The short version?
Not all “2x4s” are truly 2 inches by 4 inches.

That’s because lumber is usually sold in one of two ways:

• Nominal size
• True (actual) size

Knowing the difference can save you a lot of frustration when you're planning a project, buying materials, or trying to make hardware, post caps, brackets, or decorative pieces fit correctly.

In this guide, we’ll break down exactly what nominal vs. true sized lumber means, why it matters, and how to tell which type of lumber you have.

What Is Nominal Sized Lumber?

Nominal lumber is lumber that is referred to by its name or trade size, not its exact finished measurement.

For example:

• A 2x4 is called a 2x4
• A 4x4 is called a 4x4
• A 1x6 is called a 1x6

But those are not the actual finished dimensions.

Why is it called “nominal”?

The term nominal means “in name only.”

Years ago, lumber was originally cut to its rough dimensions — meaning a board may have truly started out close to 2 inches by 4 inches. But after it was:

• dried
• planed
• surfaced
• smoothed for consistency

…it became smaller than its original rough-cut size.

That smaller finished size is what you actually end up using in your project. This is the standard way dimensional lumber is sold today.

What Is True Sized Lumber?

True sized lumber (also called actual sized lumber) is lumber that measures the full stated dimensions.

So, for example:

• A true 2x4 actually measures 2" x 4"
• A true 4x4 actually measures 4" x 4"
• A true 6x6 actually measures 6" x 6"

This type of lumber is often used in specialty applications where exact dimensions matter more, such as:

• rough sawn lumber
• timbers
• custom milling
• certain fence and pergola applications
• post caps and decorative hardware fitment
• rustic or heavy-duty outdoor builds

If you're ordering custom accessories or metalwork to fit over wood posts, this distinction matters a lot.

A cap made for a nominal 4x4 will not fit a true 4x4 — and vice versa.

Why the Difference Matters

This isn’t just a technicality — it can make or break your project.

1. Hardware Fitment

If you're installing:

• post caps
• fence brackets
• base plates
• rail connectors
• decorative sleeves
• metal post wraps

…you need to know whether your wood is nominal or true size.

For example:

• A cap made for a nominal 6x6 is designed to fit a post measuring around 5.5" x 5.5"
• A cap made for a true 6x6 is designed to fit a post measuring 6" x 6"

That half inch difference is a very big deal.

If you order the wrong one, it may:

• not slide on at all
• fit too loosely
• wobble
• require shimming or grinding
• look sloppy once installed

2. Project Planning

When building things like:

• pergolas
• privacy screens
• planter boxes
• benches
• gates
• railing systems

…using nominal measurements in your cut list without checking actual dimensions can throw your entire build off.

A project that looks perfect on paper can suddenly be off by:

• 1/2"
• 3/4"
• or even more across multiple boards

That adds up quickly.

3. Custom Fabrication

If you’re ordering custom metal or wood products, the manufacturer usually needs actual dimensions, not what the board is “called.”

That’s especially important for:

• custom fence panels
• made-to-fit post caps
• gate frames
• pergola brackets
• sign mounts
• architectural trim pieces

How to Determine Which Lumber You Have

Here’s the good part: it’s actually very easy to tell.

Step 1: Measure It

The simplest and most accurate way is to use a:

• tape measure
• caliper
• combination square

Measure the width and thickness of the board or post.

Example:

If your “4x4” measures:

• 3.5" x 3.5" → you have nominal 4x4 lumber
• 4" x 4" → you have true sized 4x4 lumber

That’s it.

Another Example:

If your “2x6” measures:

• 1.5" x 5.5" → nominal 2x6
• 2" x 6" → true 2x6

When in doubt, measure the wood — not the label.

Step 2: Check Where You Bought It

Where the lumber came from can give you a strong clue.

Usually Nominal Sized Lumber:

You’ll most often find nominal lumber at:

• big box stores
• common lumber yards
• framing suppliers
• home centers

Examples include standard:

• construction lumber
• kiln-dried boards
• pressure treated dimensional lumber

Often True Sized Lumber:

You’re more likely to find true sized lumber at:

• sawmills
• specialty lumber yards
• timber suppliers
• rough cut lumber dealers
• custom milling shops

This is especially common with:

• rough sawn posts
• rustic beams
• timber frame materials
• agricultural lumber

If it looks more rustic, rougher, heavier, or custom-milled, there’s a good chance it may be true size — but always measure to confirm.

Step 3: Look at the Product Listing or Tag

Sometimes the answer is right on the product page or lumber tag.

Look for wording like:

Signs It’s Nominal Lumber:

• “Nominal dimensions”
• “Actual size may vary”
• “Dressed lumber”
• “Surfaced four sides (S4S)”

Signs It’s True Sized Lumber:

• “True dimensions”
• “Rough cut”
• “Rough sawn”
• “Full dimension”
• “Actual 4x4”
• “Milled true to size”

Manufacturers and suppliers often specify this, but it’s easy to overlook if you’re in a hurry.

Step 4: Check the Surface Finish

This isn’t foolproof, but it helps.

Nominal Lumber Usually Looks:

• smooth
• planed
• clean-edged
• uniform
• machine-finished

True Sized or Rough Lumber Usually Looks:

• rougher
• more textured
• more rustic
• heavier and less “refined”
• less rounded on the corners

Again, don’t rely on looks alone — but it can help point you in the right direction.

Nominal vs. True Sized Posts: Why This Causes So Many Problems

Fence and pergola posts are where this issue causes the most headaches.

A lot of people assume a 4x4 post is always the same size.

It isn’t.

Example:

You order a decorative metal post cap for a 4x4 post.

But your post is actually:

• 3.5" x 3.5" (nominal)
• or 4" x 4" (true)

That difference means the cap either:

• won’t fit at all
• rattles around
• looks wrong once installed

The same problem happens with:

• 6x6 post caps
• post skirts
• fence post sleeves
• base covers
• mounting hardware

That’s why manufacturers often ask for actual post measurements before you order.

And honestly, that’s the right move.

Best Practice: Always Measure Before You Order Anything

If your project involves anything that needs to fit over, around, or against wood, don’t guess.

Before ordering, always measure:

• width
• thickness
• outside dimensions
• any inconsistencies if the wood is rough cut

Pro Tip:

Measure in more than one spot.

Some lumber — especially rough sawn, pressure treated, or outdoor posts — may vary slightly from one end to the other.

Take measurements:

• near the top
• near the middle
• near the bottom

Then use the largest dimension if you’re ordering something that has to slide over the wood.

That little extra effort can save you from a very annoying return.

Quick Rule of Thumb

If you want the fastest possible answer:

If it measures smaller than the name…

…it’s probably nominal lumber.

If it measures exactly what it’s called…

…it’s probably true sized lumber.

Examples:

• “4x4” measuring 3.5" x 3.5" = Nominal
• “4x4” measuring 4" x 4" = True
• “6x6” measuring 5.5" x 5.5" = Nominal
• “6x6” measuring 6" x 6" = True

Simple. Very important. Weirdly easy to overlook.