Why a 4x4 Post Measures 3.5 Inches

A 4x4 post is often actually 3.5 inches because the 4x4 name is a nominal lumber size, not the dressed top a cap must fit. Solar Post Cap Lights ship as a 6-pack with multiple bases for nominal 4x4, 5x5, and 6x6 posts and for true tops at 3.5, 4, 4.5, 5, and 5.5 inches on wood, vinyl, and plastic. Tape the finished plane in both directions. The stamp on the side is a catalog name. The tape is the base you buy.

Nominal size is the name; dressed size is the cap

Sawn lumber is named for a rough size. Planing takes it down. A post sold as 4x4 commonly finishes near 3.5 by 3.5 inches. That is why a cap listed only for a full 4-inch opening rocks on a typical deck newel. The same pattern shows up on larger posts: a 6x6 often lands near 5.5 inches, which is why this 6-pack includes a 5.5 inch true-size base.

The housing is 5.8 x 5.8 x 3.54 inches. The interchangeable bases are what meet 3.5 through 5.5. Fitment is the base, not a hope that plastic will stretch.

The nominal versus actual post dimensions measuring guide walks the tape. This FAQ is the why behind the 3.5 inch surprise.

Vinyl and plastic change the number again

Close-up of a rustic barbed wire fence post in a rural field, with grass and cloudy sky in Samsun, Turkey.
Photo by funda izgi on Pexels

A wood 4x4 inside a vinyl sleeve is no longer 3.5 inches on the outside. Measure the sleeve. A factory vinyl lid is not the solar cap; if it comes off, measure the plane that remains. Plastic posts follow the finished outside, not the catalog 5x5 printed on an old invoice.

Out-of-square tops are a second FAQ. If one direction is 3.5 and the other is 3.75, write both. A matching base in the listed true sizes still works when the rim can sit complete. A parallelogram that leaves two corners in the air needs a recut before any 15-lumen 360° head goes on.

Rot and crown lie as much as stamps. A punky 4x4 can measure 3.5 and still crush under a seated cap. Square, sound fiber is part of size.

What to buy once 3.5 inches is the real number

Buy the base that matches the tape, from the set that covers 4x4, 5x5, and 6x6. A 6-pack of Solar Post Cap Lights is six IP44 plastic heads, each with eight warm white SMD LEDs, 15 lumens, auto on/off, and a 1000mAh Ni-MH AA. Mixing a 3.5 inch true 4x4 with a 5.5 inch true 6x6 on one deck is normal. That is why multiple bases ship together.

Do not force a 4-inch-only cap onto 3.5 inch timber and then call the light defective when it leaks or looks dim. IP44 sheds rain when the rim is closed. A gap at 3.5 inches is a weather path.

DIY remains $0 when the measurement is honest. An electrician at $50-$100 per hour does not convert a 3.5 inch post into a 4 inch post.

Nominal labels versus the top a solar cap must fit

| What the stamp says | True top often is | Base to match | | --- | --- | --- | | 4x4 | About 3.5 in dressed wood | 3.5 or 4 in true-size base | | 5x5 | Often near 4.5 to 5 in | 4.5 or 5 in true-size base | | 6x6 | Often near 5.5 in | 5.5 in true-size base | | Vinyl over a 4x4 core | Sleeve outside, not the core | Measure the sleeve | | Round or oversize column | Outside this 6-pack | Do not force a base |

A 3.5 inch top still gets the same light

Brightness does not change with post name. Fifteen lumens, eight SMD LEDs, 360° viewing, 30,000-hour LED life. The 3.5 inch FAQ is only about whether that head sits. A well-seated 3.5 inch 4x4 looks the same at dusk as a seated 5.5 inch 6x6. A loose 4x4-in-name-only looks darker because it is tilted.

Keep the panel in the 6-8 hour full-sun window either way. Size does not create shade, and shade does not care what the stamp said.

4x4 versus 3.5 inch checklist

Close-up shot of an insulator on a barbed wire fence with wooden post in a green field.
Photo by Mateusz Feliksik on Pexels

[ ] Ignore the side stamp until the top is taped both ways.
[ ] Expect many 4x4s to read about 3.5 inches.
[ ] Check 5x5 and 6x6 toward 4.5 and 5.5 inch true sizes.
[ ] Measure vinyl and plastic on the finished outside.
[ ] Pick the matching base from the 6-pack set.
[ ] Recut rocking or rotten tops before seating the cap.

Frequently asked questions

Why is a 4x4 post actually 3.5 inches?

The 4x4 name is nominal. Dressed lumber is smaller. Many fence and deck tops finish near 3.5 inches.

Is every 4x4 exactly 3.5 inches?

No. Some true 4-inch tops exist, and sleeves change the number. Always tape the finished plane.

Does a 6x6 follow the same rule?

Yes. Nominal 6x6 posts often measure near 5.5 inches, which is why a 5.5 inch true-size base is in the set.

Will a 4-inch cap still clip onto 3.5 inch timber?

Not as a complete rim. Use the matching 3.5 inch base so the IP44 lid can shed water.

Can one 6-pack cover mixed 3.5 and 5.5 inch posts?

Yes. Multiple bases are for that mix on wood, vinyl, and plastic posts in the listed true sizes.

What to do next

Tape the true top of every 4x4, 5x5, and 6x6 on the line. If a 4x4 reads 3.5 inches, that is expected, not a defect. Match the Solar Post Cap Lights bases to those true sizes and seat a 6-pack only where the rim can sit complete on sunlit posts.

Cover photo by Catalin Buescu on Pexels.

Why Is a 4x4 Post Actually 3.5 Inches? Post Sizing FAQs Answered