Deck and Fence Post Sizing FAQ: Finding the Perfect Solar Light Compatibility
Post size confusion comes from one fact: fence and deck posts are named by their rough-cut dimension and sold at their finished dimension. A 4x4 wood post measures 3.5 inches across the top, a 6x6 measures 5.5 inches, and vinyl posts usually measure close to their stated size. The Solar Post Cap Lights include several base sizes covering true top widths of 3.5", 4", 4.5", 5", and 5.5", which is why a cap advertised for 4x4 posts is the correct cap for a post that measures 3.5 inches. Below are the sizing questions that come up most often, answered directly.
Will a 4x4 solar post cap light fit a post that measures 3.5 inches?
Yes, and that is the normal case. Pressure-treated and cedar 4x4 posts are milled to four inches and then dried and planed to a finished 3.5 inches square. The lumber industry keeps calling it a 4x4 because that is the size it was cut at. Any post cap sold for 4x4 wood posts is built around the 3.5-inch reality.
The exception is a 4x4 vinyl or plastic post, which is usually extruded to a true four inches. Same name, half an inch of difference. This is exactly why the pack includes more than one base size rather than assuming a single measurement.
What are the standard deck and fence post sizes?
Photo by Michael Morse on Pexels
| Post as sold | Material | True top width | Typical use | |---|---|---|---| | 4x4 | Pressure-treated wood, cedar | 3.5" | Deck rail posts, fence line posts | | 4x4 | Vinyl, plastic sleeve | 4" | Privacy and picket fence systems | | 4.5x4.5 | Vinyl | 4.5" | Gate and corner posts on vinyl fences | | 5x5 | Vinyl, plastic | 5" | Ranch rail, semi-privacy, heavier gates | | 6x6 | Pressure-treated wood | 5.5" | Structural deck posts, pergola posts, heavy gate posts | | 2x2 to 3x3 | Aluminum, steel | Under 3" | Ornamental metal fencing |
The first five rows are all within the compatible range for these caps. The last row is not, because ornamental metal posts are far slimmer than any wood or vinyl cap standard and need a flat mounting surface or an adapter plate instead of a post cap base.
How do I measure a post top correctly?
Measure the flat top surface at its widest point, then rotate ninety degrees and measure again. Use the larger of the two readings, because posts are rarely perfectly square and the base has to clear the widest dimension.
Clear the top first. Grit, old caulk, flaking paint, and a raised nail head all add width in a way that makes a correctly sized base feel wrong. Do not measure over a decorative trim collar, since the base seats on the post top itself.
If a reading falls between two available base sizes, round up. A post measuring 3.6 inches takes the 4-inch base, not the 3.5-inch base it will not slide onto.
Why do my posts measure differently at different heights?
Wood moves. A post that was a clean 3.5 inches when the fence was built absorbs and releases moisture with the seasons, so it swells across the grain in humid weather and shrinks in dry weather. Differences of a sixteenth to an eighth of an inch between posts on the same fence are completely normal.
This is also why a cap that fit snugly in August can feel slightly loose in February. Nothing has failed. If the seasonal movement on your fence is significant, fasten the base rather than relying on friction alone, which the walkthrough in our 4x4 post cap light installation guide covers.
Do vinyl fence posts need a different base than wood?
Usually yes, for two reasons. The first is the size difference already described: vinyl runs true to its stated dimension while wood does not. The second is the top edge. Many vinyl posts have a slight lip, bevel, or internal reinforcing rib at the top, which changes where the base makes contact.
Vinyl posts are also hollow, which means fastening works differently and the post top may not be perfectly flat. Our guide to solar cap lights on vinyl fencing goes through the specific considerations for hollow post tops.
Can these caps go on a 6x6 post?
Yes, if the post is nominal 6x6 wood, which finishes at 5.5 inches square and sits at the top of the compatible range. Confirm with a tape measure before ordering, because some rough-sawn and reclaimed 6x6 timbers really do measure a full six inches and fall outside what the included bases cover.
Nominal 6x6 posts are common on decks as structural corner posts and on pergolas. Since the cap body measures 5.8 x 5.8 x 3.54 inches, on a 5.5-inch post the overhang is minimal, which produces a tight, flush look that suits heavier posts well.
What if my posts are different sizes on the same property?
That is normal and it is not a problem. Gate posts and corner posts are often one size larger than line posts, and a deck built at a different time from the fence frequently uses different stock.
Because the pack ships with multiple base sizes, a single order can serve more than one post width. Every cap has the same footprint, so a 5.5-inch corner post and a 3.5-inch line post end up wearing caps that look identical from any normal viewing distance. Count your posts by measured size before ordering so you know how the bases need to be allocated.
Do round or octagonal posts work?
Photo by David Brown on Pexels
Post cap bases are designed for square post tops, so round and octagonal posts are outside the intended fit. An octagonal post can sometimes work if the flat-to-flat measurement across the top falls inside the range and the corners do not interfere with the base, but this needs checking with the actual post rather than assuming.
Round posts, including landscape timbers and rustic split-rail posts, do not have a square top for the base to enclose. Those installations need a flat plate fastened to the post top first, at which point you are building a mount rather than using a cap.
Does the cap have to match the post exactly?
The base has to match. The cap does not.
The cap intentionally overhangs the post, which is standard for post caps and serves a purpose: the overhang sheds rain away from the exposed end grain of a wood post, which is the part of a post most vulnerable to rot. It also makes small differences in post width invisible. A profile that sits close to the post top rather than tall and boxy is what keeps that overhang from looking oversized, a point our piece on low profile solar deck caps examines in detail.
What if the post top is not flat or not square?
Trim it. A post top that is cupped, split, out of square by more than about a quarter inch, or sloped from weathering will tilt the cap. That is a cosmetic problem on a fence and a functional one on a solar light, because a tilted top means the solar panel no longer faces straight up and collects less of the 6-8 hours of direct sun it needs for a full charge.
A circular saw and a square will flatten a post top in under a minute. On vinyl posts, a warped top is more often a sign the post cap that came with the fence was removed roughly, so check for damage around the edges before assuming the post itself is at fault.
Quick answers
Is a nominal size ever the same as the true size? On vinyl and plastic posts, usually yes. On dimensional lumber, almost never. Always measure rather than assume.
Can I use a base one size larger if the right one is missing? It will physically go on, but the cap will rock and eventually work loose in wind. Use the size that actually matches the post.
Do I need to remove an existing post cap first? Yes. The solar cap base needs to seat on the post top itself, not on top of another cap, and stacking raises the light and makes it unstable.
Will paint or stain change the fit? A single coat will not. Several heavy coats on the top inch of a post can add enough thickness to make a snug base tight, so measure a painted post rather than measuring an unpainted spare.
What to check before ordering
Take a tape measure to the fence and record the widest top reading for each style of post you own, then group your posts by that number. Five minutes of measuring settles every question above, and it means the bases in the pack get matched to real dimensions instead of the names on the original fence order.