Timber Calculator UK

British merchants sell structural softwood in millimetres, and the size on the label is the size you get. A 47 × 150 mm joist really measures 47 mm by 150 mm, because it's been regularised — machined to a consistent section after sawing.

That trips people up when they've grown up calling it "six by two". The imperial names survive in conversation but not on the invoice, and the conversion isn't clean: nominal 6" × 2" lands at 150 × 47 mm, not 152 × 51 mm. Order by the metric figure and you'll get what you expect.

Planed timber — sold as PAR (planed all round) or PSE (planed square edge) — loses roughly 3 to 5 mm per face during machining. A piece described as 25 × 150 mm PAR typically arrives closer to 20 × 145 mm. If the finished dimension matters, ask the merchant for the actual planed size rather than assuming the nominal one.

Structural softwood in the UK is graded to BS EN 338, and the two classes you'll be offered are C16 and C24. The number is the characteristic bending strength in newtons per square millimetre, so C24 is the stronger of the two.

C24 costs roughly 10 to 15% more than C16 at merchant prices, and for most domestic joists and rafters it lets you either span further at the same section or use a shallower section over the same span. Grading itself is carried out under BS 4978 for visual grading, and machine-graded timber falls under BS EN 14081. Look for the grade stamp on the face of the timber — if there isn't one, it isn't graded, and it shouldn't be used structurally.

This calculator doesn't care which grade you pick. Volume and length are identical either way. The grade only changes the price and what the timber is allowed to do.

A single-storey rear extension needs 12 floor joists in 47 × 150 mm C24, each cut to 3.6 m. Add 5% wastage for cutting and the odd damaged length.

Piece count with wastage: 12 × 1.05 = 12.6, rounded up to 13 pieces.

Now the part most calculators skip — which stock length to buy.

Option A — buy 4.8 m lengths. Only one 3.6 m piece comes out of a 4.8 m length, so that's 13 lengths, or 62.40 m of timber purchased. You use 46.80 m of it. The remaining 15.60 m is offcut — 25% of everything you paid for. At around £4.10 a metre for 47 × 150 C24, that's £255.84.

Option B — buy 3.6 m lengths. One piece per length again, 13 lengths, but now you've bought 46.80 m and used all of it. Offcut: zero. Cost: £191.88.

Same job, same timber, same wastage allowance — £63.96 difference, purely from the length on the order.

For the record, Option B works out at 0.330 m³, or 139.8 board feet if you need that conversion.

Nearly every timber calculator online gives you a linear metre total and stops. That figure is close to useless at the trade counter, because no merchant sells you 46.8 m of timber. They sell 3.6 m lengths, or 4.8 m lengths, and you buy whole ones.

The gap between what you need and what you have to buy is real money, and it doesn't shrink by lowering your wastage percentage. Wastage covers cuts and damage. Offcut waste is a separate thing entirely — it's the leftover created by the mismatch between your cut length and the stock length.

Sometimes the longer stock length wins. If you need 2.7 m pieces, a 5.4 m length gives you exactly two with nothing left, while a 3.0 m length leaves 300 mm on the floor every time. The rule isn't "buy short" or "buy long" — it's buy the length your cut divides into cleanly.

In practice most UK merchants stock 2.4, 3.0, 3.6, 4.2, 4.8 and 5.4 m in common carcassing sections, and anything over 6 m is usually a special order with a lead time.

Almost never. The board foot is a US timber unit equal to a piece 12 inches square and one inch thick, and no British merchant prices in it. One cubic metre is 423.78 board feet.

The conversion is here for two situations: you're working from an American woodworking plan, or you're comparing a quote from a specialist hardwood importer who still quotes in board feet. For anything bought from a UK builders' merchant, work in metres and cubic metres and ignore the board foot output entirely.

This tool works out quantity, volume and cost. It does not tell you what size timber your job needs structurally.

Sizing a joist or rafter depends on span, spacing, load, and grade, and it's governed by Approved Document A of the Building Regulations. The published span tables — TRADA's, and those in the relevant Eurocode guidance — are the right reference for that, and anything structural on a domestic project should be signed off by a structural engineer or building control. Do not size a joist from a quantity calculator, including this one.