Timber Calculator — Board Feet & Cost

Timber in the UK is sold by the linear metre for structural work such as joists, studwork and battens, or by the cubic metre once quantities get large. Board feet, the unit used on North American plans, convert easily enough, since one board foot is a twelfth of a cubic foot, or 0.00236 m³. The common structural sections here are 47 by 100 mm, used for C16 joists, along with 47 by 150 mm and 47 by 200 mm.

This calculator works in linear metres, cubic metres and board feet at the same time. Enter the cross-section dimensions and the total length you need, and it converts between all three units. For a stud wall it also estimates the number of studs, spaced at either 400 mm or 600 mm centres, and adds the top and bottom plates the wall needs.

Structural timber sold in Britain must be strength-graded to BS EN 14081, and C16 and C24 are the classes you will meet in a merchant's yard. C16 is the standard load-bearing grade, while C24 is stronger and specified for joists and rafters carrying longer spans. Anything outdoors should be pressure-treated to BS 8417, Use Class 3 above ground and Use Class 4 where it sits in ground contact. Merchants including Travis Perkins and Jewson increasingly want FSC certification for sustainable sourcing, with PEFC as the alternative scheme, while specialists such as ReSawn UK deal in reclaimed stock. Building Regulations normally call for structural calculations on joists and rafters spanning more than 2.4 m.

Species matters as much as grade. CLS, the Canadian Lumber Standard section planed on all sides and typically 38 by 89 mm, is what most stud walls are built from. Pressure-treated softwood, the green Tanalised timber sold everywhere, carries a 15 to 30 year rating in ground contact. Cedar is naturally durable and weathers without treatment, larch is the usual choice for outdoor cladding, oak is the premium hardwood for framing and feature beams, and spruce or pine is the cheapest option for framing indoors.

Quantities come from a simple volume calculation of length by width by thickness, or in linear metres from the total run divided by the standard length. British merchants stock 2.4 m, 3 m, 3.6 m, 4.2 m, 4.8 m and 6 m lengths, and cut waste falls the longer the pieces you buy. Take a partition wall 4 m long and 2.4 m high at 600 mm centres: it needs 8 studs of 2.4 m plus 2 plates of 4 m, which is 27.2 m of CLS, or about 29 m once waste is allowed for. Add a header over any doorway and double the studs at its sides. Insulation between studs is 100 mm Rockwool at about £10 per m², or PIR at £20 to £25.

On price, a 2.4 m length of 38 by 89 mm CLS costs £4 to £8. A 3 m length of C16 47 by 100 mm is £8 to £14, and a 4.2 m length of C24 47 by 150 mm £20 to £35. Pressure-treated decking of 32 by 144 mm in 3.6 m boards costs £15 to £25 each, cedar cladding £40 to £80 per m², and an oak structural beam £80 to £200 per linear metre. Sheet materials run £40 to £70 for 18 mm plywood at 2.4 by 1.2 m and £25 to £40 for 18 mm OSB. Prices rose 30 to 50% through the supply crisis of 2021 and 2022 and have been steadier since 2024.