Sawn versus finished timber sizes

At a glance

Millimetres. The UK figures are one or two merchants’ planed softwood listings and one merchant’s advice for rough oak, not a rule; we could not read BS EN 1313. The US figures are from the Wood Handbook and show the pattern. Ask your partner workshop for the allowances it works to.

Three states of a timber size

A board has several sizes in its life, and the one on the label is rarely the one on the drawing. Four words do most of the damage:

Two different losses are easily mixed up. Planing a sawn section to PAR takes a few millimetres off each dimension: 4 to 6 mm in the planed softwood listings, 4 to 7 mm in the American hardwood table. Making a finished component from rough sawn hardwood takes more, because the rough board is uneven and, if wide, may cup or twist: the merchant advice recorded on our standard-sizes sheet is to buy at least 10 mm wider and 10 to 15 mm thicker than the finished part. When a quote says ‘25 mm oak’, ask which of these it means.

Why 25 mm is not 25 mm

• Planing makes faces flat, smooth and square, and the sawn surface and its saw marks and distortion have to come off first. The listings take 2 to 2.25 mm from each face and 2.5 to 3 mm from each edge (our arithmetic, assuming the same is taken from both sides: 25 − 20.5 = 4.5 in thickness, so 2.25 a face; 75 − 69 = 6 in width, so 3 an edge).
• The sawn size is a target with a tolerance. BS EN 1313-1 sets permitted deviations on the thickness and width of softwood sawn timber at a reference moisture content, and BS EN 1313-2 does so for hardwood. We read only their listings, not the numbers, so we give none: ask the partner what it allows.
• A size belongs to a moisture content. The Wood Handbook defines US dry sizes so that green lumber surfaced to the green size shrinks to about the dry size at roughly 15 per cent moisture content. Wood goes on moving after planing: with the figures on our timber movement sheet (American white oak, plain sawn, a 5.1 point swing in moisture content) a finished 20.5 mm thickness changes by 0.39 mm, and a 150 mm width by far more.
• The name stays while the piece shrinks. A US 2 x 4 is 38 x 89 mm, not 50.8 x 101.6, and a nominal 1 in. board is 19 mm: the Wood Handbook shows the American standard fixing the actual sizes behind the old names.

The American hardwood table shows the pattern across the thickness range, rough against surfaced on two sides:

It is a US table for hardwood lumber, not a UK one. The point it makes carries over: the planed size is a few millimetres under the rough size, and the planed sizes fall on a ladder of their own.

From the finished size back to the order size

The partner workshop works backwards: finished size, plus what planing takes, up to the next size it can buy. A drawing should be made with that in mind, because the cost of a part follows the section bought, not the section drawn.

Example 1: a finished 40 x 40 mm leg. With the planed softwood allowance above (4.5 mm in the 25 x 75 listing) the section needs at least 44.5 mm each way, so a 50 x 50 mm sawn section is bought and 36 per cent of its cross-section is planed away ((50 × 50 − 40 × 40) ÷ (50 × 50)). If the leg is rough sawn oak and the merchant’s advice of 10 to 15 mm extra in thickness holds, 50 to 55 mm is needed in the one direction.

Example 2: the stock step. Using the American hardwood ladder as the model, a part drawn 21 mm thick comes from the 25 mm stock, and a part drawn 22 mm needs the 32 mm stock:

One millimetre of drawing is a whole step of stock, and about twice the wood is planed away. The partner’s own thicknesses will differ from the US ladder, so ask for them and draw just under a step rather than just over it. The sheet on designing to standard sizes does the same for boards.

Mouldings, trim and boards

Mouldings and trim are sold by a name and a finish size beside it: 19 x 50 with a finish size of 15 x 45 is the common trap, and the two are not interchangeable on a drawing. The old British Standard for softwood trim, BS 584:1967, was superseded in 1990 by BS 1186-3 (a distributor’s listing gives its abstract as the quality, design and dimensions of architraves, skirtings, picture rails, cover fillets, quadrants, half-round beads and scotias); we did not read it. Our architrave and skirting sheets deal with the sections themselves.

Boards are the opposite case: a sheet sold as 18 mm is made to a thickness tolerance, not planed down. The data sheets we read give 17.1 to 18.1 mm for an 18 mm birch plywood, 18 mm plus or minus 0.2 (sanded) for MDF.H and plus or minus 0.3 mm for P2 chipboard. If an 18 mm board is let into an 18 mm groove, plan for those tolerances. Veneering and edge banding add to the finished thickness again.

What to write on the drawing

• State that sizes are finished, and mark any nominal size ‘nom.’ with its finished size beside it.
• Give the order thickness x width x length. The US Architectural Woodwork Standards use that order for lumber, and a section drawn as 40 x 20 is read two ways without it.
• Ask for the sawn size and allowance the maker uses, for each timber section, before the drawings are approved.
• Say the moisture content the size is for, and the finish applied: sanding and lacquer change the last tenth of a millimetre.
• Leave a margin to the next step of stock when the design has freedom, and flag the dimension in the drawing.

All timber sizes on drawings and schedules are finished sizes after planing, sanding, veneering and edge banding. A nominal size is marked 'nom.' and its finished size is given beside it. Dimensions are thickness x width x length.

[Oak] sections finished [40 x 40] mm, from [kiln-dried] stock at [8 to 12] per cent moisture content. The maker is to state the sawn size it will buy for each section and the planing allowance it works to, before drawings are approved.

[Skirting / architrave] finished [15 x 45] mm, [planed softwood / hardwood], to drawing [number]. A section of the same nominal size is not to be substituted without the designer's approval.

Finished thickness and width within [+/- 0.5] mm and length within [+/- 2] mm of the drawing, at [ ] per cent moisture content. The maker is to state the tolerance it works to if it differs.

Standards and sources

• USDA Forest Products Laboratory, Wood Handbook FPL-GTR-282 (2021), Chapter 6: Tables 6-4 and 6-6 and the text on dry and green sizes. US practice. Read 5 Nov 2026.
• BS EN 1313-1:2010 and BS EN 1313-2:1999 Round and sawn timber: permitted deviations and preferred sizes (softwood; hardwood): BSI listings, and the contents of -1 from BSI's preview; the numbers were not read.
• BS 584:1967 Specification for wood trim (softwood), superseded by BS 1186-3:1990: a distributor's listing; BS 1186-3 not read.
• Architectural Woodwork Standards, 1st edition (2009), Section 3 Lumber: 1.2.2, 1.5.1. US practice.
• Merchant listings and product data sheets, read 5 Nov 2026: two planed softwood listings; plywood, MDF.H and chipboard data sheets. Names in the studio's notes file.

Questions we are asked

What does PAR mean on timber?

PAR means planed all round: all four faces have been planed, so the piece is smaller than its sawn size in both thickness and width. It is trade shorthand, not a term we found defined in a source we read. A UK merchant lists such sections with a finish size beside the name, for example 19 x 50 with a finish size of 15 x 45.

Why is a 2 x 4 not 2 inches by 4 inches?

Because the name describes the board as it was once sawn, not as it is sold. The American Softwood Lumber Standard, summarised in the Wood Handbook, gives the actual dry size of a 2 x 4 as 38 x 89 mm, 1.5 x 3.5 inches, and a nominal 1 inch board as 19 mm. UK planed timber follows the same logic with different numbers.

How much does planing take off timber?

In the sources we read, between 4 and 7 mm in each dimension of a section: 4 and 5 mm for a 19 x 50 architrave, 4.5 and 6 mm for a 25 x 75, 4 and 6 mm for a 22 x 100, and 4 to 7 mm in thickness for American hardwood. That is about 2 to 3.5 mm from each face or edge. Rough sawn hardwood for a finished component needs more again. Ask the supplier for its own allowance.

Should I draw timber to nominal or finished size?

Finished size, and say so on the drawing. The partner workshop works back to the sawn size it buys. If a nominal size appears on a schedule, mark it nominal and give the finished size beside it.

Is an 18 mm board really 18 mm thick?

A sheet board such as plywood, MDF or chipboard is made to a thickness tolerance rather than planed down, so it is close to its stated thickness: one plywood data sheet gives 17.1 to 18.1 mm for 18 mm, and MDF.H and chipboard data sheets give plus or minus 0.2 and 0.3 mm. Solid timber sold by a nominal or sawn size is different, and is smaller once planed.