Solar Post Cap Lights for 4x4, 5x5 and 6x6 Posts: How to Get the Fit

Solar post cap lights fit by the actual size of the post top, not the lumber label. A dry wood 4x4 measures 3 1/2 inches and a 6x6 measures 5 1/2 inches, while sleeves over them run larger. Measure the post, then pick a cap that lists that exact size or comes with the adapter for it.

Why a 4x4 post is not 4 inches wide

The size on lumber is its nominal size, set before the wood is dried and planed smooth. The Southern Forest Products Association lists a dry nominal 4 inch dimension at 3 1/2 inches actual and a nominal 6 at 5 1/2 inches. A vinyl or composite sleeve slid over a wood post is wider again, because of the gap and the sleeve wall.

So a deck can have “4x4” posts that measure 3 1/2 inches bare, wider with a vinyl sleeve, or 4.45 inches with a Trex 4 inch composite sleeve. A cap that slides over one will rattle on another.

Measure across the top of the post with a tape, side to side, at the height the cap will sit. Posts can swell or be out of square, so measure two sides.

How post cap adapters work

An adapter is a collar inside the cap that takes up the gap on a smaller post. Classy Caps, a post cap maker, shows how much this varies by model:

• Windsor (white PVC): fits an actual 4 inch or 3 1/2 inch post, with the 3 1/2 inch adapter included and a 5 inch adapter sold separately.
• Imperial SL211R (bronze aluminum): fits 4 inch and 3 1/2 inch posts with the 3 1/2 inch adapter included; a 4 1/2 inch adapter is sold separately.
• Oxford (black aluminum): includes an adapter to fit actual 6, 5 1/2 or 5 inch posts.
• Edison lantern and post cap: adapters included for actual 5, 5 1/4, 5 1/2 and 6 inch posts.

The lesson holds for any brand: read the listing for the actual post sizes and whether each adapter is in the box. For a 5x5 post, look for a cap that lists the exact width you measured, or one whose adapter covers it.

How to pick a solar post cap light for brightness and run time

Solar post cap lights are mostly accent lights for decks, fences and stairs, not area lights. Classy Caps lists 10 to 50 lumens across the four models above. Check the listing for the lumens, the battery type and the run time on a full charge. For a dark driveway or a yard, a solar flood light or motion light suits the job better.

The panel on a post cap is small and faces straight up, so it needs open sky. A post under a tree or a roof overhang will charge slowly, and winter days give it fewer hours of sun. Our outdoor solar guide covers placement and why a light can fade in December.

Matching post caps to wood, vinyl and composite posts

How a cap attaches depends on the cap and the post, so follow the cap maker’s instructions for your post material. Composite deck brands often sell caps sized for their own sleeves; Trex, for example, sells flat caps for its 4 inch and 6 inch post sleeves. Check the sleeve brand’s dimensions before buying a cap from another maker.

Buying for a whole deck? Count the posts, measure each type, and buy one size per post type rather than relying on adapters for every post.

More solar lights for the yard

For bright, motion-triggered light at doors and paths, a solar security light pairs well with post caps on the deck. Lights that keep working in a blackout also belong in an outage plan; our guide to solar generators for home backup covers power inside the house. Gift ideas that combine both are in solar generator gifts. Solar flood lights cover a wider area, and solar light batteries explains how to replace a worn cell.