How to Measure Fence Posts for Solar Cap Lights: 3.5x3.5 vs 4x4 Fit Guide
Measure fence posts for solar cap lights on the finished top the cap will sit on, then choose 3.5x3.5 or 4x4 fit from that number. MATULSA black 3-color-changing lights are listed for 4 in x 4 in posts. White 3-color-changing lights are listed for 3.5 in x 3.5 in or 4 in x 4 in posts. The lumber stamp is not a measurement. The tape on the remaining plane is.
Measure the seat, not the memory of the lumberyard
A post sold as 4x4 is often closer to 3.5 inches across the dressed top. That single fact is why black caps get returned. Put a tape on the width and the depth at the surface that will remain after any decorative lid comes off. Write both numbers. If they differ, the top is not square enough yet.
Measure at the top, not at the ground, and not through a rail. Ground line and top can disagree on old fences. The solar post cap only knows the top.
Wood and vinyl both get this treatment. A vinyl sleeve over a 4x4 core is whatever the sleeve measures outside. The core is history.
3.5 versus 4 is a SKU choice, not a squeeze
If the top is 4 in x 4 in, black or white can be listed fits. Choose housing color for the fence, then a 4-pack or 8-pack. If the top is 3.5 in x 3.5 in, white is the listed opening. Do not force a 4-inch black cap onto it.
Close is not a size. A 3.75-inch oddball is not on the chart. Square the top with a trim block that finishes at 3.5 or 4, or leave that post uncapped. Shims that hide the solar face are not a fit guide.
Round posts, 5x5 columns, and 6x6 newels are outside this pair of openings. This product is a post cap for the listed sizes on decks, fences, gardens, patios, and porches.
What to remove before the tape comes out
Take off a factory vinyl lid if the solar cap will replace it. Do not measure the lid and then the post and average them. Measure the plane that stays.
On wood, knock off a rotten crown or a nailed-on decorative cap that will not share the post. Cut square. Then measure. A tape on a peaked top lies.
If a metal skirt or solar ring is already there and will stay, this fit guide does not apply until that hardware is gone. Two caps do not stack.
Walk the line with a two-column list
Column one: 3.5. Column two: 4. Every sunlit post goes in one column. Mixed columns mean white can cover both listed sizes, or two styles if the 4-inch posts should be black for looks.
Shade posts still get measured if they might be pruned later. They do not go in the cart until they can charge. Check listing photos so the 4-pack or 8-pack style matches the measured tops.
What the tape says versus which cap to use
| Finished top | Black 3-color-changing | White 3-color-changing | | --- | --- | --- | | 4 in x 4 in square | Listed fit | Listed fit | | 3.5 in x 3.5 in square | Not the listed size | Listed fit | | Nominal 4x4 that tapes 3.5 | Not the listed size | Listed fit | | Vinyl sleeve over a 4x4 core | Only if the sleeve is 4x4 | If the sleeve is 3.5 or 4 | | Round, 5x5, or odd leftover | Outside this guide | Outside this guide |
Measure twice after any repair
A new wood plug changes the number. A replaced vinyl insert changes the number. Measure again after the repair, not from the note on last year’s envelope.
If width and depth still disagree by more than a thin gap, the cap will perch. Plane or recut until the seat is a square that matches 3.5 or 4.
Then count the sunlit posts in each column and buy 4-packs or 8-packs to those counts. Fit is the whole guide. Color-changing light does not stretch an opening.
3.5 versus 4x4 measurement checklist
Photo by David Brown on Pexels
- Remove lids and extra caps so the seat is the true plane.
- Tape width and depth at the top, not at the ground.
- Square any crowned or split wood; firm any vinyl sleeve.
- Mark 3.5 or 4 beside each sunlit post.
- Assign black only to true 4x4; white to 3.5 or 4.
- Recount after repairs before choosing a 4-pack or 8-pack.
Frequently asked questions
Is a 4x4 post always a 4-inch top?
Often not. Many finish near 3.5 inches. Use a tape on the seat.
Can white caps go on both 3.5 and 4-inch posts?
White is listed for 3.5 in x 3.5 in or 4 in x 4 in. That is the mixed-line option.
Should the tape include the rail cap?
No. Measure the post top the solar cap will sit on after extra hardware is removed.
What if one side is 3.5 and the other is 4?
The top is not square. Recut or add a block that finishes at one listed size.
Do gardens and porch posts use a different measurement?
No. Same 3.5 or 4 rule on wood or vinyl if the fixture is this post cap.
Related reading
Nominal-versus-actual and size pages that sit next to this 3.5 vs 4x4 tape work.
- nominal versus actual size when measuring posts for solar lights
- 3.5x3.5 versus 4x4 solar post cap size and measurement guide
- choosing 3.5x3.5 vs 4x4 caps for wood and vinyl posts
- what to do if a solar post light is too loose or too tight
What to do next
Carry a tape along the fence and deck. Write 3.5 or 4 on each sunlit top after lids are off. Assign black 3-color-changing caps only to true 4 in x 4 in posts, and white to 3.5 or 4, then count a 4-pack or 8-pack. That is the fit guide: the number on the seat, not the stamp on the post.
Cover photo by Erik Mclean on Pexels.