The formula
boards = area × (1 + allowance) ÷ (covering width × length)
Where it comes from
The catch with tongue and groove is that every board covers less than it measures. The tongue of one sits inside the groove of the next, so the width you see — the covering width — is the nominal width minus that overlap. A 10 cm board with an 8 mm tongue covers 9.2 cm, 8 % less, and that 8 % is eight extra boards in every hundred. It is exactly the shortfall when someone works out the material by dividing by the catalogue width.
How to work it out by hand
- Subtract the tongue from the board width: that is the covering width
- Multiply it by the length for the area each board covers
- Add the cutting allowance to the area you need to cover
- Divide by the area of one board
What is worth knowing
Timber has to acclimatise before it goes up: two or three days in the room it will live in, with the packs open. It moves with humidity, and a board fitted straight from a cold warehouse opens gaps as it dries or buckles as it swells. Staggering the end joints so they do not line up in adjacent rows is not just cosmetic: it spreads the stresses. And when it goes on a ceiling, work out the direction of the boards first: running parallel to the longest wall makes the room look bigger and cuts fewer pieces.