How to Measure Your Fence Post (and Choose the Right Post Cap Size)
Choosing the right post cap starts with one simple but important step: accurately measuring your fence post. Whether you're building a new fence or upgrading an existing one, correct measurements ensure a perfect fit—no gaps, no wobbling, and no sizing surprises on installation day.
At Ledro House, all our post caps are designed to sit securely on top of the post, secured with an included lag bolt. Because they cap the post rather than slide over it, the size you order is the actual base size of the cap—so it's important to order the size that matches your post's true measurement. This guide explains how to measure your post properly and match it to the correct size.
Why Accurate Measuring Matters
In the US, a post's name and its actual measurement are often very different. A wood 4x4, for example, is usually planed down to a true 3-1/2", and a wood 6x6 is really 5-1/2"—while vinyl, composite, and rough-sawn posts are typically true to their name. Metal and aluminum posts can vary slightly too.
Accurate measuring ensures:
- Your post cap fits securely
- Your chosen design sits correctly
- Installation is fast and frustration-free
- You achieve a professional, finished appearance
Step 1: Identify Your Post Type
Before measuring, determine what kind of post you have:
Wood post
Remember that a nominal size is not the true size—a "4x4" usually measures 3-1/2", and a "6x6" measures 5-1/2". Measure the actual post rather than relying on the label.
Vinyl, composite, or rough-sawn post
These are usually true to name—a 5x5 vinyl post really measures about 5". Still, always confirm with a measurement.
Steel or aluminum post
Measure the outside dimensions across one flat face.
Step 2: Use Accurate Tools
For best results use:
- A tape measure
- A ruler or calipers (for the highest accuracy)
Measure to the nearest 1/8 of an inch.
Step 3: Measure the Width of Your Post
Measure flat across one face of the post—not corner to corner—and measure near the top, where the post hasn't swollen from ground moisture. Then simply order the size that matches your measurement.
Because every Ledro House post cap sits on top of the post, there's no internal allowance to calculate—the size you select is the base size of the cap.
Common posts and the size to order
- Wood 4x4 (planed to 3-1/2") → order 3-1/2"
- True 4x4 (vinyl, composite, rough-sawn) → order 4"
- 5x5 vinyl post → order 5"
- Wood 6x6 (planed to 5-1/2") → order 5-1/2"
- True 6x6 (rough-sawn, vinyl, composite) → order 6"
- Wood 8x8 (planed to 7-1/4") → order 7-1/4"
Always select the post cap based on your measured post size, not the nominal name printed on the lumber.
Step 4: Check the Size Guide on the Product Page
Every post cap includes a simple size guide showing:
- The size to select
- The common post name it matches
- The post types that size typically fits
This removes the guesswork and ensures you select the correct size.
If you're unsure, just email us at [email protected] and we'll confirm it for you.
Step 5: If Your Post Isn't Standard, Custom Sizes Are Available
Many older fences or custom-built fences have non-standard post sizes. To accommodate this, Ledro House offers custom fit options so your post cap sits perfectly—no wobble, no gaps.
Simply send us your precise measurement and we'll take care of the rest.
Tips for Accurate Measuring
- Measure both directions (e.g., left–right and front–back).
- If your post varies slightly, use the largest measurement.
- Avoid measuring over thick paint—measure the actual wood or metal where possible.
- Double-check before ordering.
Choose the Perfect Fit at Ledro House
With accurate measurements and an understanding that a post's name and its true size are often different, you'll be able to select the perfect product with confidence.
Browse our full range of post caps, or reach out to us at [email protected] for personalized sizing help or custom orders.
Your fence deserves a perfect finish—and it starts with choosing the right size post cap.