How to Measure Posts for Solar Cap Lights

Measure fence and deck posts for solar caps on the true top, then buy bases that match that number. Solar Post Cap Lights ship as a 6-pack with multiple bases for nominal 4x4, 5x5, and 6x6 posts and for true sizes from 3.5 inches to 5.5 inches on wood, vinyl, and plastic. A lumber stamp is not a measurement. Tape width and depth at the plane the 5.8 x 5.8 x 3.54 inch cap will sit on, drop posts that never see sun, and order only the heads that will seat level.

The tape goes on the top, not on the label

Nominal 4x4 lumber is often closer to 3.5 inches after it is dressed. A 6x6 newel is often near 5.5 inches. Vinyl sleeves add thickness. Decorative rings that will be removed do not count. The buying measurement is the rectangle left when the cap arrives.

Measure both directions. A post that is 3.5 by 4 inches is not a square 4x4. Multiple bases in the 6-pack exist for 3.5, 4, 4.5, 5, and 5.5 inch true tops. Write the pair of numbers on painter's tape on each newel before opening a box.

The nominal versus actual post dimensions measuring guide is the lumber map. This page is the buy that follows the tape.

How to take the number in five minutes

Black and white photo of a snow-covered fence in a serene winter landscape.
Photo by Michael Nowarra on Pexels

Stand a ruler or tape across the finished top, edge to edge, in the direction of the rail, then again across the rail. Read to the nearest eighth. If a factory vinyl lid is still on, decide whether the solar cap replaces it. Measure the plane that will remain.

If the top is crowned, split, or punky, the number is not buy-ready. Square or replace that top first. A 15-lumen 360° head on a tilt looks dim and sheds water into the post instead of off the plastic rim.

Wood, vinyl, and plastic posts are all in range when the true size matches. Metal round posts and oversize columns are outside this 6-pack. Do not order hoping a base will stretch.

Turn measurements into a 6-pack order

Count sunlit posts whose true tops sit in the 3.5 to 5.5 inch band. A 6-pack is six heads, each with eight warm white SMD LEDs, 15 lumens, IP44, auto on/off, and a 1000mAh Ni-MH AA. It is not six guesses. If four posts fit and two live under an eave, buy for four and leave the eave dark, or recut the problem tops later.

Mixed 4x4, 5x5, and 6x6 lines are why multiple bases ship in the box. Group the tape notes: all 3.5, all 5.5, or a mix. The housing is the same 5.8 inch cap; the base is the SKU that changes.

DIY install is $0 once the measurement is honest. An electrician at $50-$100 per hour cannot fix a cap that was bought to a stamp instead of a tape.

What the tape should say before a 6-pack is bought

| What was assumed | What to measure | Buy if | | --- | --- | --- | | Stamp says 4x4 | True top, both ways | About 3.5 to 4 inches and square | | Stamp says 5x5 | Finished sleeve or timber | About 4.5 to 5 inches, matching base | | Stamp says 6x6 | Dressed newel top | About 5.5 inches, matching base | | Vinyl with a lid | Plane after the lid is off | Still 3.5 to 5.5 and firm | | Shaded porch post | Sky as well as size | Only if the panel still sees sun |

A wrong measurement is a weather leak

A loose base is a wind rattle and a path for rain under the cap. IP44 protects the head, not a gap you bought by rounding 3.5 up to 4. Seat the matching base so the rim is complete. Then the 6-8 hour full-sun charge and the dusk sensor can do their jobs.

If two adjacent posts disagree by half an inch, they may still share the pack because bases cover that band. If one is 3.5 and one is a round 6-inch column, only the 3.5 is in this product.

Post-measure buying checklist

A serene evening scene with someone reading on a wooden deck by a river, lit by string lights.
Photo by Daniil Ustinov on Pexels

[ ] Tape each finished top in both directions.
[ ] Write true size on the post; ignore the side stamp.
[ ] Square rotten, crowned, or split tops before ordering.
[ ] Keep only sunlit posts on the shopping list.
[ ] Match 3.5 to 5.5 inch bases for 4x4, 5x5, and 6x6.
[ ] Order a 6-pack to the count that actually fits.

Frequently asked questions

Is a 4x4 post always 4 inches on top?

Often not. Many finish near 3.5 inches. Measure the true top before buying a base.

Can one 6-pack mix 4x4 and 6x6 posts?

Yes when each top has a matching base in the 3.5 to 5.5 inch range. The heads are the same; the bases change.

Do vinyl posts measure like wood?

Measure the sleeve, not the wood core inside. Remove a factory lid if the solar cap replaces it.

What if the two sides of the top disagree?

Record both numbers. A rectangle that still sits in the listed true sizes can use the matching base. A wildly out-of-square top needs a recut.

Should shaded posts be measured at all?

Only if the panel will still see a 6-8 hour full-sun window. Size without sky is a cap that stays dim.

What to do next

Walk every fence and deck post with a tape. Write the true top on each sunlit newel, square any top that would rock, and buy a 6-pack of Solar Post Cap Lights only for posts in the 3.5 to 5.5 inch band. Match the bases to 4x4, 5x5, and 6x6 as measured, not as stamped.

Cover photo by zhang kaiyv on Pexels.

How to Measure Fence & Deck Posts for Solar Caps: Step-by-Step Buying Guide