How Much Timber Do I Need? UK Sizes, Stock Lengths and Costs
Published 23 July 2026 · Browse all tools
Working out the linear metres is straightforward. Measure each piece, count the quantity, multiply and add. The part that actually costs money is converting that list into the stock lengths UK merchants sell. Pick the wrong stock length and you're paying for offcuts that go straight into the skip. The UK Timber Calculator handles the arithmetic automatically, but understanding the logic behind the numbers means you'll order smarter every time.
Start with the piece list, not the total
Most ordering mistakes start at the same place. Someone writes down a total linear metre figure without recording the individual pieces, drives to the merchant, asks for sixty metres of 47 × 150, and only realises at the cutting stage that the stock lengths they bought don't divide neatly into their joist length.
The right starting point is a cutting list. Write down every piece you need, with its length and section size. For a deck frame, that might read: 6 pieces at 4.2 m, 9 pieces at 2.7 m, 2 pieces at 3.0 m. Only when you have that list can you ask the right question: which stock length minimises offcut waste given these specific cuts?
A joiner I spoke to puts it simply: a total metres figure tells you the minimum timber you need, not the amount you should buy. The difference between the two is offcut waste, and it's decided entirely by how your piece lengths divide into the available stock lengths. Get it right and you'll save 5% to 15% on your order. Get it wrong and that saving goes into a skip.
Takeaway: build the cutting list first. Total linear metres is a check figure, not an ordering number.
UK timber sizes, and why "four by two" isn't 4 by 2
Site language in the UK is still largely imperial names for metric sections. A "four by two" means a piece 4 inches by 2 inches, which would be 101.6 × 50.8 mm. In practice it's 47 × 100 mm in regularised carcassing timber, or 38 × 89 mm in CLS (Canadian Lumber Standard) studwork. The imperial name has stuck. The metric reality is what you order.
| Section (mm) | Called on site | Typical use |
|---|---|---|
| 38 × 89 | 4 by 2 CLS | Internal stud walls |
| 38 × 140 | 6 by 2 CLS | Deeper studwork, insulation depth |
| 47 × 100 | 4 by 2 | Decking joists, stud walls, wall plates |
| 47 × 150 | 6 by 2 | Floor joists, rafters, deck frames |
| 47 × 200 | 8 by 2 | Longer-span floor joists |
| 47 × 225 | 9 by 2 | Long-span joists, loft conversions |
There are three surface conditions you'll encounter.
Sawn timber is rough off the saw, with natural moisture variation of around plus or minus 3 mm in the section. It's cheaper than planed timber and used where surface finish doesn't matter: joists, rafters, carcassing.
Regularised timber (machine regularised, sometimes stamped MR) is sawn but run through a machine to hit a consistent depth. The 47 × 150 regularised joist you pick up from Travis Perkins will measure 47 × 150 to within about a millimetre. It's the standard for structural carcassing where consistent framing matters.
PAR (planed all round) or PSE (planed square edge) is finished timber with four faces planed smooth. You lose 3 to 5 mm per face in the planing, so a nominal 50 × 100 mm sawn section becomes roughly 44 × 94 mm PAR. Specifying PAR and expecting sawn dimensions in the finished piece is a common and expensive mistake.
Takeaway: check whether your section size is sawn, regularised or PAR before you order. Nominal dimensions and finished dimensions are not the same.
Stock lengths — the bit that costs you money
UK merchants stock softwood in 300 mm increments: 1.8, 2.1, 2.4, 3.0, 3.6, 4.2, 4.8, 5.4, and 6.0 m. Not every length is available from every merchant at any given time. Jewson, Travis Perkins, MKM and Selco all carry broadly similar ranges, but spot availability varies by branch and by what's on the delivery that week. If you need a particular length that's out, some merchants will cut to order.
The standard advice is to buy the shortest stock length that covers your piece. Lower cost per length, less to carry, less to waste. That's right most of the time. It isn't universally right, and the exceptions are where the ordering money is.
Offcut waste is not the same as cutting waste. Cutting waste is the thin strip lost to the saw kerf, a few millimetres per cut. Offcut waste is the length left over when your cut doesn't divide evenly into the stock, and it can easily be 20% to 30% of your order if you've picked the wrong stock length.
The right stock length is the one your cut length divides into most cleanly. If your pieces are 2.7 m, a 3.0 m stock gives you one piece per length with 300 mm of waste each time. A 5.4 m stock gives you two pieces exactly, zero waste. The 5.4 m lengths cost more individually, but you buy fewer of them and spend nothing on offcuts.
Merchants will tell you that most customers default to 3.0 m lengths for anything under 3 m, without ever checking whether a longer stock length would halve the order. It's one of those things that's obvious after you've done the comparison once, and it never occurs to most people until after they've done it the expensive way.
Use the timber calculator to compare stock length options before you order, not after.
Takeaway: check whether your cut length divides into two or three per stock length before deciding which stock to buy.
Worked example: a 4.2 m × 3.0 m deck frame in Wakefield
The scenario: a raised deck frame for a terrace in Wakefield. Deck measures 4.2 m × 3.0 m. Joists in 47 × 100 mm C16 at 400 mm centres, spanning the 3.0 m direction. Each joist cuts to 2.7 m after allowing for the perimeter frame. Nine joists needed.
Adding 10% wastage: 9 × 1.10 = 9.9, rounded up to 10 pieces at 2.7 m.
- Option A, 3.0 m stock lengths: one 2.7 m piece per length, so 10 lengths. Timber purchased: 30.00 m. Timber used: 27.00 m. Offcut waste: 3.00 m, or 10% of the order.
- Option B, 5.4 m stock lengths: two 2.7 m pieces per length exactly, so 5 lengths. Timber purchased: 27.00 m. Timber used: 27.00 m. Offcut waste: zero.
- Volume for Option B: 0.047 × 0.100 × 27.00 = 0.127 m³.
- At £2.85 per metre for 47 × 100 C16: Option A costs £85.50, Option B costs £76.95. The longer stock length saves £8.55.
Here the longer stock length is the cheaper one, which is the opposite of what most people expect. It wins because 2.7 m divides into 5.4 m exactly. The saving on nine joists is under a tenner. Scale that logic to a 40-joist suspended floor and it's real money, and the principle is exactly the same: find the stock length your cut divides into cleanly.
Run your own figures in the timber calculator to find the optimal stock length for your job before you drive to the merchant.
Takeaway: stock length is a variable, not a fixed choice. Run the comparison for your cut lengths before you order.
C16 or C24 — does it change the quantity?
Structural timber in the UK is strength-graded to BS EN 338. The grade describes characteristic bending strength: C16 is 16 N/mm², C24 is 24 N/mm². Higher grade means the timber can span further or carry more load at the same section size.
Grading is carried out visually under BS 4978, which sets limits on knot size, slope of grain and other features, or by machine stress grading. Either method produces a grade stamp that should be visible on the timber before you buy it.
The question that comes up is whether C24 means you need fewer or smaller pieces. Sometimes, yes. If TRADA span tables or a structural engineer's calculation show that C24 at 47 × 150 can do what C16 at 47 × 200 would need, you'd use the smaller section. That's a structural design decision, not a quantity one.
For a given section size and piece count, C24 doesn't change the number of lengths you need. It changes the price and the structural performance. C24 typically runs 10% to 15% above C16 in the same section. For ordinary DIY carcassing, non-structural decking and general framing, C16 is the standard choice.
Takeaway: C16 is the default for ordinary framing. Step up to C24 only when span tables or an engineer says you need the extra strength.
What timber costs in 2026
Approximate shelf prices at UK builders' merchants for structural softwood in 2026:
- 47 × 100 mm C16: around £2.85 per metre
- 47 × 150 mm C24: around £4.10 per metre
- 38 × 89 mm CLS: around £2.20 per metre
A typical 2026 quote from a Yorkshire merchant for a pallet of 47 × 150 C24 in 4.8 m lengths runs about £210 to £240 plus VAT for 20 lengths, depending on the day and the account. Softwood prices have been more volatile since 2021 than in the previous decade, linked to European supply, so any figure should be treated as a starting point rather than a fixed number.
Regional variation is real. The North of England typically runs 3% to 5% below the South East on timber, with London and the South West sitting higher. A trade account at Travis Perkins, MKM or Jewson usually beats shelf price by 10% to 20%, and the larger the order the more room there is to negotiate. For a single small job, buying off the shelf is fine. For anything over a cubic metre, an account pays for itself quickly.
Takeaway: shelf prices are the ceiling. A trade account is the floor. If you're buying more than a cubic metre at a time, the account application is worth the afternoon it takes.
What this doesn't answer: what size timber you need
This guide covers quantities: linear metres, stock length selection, cost. It doesn't and can't tell you what section size your timber needs to be.
That question is structural. The answer depends on your spans, your loads, your floor or roof construction, and the conditions the timber sits in. The starting point for most structural timber in England and Wales is Approved Document A, the Building Regulations document covering structure. TRADA publishes span tables that set out which sections work for which spans and spacings, and those tables are the reference for any structural engineer's calculations.
If you're sizing joists, rafters or a load-bearing frame without a structural engineer or building control review, you're outside what any calculator or guide can safely cover. Undersized structural timber doesn't fail gradually. Get a structural engineer to confirm section sizes before you order. Then use the quantity calculation once the sizes are locked.
Takeaway: a quantity calculator tells you how much to buy, not what to buy. Section size is a structural question, not a maths question.