I've been looking into why a 47 × 100 never measures 47 by 100, and it isn't a trick. The ticket is a name. Finished is what a tape reads. Buy, mark, and cut from the second number.
You are at the yard with a list. The ticket says 47 × 100, or 4 × 2, or 25 × 150 PAR. You put a tape on the end of a board. The numbers do not match. The first thought is that someone has taken a few millimetres off and hoped you would not notice.
They have taken millimetres off. That is the point. The size on the ticket is often a name. The size you can hold a tape to is the one that has to fit the alcove, the bracket, or the lining.
Nominal is the name the yard uses. Finished is the size you can measure. Buy, mark, and cut from the second number.
What you'll learn
What “nominal” means when a yard quotes 47 × 100 or 2 × 4
Why a planed or regularised board is smaller — and that this is normal
Sawn versus planed, in one short pass
A worked example you can copy: measure the space, ask for finished size, check the ticket
What to say at the counter so you leave with the board you meant
The name on the ticket is not the tape reading
Yards in the UK still talk in old inch names. A “4 × 2” or “2 × 4” is the same conversation as “47 × 100”. Those numbers are the nominal size: the familiar name for that section, close to the rough-sawn size the mill started from.
They are not a promise that the piece in your hand is 47 mm thick and 100 mm wide.
The name stuck because everyone already knew it. The mill then dries the timber and puts it through a planer so the faces are consistent. A few millimetres come off. What you are left with is the finished size — also called the actual size, or the planed size. That is the number that has to match your measurement.
If the board is carcassing (the everyday stuff for studs, joists and shed frames), the ticket may also say C16. That is a strength class, not a size. C16 means the piece has been graded for a known structural performance. It does not make a 47 × 100 any closer to 100 mm.
Why the board is smaller
Two things shrink the section.
Drying. Fresh-sawn timber holds a lot of water. As it dries, the width and thickness close up a little. Length barely moves. Structural sizes are talked about at around 20 per cent moisture — a useful reference, not something you need to measure in the car park.
Planing. A machine then takes a thin shaving off the faces so the section is even along the length, the edges are square, and — on carcassing — the corners are often eased (slightly rounded) so the piece is kinder to handle. Those shavings are the millimetres you can see on the tape.
On everyday UK regularised carcassing, a common pattern is about 2 mm off the thickness and about 5 mm off the width. So a piece sold as 47 × 100 typically finishes around 45 × 95. Independent merchants and the big sheds both print that pair. It is not one yard being mean.
Joinery boards sold as PAR (planed all round) or PSE (planed square edge) lose a bit more, because they are planed for a smooth face you will see and paint or oil. A nominal 25 × 150 often finishes around 21 × 144. Treat 3 to 5 mm off each dimension as a working habit, then confirm on the piece in front of you. Yards do not all set the planer to the same last millimetre.
The British standard behind structural sizes (BS EN 336) talks about a target size and two tolerance bands: a looser one for sawn faces, a tighter one for planed faces. You do not need the standard at the counter. You need the habit: the planed target is a few millimetres smaller than the sawn name, and a little variation around that target is allowed.
Sawn, planed, regularised — only as far as the tape
You do not need a full lesson on PAR and PSE to buy the right board. You need to know which finish you are looking at, because the finish decides how much has been taken off.
Sawn (sometimes “rough sawn”) is cut and left. The faces are ridged. The section is closer to the named size. It is the honest choice for something hidden — a plate under a shed floor, a batten behind cladding.
Planed — PAR or PSE on the joinery racks — is smooth on the faces you will see. It is the right family for a shelf, a lining, a door stop. It is also the smaller family.
Regularised carcassing sits in the middle. It has been planed just enough to be consistent, often with those eased edges. A 47 × 100 regularised C16 is the everyday “4 × 2” on a UK rack. It is not the same product as CLS (Canadian Lumber Standard), which is a different, smaller section — a CLS “4 × 2” is typically 38 × 89. If a website and a yard both say 4 × 2, ask which one they mean.
If you need the full PAR / PSE comparison, that is a different page. For this job, ask one question: is this the sawn name, or the size after the planer?
Worked example: a shelf, then a simple frame
1. Measure what you need
An alcove is 920 mm wide. You want a single shelf that sits on two slim battens, about 20 mm thick so it does not look clumsy, and deep enough for a row of jars — say 140 mm of useful depth.
Write the useful numbers, in millimetres, before you leave the house:
Width of the opening: 920 mm (check both ends; alcoves are rarely square)
Thickness you actually want: about 20 mm
Depth you actually want: about 140 mm
Do not write “1 × 6”. That is a name. It will not tell you whether the board will drop into 920 mm, or sit 4 mm proud of a housing you already cut.
2. Ask for finished size
At the rack the familiar joinery size is 25 × 150 PAR. That is the nominal name. A typical finished section is about 21 × 144.
That is useful:
21 mm thick is close to the 20 mm you wanted
144 mm deep is close to 140 mm
144 mm will not span a 920 mm opening on its own — you still need the battens, or a thicker section, if the shelf will take weight. The size on the ticket is the section, not a promise about span.
Say it in millimetres: “I need a planed board about 21 mm thick and about 144 mm wide. What does your 25 × 150 actually finish at?”
If they only have 25 × 100 PAR, the finished width is often around 94 mm. That is a different shelf. Do not force the name to do the job.
3. Check the ticket, then the board
The ticket should show both numbers, or the person on the counter should be able to say the finished pair. Then put a tape on one board, mid-length, not on a chewed end.
If it reads 21 × 144, you can cut the length to 918 mm and leave a whisper each side for a tight alcove. If it reads 19 × 140, your housing or bracket holes need to follow 19 mm, not the 25 on the list.
Same habit on a simple shed partition. You want a frame about 100 mm deep so a sheet of 12.5 mm plasterboard each side makes a tidy store wall.
A 47 × 100 regularised C16 typically finishes 45 × 95. The cavity is 95 mm, not 100.
A CLS 4 × 2 is 38 × 89. Mix the two in one wall and the face will step.
Work out the lining from 95 mm (or 89 mm), not from the name.
Measure the space. Ask for the finished pair. Tape one piece. That is the whole method.
A few sizes you will see on a UK rack
These are typical, not a guarantee. Always read the ticket on the pack you are buying.
What people say | Nominal name (mm) | Typical finished size (mm) | What it usually is |
|---|---|---|---|
4 × 2, 2 × 4 | 47 × 100 | ~45 × 95 | Regularised carcassing, often C16 |
CLS 4 × 2 | — | 38 × 89 | Different product; do not swap it for 45 × 95 |
6 × 2 | 47 × 150 | ~45 × 145 | Regularised carcassing |
1 × 6 PAR | 25 × 150 | ~21 × 144 | Planed joinery / shelf stock |
1 × 4 PAR | 25 × 100 | ~21 × 94 | Planed joinery |
A millimetre or two either way is normal. Moisture will move the section a little more once the board sits in a heated room. If the fit is tight — a housing, a metal hanger, a sheet that must land flush — design to the tape, not the name.
What to say at the counter
Take the list. Say it in millimetres. Ask for the finished size out loud.
“I need this finished size: __ mm thick by __ mm wide, and __ mm long.”
“Is 47 × 100 the name, or the size after the planer?”
“What does this pack actually finish at? Can I tape one?”
“Is this regularised carcassing or CLS? I do not want to mix 45 × 95 with 38 × 89.”
If it is a stud, joist or anything a drawing called structural: “Is this C16 (or C24), and is the stamp on the piece?”
“I measured an opening of __ mm. Will this length cover it after I square the ends?”
Then three checks before it goes in the car:
Ticket — both sizes, or a clear finished pair.
Tape — mid-length, thickness and width.
One honest look down the edge — a banana in the rack will still be a banana at home.
Independent yards are used to this conversation. They will often write the finished size on the ticket, pull a piece so you can measure it, or tell you if today’s pack is running a millimetre under. That is the useful half of knowing a merchant: not a cheaper board, a board that matches the gap you measured.
What this page is not
This is how to read a size. It is not a span table, a strength-class lesson, or a full PAR / PSE guide. A 45 × 95 C16 is not “strong enough” just because you now know it is 45 × 95. If a drawing, Building Control, or a joist hanger needs a size and a grade, use those figures — the finished ones.
American pages will tell you a 2 × 4 finishes at about 38 × 89. That is the CLS / North American story. A UK 47 × 100 on a regularised rack is the 45 × 95 story. Ask which rack you are standing in.
The glossary entry for nominal vs actual dimension still uses the American 2 × 4 example. This page is the UK yard version. Movement is a different job again — if a board has to sit tight in a heated room, read Why Timber Moves.
Key points
Nominal is the name. Finished is the tape reading. Cut and fit from the tape.
A UK 47 × 100 (4 × 2) regularised board typically finishes around 45 × 95. That is normal.
Planing and drying take the millimetres off. PAR and PSE lose a little more than regularised carcassing.
CLS 4 × 2 is 38 × 89. It is not the same board.
Measure the space in millimetres, ask for the finished pair, check the ticket, tape one board mid-length.
Independent yards will usually help if you ask for finished size out loud.
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