Dimensions / Building materials / Lumber / 4×4 post
A 4×4 is at least 3-1/2 × 3-1/2 in dry (88.9 × 88.9 mm, PS 20-25) and 3-9/16 in square green; PS 20 counts it as dimension lumber, not a timber. As a deck post, the 2021 IRC sets its height by the deck area it carries: a Southern pine 4×4 may be 14 ft at 20 sq ft, 8 ft 4 in at 100 sq ft and 6 ft 2 in at 160 sq ft, from the underside of the beam to the top of the footing. The 2018 IRC allowed 8 ft (6 ft 9 in under a three-ply beam), and AWC’s DCA 6 deck guide asks for 6×6 posts. Posts in the ground need ground-contact treatment (AWPA UC4A), such as MCA at 0.15 pcf.
| Dry 4×4, actual (PS 20-25) | Southern pine deck post carrying 100 sq ft (2021 IRC) | Ground-contact MCA retention (UC4A) |
|---|---|---|
| 3-1/2 × 3-1/2in88.9 × 88.9 mm | 8-4ft-in | 0.15pcf |
Sources: NIST, IRC, YellaWood. Checked October 2026.
A run of Southern pine 4×4 posts under a small deck is filled in. Enter your run, the deck area on a post, its height and a measured size; each check follows PS 20 or the IRC table on this page.
Post height runs from the underside of the beam to the top of the footing or pier; the deck area is the deck surface this post and its footing carry.
24 ft at 8 ft spacing takes 4 pieces, one at each end: 42.67 board feet at 8 ft each, and a mill unit holds 91. Your 4 are enough.
A 24 ft run with posts 8 ft apart, an example, is filled in; Roseburg packs 91 green Douglas-fir 4×4s to a unit.
A Southern pine post carrying 60 sq ft of deck may be 11 ft 0 in tall; yours is 6 ft 0 in.
A Southern pine 4×4 carrying 60 sq ft of deck: the 2021 IRC allows 11 ft; 6 ft is an example height.
A dry 4×4 must be at least 3 1/2 × 3 1/2 in; yours measures 3 1/2 × 3 1/2 in.
A dry 4×4 is 3-1/2 in square at least (PS 20-25), filled in as the measurement.
Selling treated posts? Photograph a post and its end tag, mark the actual size, the length and the use category in SizeMarker and send it with your quote.
Mark a photoTreaters’, mills’ and retailers’ pages; Do it Best and Travis Perkins are retailers.
| Product | Size; length | Grade; treatment | Pack |
|---|---|---|---|
| YellaWood treated timbersThis page | 8’, 10’, 12’, 14’, 16’, 18’ and more | #1, #2; MCA at 0.15 pcf for ground contact | — |
| ProWood (UFP) | 3-1/2” × 3-1/2” | End tag: AWPA U1 UC4A, MCA retention 0.15 | — |
| Do it Best 4×4×10 ground contactRetailer | 3-1/2 × 3-1/2 in actual | #1 Prime; ground contact | — |
| Interfor Southern yellow pine | 3-1/2 × 3-1/2 in (89 × 89 mm); 8–16 ft | #1, #2 | — |
| Weyerhaeuser Southern pine | 4x4 timbers also in 8’ lengths | — | — |
| Roseburg green Douglas-fir | PET 92-1/4”, 96”, 104-1/4”, 116-1/4”, 120” | #2 & Better | 91 |
| Viance Preserve CA-CPreservative | — | 0.15 lb/cu ft (2.4 kg/m³) for UC4A; 0.25 lb/cu ft (4.0 kg/m³) for UC4B | — |
| Travis Perkins Postsaver postRetailer, UK | 100 × 100 mm × 2.4 m (4 × 4” × 8 ft) | UC4; 13.20 kg | — |
Sources: YellaWood · YellaWood · ProWood · ProWood · Do it Best · Interfor · Weyerhaeuser · Roseburg · Viance · Travis Perkins
Feet-inches, from the underside of the beam to the top of the footing or pier; 40 psf live load, No. 2 grade, wet service. Interpolation permitted, extrapolation not. Washington State replaced this table; the values are as struck through in Seattle’s 2021 code.
| Deck area on the post | Southern pine | Douglas fir, hem-fir, spruce-pine-fir | Redwood, Western cedars, Ponderosa pine |
|---|---|---|---|
| 20 sq ft | 14-0 | 14-0 | 14-0 |
| 40 sq ft | 13-8 | 13-6 | 13-2 |
| 60 sq ft | 11-0 | 10-10 | 10-3 |
| 80 sq ft | 9-5 | 9-3 | 8-1 |
| 100 sq ft | 8-4 | 8-0 | 5-8 |
| 120 sq ft | 7-5 | 7-0 | Not permitted |
| 140 sq ft | 6-9 | 6-2 | Not permitted |
| 160 sq ft | 6-2 | 5-3 | Not permitted |
Sources: IRC
| Item | Figure |
|---|---|
| 2018 IRCSeattle 2018 code; Brainerd, MN | 4×4 posts up to 8 ft with one- or two-ply beams, 6 ft 9 in with a three-ply beam on a post cap |
| AWC DCA 6 | All deck posts 6×6 or larger: a 4×4 has too little area for the let-in notch, and its connectors carry only 2-ply 2 in or 4 in beams |
| Local guidesWentzville, MO; Gillette, WY | 4×4 posts up to 8 ft |
| 6×6 posts2021 IRC Table R507.4 | 14-0 at every deck area from 20 to 160 sq ft |
| Minimum columnIRC R407.3 | Wood columns at least nominal 4 × 4 in |
| TreatmentAWPA U1-25; IRC R317 | Sawn fence and deck posts are UC4A, ground contact; ground-contact wood must be labelled for it, and field cuts treated to AWPA M4 |
| Fixing the footIRC R507.4 | Connectors, or at least 12 in of embedment in soil or a concrete pier |
| Dry and greenPS 20-25 Table 3; PS 20-20 | Dry 3-1/2 in (88.9 mm), green 3-9/16 in (90.5 mm); under 5 in thick it is dimension lumber |
| WeightAWC NDS 2024 Supplement Table 1B | 2.127 lb per foot at 25 lb/ft³, 2.977 lb at 35 lb/ft³, 4.253 lb at 50 lb/ft³ |
| An 8 ft postOur calculation | 2.977 lb × 8 ft = about 23.8 lb at 35 lb/ft³; wet treated wood weighs more |
| In the check aboveOur calculation | 24 ft ÷ 8 ft = 3 spaces, so 4 posts of 10.67 board feet each; 60 sq ft allows 11-0, against a 6 ft post |
Sources: IRC · Brainerd · AWC · Wentzville · Gillette · IRC · AWPA · IRC · IRC · NIST · ALSC · AWC
3-1/2 × 3-1/2 in when dry: 88.9 × 88.9 mm under PS 20-25. A green 4×4 is 3-9/16 in square.
It depends on the deck area it carries: under the 2021 IRC a Southern pine 4×4 may be 14 ft at 20 sq ft, 8 ft 4 in at 100 sq ft and 6 ft 2 in at 160 sq ft. The 2018 IRC allowed 8 ft, and Wentzville and Gillette cap 4×4 posts at 8 ft.
AWC’s DCA 6 deck guide asks for 6×6 or larger: a 4×4 has too little area for the let-in notch. The IRC accepts 4×4 posts within its height table; a 6×6 may be 14 ft at every deck area in it.
Ground contact, AWPA UC4A, which names sawn fence and deck posts; for example MCA at 0.15 pcf (YellaWood) or CA-C at 0.15 lb/cu ft. Field cuts are treated to AWPA M4 (IRC R317).
About 2.977 lb per foot at 35 lb/ft³ (AWC NDS Table 1B), so about 23.8 lb for an 8 ft post; wet treated wood weighs more. Travis Perkins gives 13.20 kg for its 100 × 100 mm × 2.4 m post.
Buyers ask for the nominal and actual size, the length, the species and grade, the preservative, its retention and use category, whether the end tag says ground contact, the moisture after treatment, and pieces per unit and the unit’s weight. Mark the real post, its end and its tag on your own photo in SizeMarker, as in the picture at the top, and send one product card for WhatsApp or WeChat.
