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What Size Post Cap Light Do I Need? A Simple Sizing Guide

Figuring out what size post cap light you need is one of the most frustrating parts of a deck upgrade. You’ve decided to upgrade, you’ve picked a style, and you’re ready to add that beautiful, ambient glow. You click “buy” on a “4×4 Post Cap Light,” and when it arrives… it doesn’t fit.

It slides around on top of the post, or worse, it won’t even go on.

This is the single most common mistake homeowners make, and it stems from a confusing secret of the lumber industry. But don’t worry. This guide will make you an expert in 5 minutes and ensure you get the perfect fit the very first time.

We’ll cover how to measure, what “nominal” vs. “actual” size means (this is the key!), and how to handle different post materials.

The Golden Rule: Always Measure Your Post (Never Guess)

If you take only one thing away from this article, let it be this: You must physically measure your post with a tape measure.

Don’t guess. Don’t assume. And please, don’t just buy the size that shares the name of your post.

Why? Because a “4×4” post is almost never actually 4 inches by 4 inches.

Ordering the wrong size is a huge hassle. You have to deal with repackaging, returns, and shipping delays. And a “close enough” fit looks sloppy. A cap that’s too big will look awkward and off-center, while one that’s too small simply won’t install. A proper-fitting post cap light should be snug, secure, and look like it was custom-made for your deck.

“Nominal” vs. “Actual”: The #1 Secret to Post Cap Sizing

This is it. This is the concept that causes 99% of all sizing mistakes.

Nominal Size: This is the name of the lumber before it’s planed and dried. When a “2×4” is first rough-cut from the log (known as “green wood”), it’s closer to that size.
Actual Size: This is the final, actual dimension of the wood you buy at the hardware store after it has been smoothed on all four sides (surfaced) and dried.

During this process, the wood shrinks. That “4×4” post becomes 3.5 inches x 3.5 inches.

This is true for almost all standard, pressure-treated pine, cedar, or redwood posts. You are not buying a 4×4. You are buying a post called a 4×4, which has an actual dimension of 3.5″ x 3.5″.

This is why that “4×4 Post Cap Light” you were looking at was probably designed to fit a 3.5″ post, or why the one you bought might be a “true” 4″ cap designed for a different type of post entirely. Understanding this difference is key to figuring out what size post cap light you need.

How to Measure Your Deck Posts (The Right Way)

This will take you less than 30 seconds per post. Let’s do it right.

Step 1: Grab Your Tools

You just need a reliable tape measure and a way to write down the measurement (your phone’s notes app is perfect).

Step 2: Measure the Width

Lay your tape measure across the very top of your post. Measure from one outside edge directly across to the other outside edge. Do not measure diagonally.

Step 3: Write It Down (and Measure the Depth)

Is it 3.5 inches? 4 inches? 5.5 inches? Write it down immediately. For square posts, the width and depth should be the same, but it never hurts to check both.

Step 4: Measure a Few Different Posts

This is a pro-tip. Don’t just measure one post at the corner and assume they’re all identical. Wood can warp, swell with humidity, or have slight variations from the mill. Measure 3-4 posts around your deck (especially a corner post and a line post) to confirm they are all consistent. If you get 3.5″ on all of them, you’re golden.

A Simple Guide: What Size Post Cap Light for Common Posts

Now that you have your actual measurement, you can shop with confidence. Manufacturers of quality lighting systems, like Tru-Scapes, will always list the actual post dimension their cap is designed to fit.

Here is a quick-reference chart for the most common post sizes in the United States.

If Your Post is Called… (Nominal Size)	Your Post Actually Measures… (Actual Size)	The Post Cap Light Size You Need
Standard 4×4 (Wood)	3.5″ x 3.5″	3.5-inch (or 3-5/8″)
Standard 6×6 (Wood)	5.5″ x 5.5″	5.5-inch (or 5-5/8″)
Standard 4×6 (Wood)	3.5″ x 5.5″	3.5″ x 5.5″ (Rectangle)
Rough-Cut 4×4 (Wood)	4.0″ x 4.0″	4.0-inch (or 4-1/16″)
Rough-Cut 6×6 (Wood)	6.0″ x 6.0″	6.0-inch (or 6-1/16″)
Vinyl/Composite 4×4	4.0″ x 4.0″	4.0-inch
Vinyl/Composite 5×5	5.0″ x 5.0″	5.0-inch

A Note on Sizing (e.g., 3.5″ vs. 3-5/8″)

You might notice that a post cap light for a 3.5″ post is sometimes listed with an interior opening of 3-5/8″. This is intentional. That extra 1/8 of an inch provides just enough room to slide the cap on easily over the wood post without having to force it or sand the post.

Always read the product description. It will clearly state “Fits a standard 3.5-inch post” or “For true 4-inch posts.”

Does the Post Material Change the Sizing?

Yes, absolutely. The “nominal vs. actual” confusion is almost exclusively a “real wood” problem. Vinyl, composite, and metal posts are typically true to their size.

Wood Posts (Pine, Cedar, Redwood)

This is the most common material and the one we’ve been focusing on.

Standard 4×4 = 3.5″ x 3.5″
Standard 6×6 = 5.5″ x 5.5″

The only exception is “rough-cut” lumber, which is less common for decks. It’s rougher, not surfaced, and is often its true nominal size (a rough-cut 4×4 is actually 4″ x 4″). You would know if you have this—it’s very rustic looking.

Vinyl/PVC Sleeves

These are hollow sleeves that slip over a smaller wood post (often a 4×4). They are manufactured to precise, consistent dimensions.

A “4×4 Vinyl Sleeve” is almost always exactly 4.0″ x 4.0″.
A “5×5 Vinyl Sleeve” is almost always exactly 5.0″ x 5.0″.

Do not buy a 3.5″ post cap for your 4×4 vinyl sleeve. It will not fit. You need a true 4.0″ cap.

Composite Sleeves (like Trex, TimberTech, etc.)

Composite posts and sleeves are similar to vinyl. They are manufactured to be consistent and are true to their stated size.

A 4×4 composite sleeve is often 4.0″ x 4.0″ or even 4.5″ x 4.5″, depending on the brand (like Trex).
Again: You must measure.

Common Sizing Questions Answered

We get these questions all the time. Here are the quick answers to the most common dilemmas.

What if my post is an “in-between” size?

You measured your 4×4 wood post and it’s 3.75 inches. This can happen if the wood is particularly swollen with moisture. A 3.5″ cap won’t fit, and a 4.0″ cap will be too loose.

You have a couple of options:

Wait for it to dry. If the deck is brand new and the wood is very wet, it may shrink as it dries.
Sand the post. For a wood post, you can use a power sander to take off that extra 1/8 inch from the top few inches of the post to get a snug fit.
Use a shim. If you have to buy a 4.0″ cap for a 3.75″ post, you can place a small, non-obvious “shim” (a thin piece of vinyl or wood) on one side inside the cap to create a tighter fit.

Do I measure the post or the old cap?

Always measure the post itself.

The old, existing cap might be warped from the sun, broken, or—most likely—it was the wrong size to begin with. The only true measurement is the post.

What’s the difference between a post cap and a post cap light?

A “post cap” is often just a decorative piece of wood, metal, or vinyl. Its main job is to protect the post’s end-grain (the most vulnerable part) from rain and sun, which prevents splitting and rot, a key factor in overall deck safety.

A “post cap light” does that and integrates a light source.

High-quality, low-voltage (12V) post cap lights, like the ones from Tru-Scapes, are a core part of a complete deck lighting system. They integrate seamlessly with stair lights and rail lights, all running on one transformer. This provides a consistent, warm, and professional glow that adds immense safety and ambiance to your space.

How do I know I’m buying a quality post cap light?

When you’re shopping, especially for a wired system, look for a few key indicators of quality:

Material: Is it powder-coated, die-cast aluminum? Or is it flimsy plastic? Aluminum construction will last for decades, while plastic can become brittle and fade in just a few seasons.
Light Source: Does it use an integrated, professional-grade LED? Or a cheap, replaceable bulb? Integrated LED systems are designed to last for 10-15 years (or 40,000+ hours) with zero maintenance.
The System: Is it part of a complete 12V low-voltage system? This is the standard for professional landscape and deck lighting. It’s safe, energy-efficient, and ensures all your lights match in brightness and color temperature.

You’re Ready to Buy

That’s all there is to it. The great mystery of what size post cap light you need is solved.

The secret is to simply ignore the name of the lumber (like “4×4”) and use a tape measure to find its actual size. For 90% of wood decks, that “4×4″ is 3.5” and that “6×6″ is 5.5”. For vinyl or composite, a “4×4″ is usually a true 4.0”.

Measuring takes 30 seconds, but it gives you total confidence and saves you the massive headache of a return.

Now that you know exactly how to size your posts, are you ready to see how a professional, durable lighting system can transform your deck’s atmosphere? Feel free to explore our full range of low-voltage post cap lights, or if you have a tricky sizing problem, share it in the comments below!