How do I work out how much timber to buy for a job?

The mistake almost everyone makes is dividing total linear metres by the length of a stick. That ignores the offcut left at the end of each length, which is where the timber actually goes. Doing it properly takes ten minutes at the kitchen table and prevents both a second trip and a garage full of surplus.

What you'll need

• A drawing or a sketch with real dimensions
• The merchant's stock lengths for the section you want
• A calculator or a spreadsheet

Step by step

1. Draw it, even roughly, and dimension it.A sketch with real numbers on it. You cannot count what you have not drawn, and the pieces people forget — noggins, plates, trimmers, packing — are always the ones that were never on a drawing.
2. Write a cutting list, one line per piece.Part name, quantity, finished length, and the section. Group by section, because you order by section. Include everything: studs, head and sole plates, noggins, battens, and anything that will be cut from the same stock.
3. Find out what lengths the merchant actually stocks.UK softwood normally comes in metric lengths from around 1.8 metres up in 300mm steps — 2.4, 3.0, 3.6, 4.2, 4.8 metres are the common ones. Some sections are only stocked in a few lengths. Check before you calculate, because the answer depends on it.
4. Fit your pieces into stock lengths, not into total metres.For each stock length, work out how many of your pieces come out of one stick and what is left over. Three 1.1 metre pieces come out of a 3.6 metre length with 300mm to spare; they will not come out of a 3.0 metre length. Count the sticks.
5. Look for combinations that use the offcuts.If you need both long and short pieces from the same section, pair them: a 2.4 metre piece and two 600mm pieces come neatly out of a 3.6 metre length. This is where the real savings are, and it is exactly what a merchant's estimator does.
6. Add a saw kerf allowance if the pieces are tight.Every cut removes a few millimetres. Three pieces cut from a stick lose two kerfs. It rarely matters, but where a stock length divides exactly into your piece length it matters entirely — you will be a few millimetres short on the last piece.
7. Add waste.Around 10 per cent on simple straight work, 15 to 20 per cent where there are mitres, angles, awkward lengths, or where you are buying unsorted stock and expect to reject some. Round up to whole lengths afterwards, because that is how it is sold.
8. Convert to the merchant's units and check the price both ways.Some sections are priced by the metre and some by the length. Both are fine, but compare like with like: a per-metre price times the stock length is what a stick costs.
9. Order the fixings and consumables at the same time.Screws, nails, joist hangers, brackets and preservative for cut ends. Counting these off the same cutting list is much more reliable than guessing at the counter.

Tips

• Ask the merchant to estimate for you. For a straightforward job like a stud wall or a fence, giving them the dimensions and letting their estimator produce the list is free, quick and usually more accurate than doing it yourself.
• Buy one extra length of each section. It costs a fraction of the trip back and it covers the board that turns out to be twisted and the cut in the wrong place.
• If you are buying for a job with lots of short pieces, buy longer sticks. Waste falls sharply as stock length rises because there are fewer end offcuts.
• Keep the cutting list and tick pieces off as you cut them. It is the only way to notice you have cut a stud to the wrong length before you cut nine more.

Common mistakes

• Dividing total metres by stock length — It ignores the unusable offcut at the end of every stick, and it consistently under-orders. Four 1.0 metre pieces need two 2.4 metre lengths, not 4 divided by 2.4.
• Forgetting that PAR is smaller than nominal — A frame designed around 50mm uprights and built from 44mm PAR comes out short. Use the measured section size when working out how many pieces come out of a width.
• Not allowing for splits at the ends of boards — End checks are normal and you lose 50 to 100mm from many boards. If your pieces use the full stock length exactly, you have no margin at all.

Questions people ask