Quick answer: Nominal size is the rough-sawn dimension of timber before it is dried and machined. Finished size is the actual planed dimension after processing — typically 3-5mm smaller in each direction. A "2×4" is sold by its nominal size (50mm × 100mm) but finishes at 47mm × 95mm. Always order to finished size when matching to drawings or existing timber.
When browsing timber products, you may notice references to both Nominal Size and Finished Size. While these terms can sound confusing, the difference is straightforward — and understanding it helps you order the right timber first time.
📏 What Is Nominal Size?
The nominal size refers to the rough-sawn size of the timber before it is dried and machined.
For example:
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2×4 timber is the nominal size
This is the size the timber starts out as, before any processing takes place.
Because timber is a natural product, it does not dry uniformly. As timber dries, it naturally loses moisture and can shrink by a few millimetres in each direction — and this amount can vary from piece to piece.
If timber were sold at its original rough size (e.g. a full 50mm × 100mm), those natural variations would make accurate planning difficult.
✂️ What Is Finished Size?
To make timber reliable and consistent, many products — especially carcassing timber — are machined to a finished size.
This means the timber is:
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Dried
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Planed
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Regularised
The result is a consistent, predictable size that builders, carpenters and designers can plan around with confidence.
This is why common sizes like 2×4 have become industry standards — not because they are exact, but because everyone knows what they finish at.
🧱 So… What Size Is a 2×4?
Let’s take a common example:
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Nominal size: 47mm × 100mm (often referred to as 2×4 or 4×2)
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Finished size: 45mm × 95mm
In imperial terms, that finished size works out at approximately:
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1¾″ × 3¾″
So in practice, a standard 2×4 ends up around ¼″ smaller overall once finished.
This isn’t a fault — it’s an industry standard that ensures consistency.
✅ Why Finished Size Matters
If your project relies on precise dimensions, the finished size is always the measurement that matters.
When ordering timber:
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Use finished metric sizes as your reference
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Treat nominal sizes as traditional naming, not exact dimensions
This avoids surprises and ensures components fit as expected.
🌲 Our Advice
At Nottage Timber Merchants:
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Nominal sizes are used as familiar product names
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Finished sizes are always the true working dimensions
📌 Top tip:
If final dimensions are critical, always measure and plan in metric, using the finished size.
If you're unsure which size is right for your project, our team is always happy to help — just ask.
FAQ
What does nominal size mean in timber?
Nominal size is the original rough-sawn dimension before the timber is dried and planed. A "2x4" is sold by its nominal size (50mm × 100mm) but finishes smaller after machining.
What is the finished size of a 2x4?
A nominal 2x4 (50mm × 100mm) finishes at 47mm × 95mm after kiln drying and planing. This is the actual size you can plan around.
Why is timber sold smaller than its labelled size?
Timber is dried and planed after rough-sawing. Drying causes shrinkage, and planing removes a few millimetres from each face. The labelled "nominal size" is the pre-processing dimension; the finished size is what you actually receive.
What's the difference between sawn and planed timber sizes?
Sawn (or "rough-sawn") timber is sold at its nominal size. Planed timber is machined to a smaller, accurate finished size — typically 3-5mm smaller in each direction. PSE (Planed Square Edge) and PAR (Planed All Round) timber are sold this way.
Should I order to nominal or finished size?
Always order to finished size when matching to drawings, existing timber, or precise dimensions. Use nominal size only as a familiar product name (like "2x4" or "4x2"). Each product page lists both.
For more on planed timber sizes, see our PSE timber range. For structural sizes, browse C24 graded timber in standard UK finished dimensions.