mm for 12.5 mm board and 42 mm for 15 mm board.
A 2400 × 1200 mm sheet of 12.5 mm Gyproc WallBoard weighs about 23 kg; 15 mm is about 28 kg.
Where a board end does not land on a joist, yes. Some fire-rated ceilings need noggins under every board edge.
It is much easier with a hired board lifter, or use smaller 1800 × 900 mm boards.
Board weights are from British Gypsum data for Gyproc WallBoard. Screw spacing and penetration follow British Gypsum ceiling specifications. Maximum joist centres are common guidance; follow the board maker's instructions. Reviewed November 2026.