The recommended width for fence posts depends on the material, height, spacing and load each post carries. You will often see nominal 4×4 posts in residential timber fencing. Check the measured width and the installation requirements before choosing one for your fence.
Recommended width for fence posts: understanding actual dimensions
Start with the post specified for your fence system. For conventional timber fencing, common US nominal sizes include 4×4 and 6×6. The manufacturer’s post-measurement guide identifies typical actual widths of 3.5 and 5.5 inches respectively and advises measuring.
| Nominal label | Typical actual square width | Approximate metric width |
|---|---|---|
| US 4×4 timber | 3.5 in | 88.9 mm |
| US 6×6 timber | 5.5 in | 139.7 mm |
| Metric timber | Use the stated measured size | Check the supplier’s dimensions |
Your 100 mm square post differs from a US 4×4 in actual size. Wet timber, rough-sawn products and proprietary posts can differ, so use your measured size when estimating concrete displacement.
Does a wider post always solve the problem?
Post width is one part of the fence assembly. Rails, connections, ground support and foundations also need to suit the load. Your permitted spacing also depends on the assembly, while gate posts need checking for the particular gate and hardware.
If you measure a 3.5-inch square post, its cross-sectional area is 12.25 square inches, while a 5.5-inch square post has 30.25. That gives you about 2.47 times the area. Your post’s strength still depends on timber species, grade, condition and the assembly. The five-foot post guide explains the separate length decision.
How wide should the post hole be?
QUIKRETE’s approximately three-times-width guidance is a preliminary reference for installations. Three times a measured 3.5-inch width is 10.5 inches. The calculator’s practical suggestion for a nominal 4×4 is 12 inches; a product specification or your entered hole diameter takes precedence.
Your post also needs room inside the hole. A 3.5-inch square post measures about 4.95 inches diagonally, so comparing only one flat face with a round opening can be misleading.
What does post width change in the concrete estimate?
Your larger post displaces more concrete inside the same hole. For 24 inches of fill in a round 12-inch hole, a 3.5-inch square post leaves approximately 1.4007 cubic feet of concrete. Substituting a 5.5-inch square post reduces that to 1.1507 cubic feet, a difference of 0.25 cubic feet per hole.
Keep the hole’s required installation size separate from this comparison. Choose the correct structural post first, then calculate its concrete displacement.
Frequently Asked Questions
Is a 75 mm post suitable for every low fence?
No. Check the system’s post and span requirements, including the post’s role.
Are hollow sleeves equivalent to timber posts?
Their wall thickness, inserts and installation details matter; equal outside dimensions don’t establish equal performance.
Should I enter nominal width in the calculator?
Enter the measured width or diameter in the selected units.
Sources & References
Research checked 10 November 2026. Primary links appear beside the facts they support above; each destination is linked once.
The tables and worked examples are independently calculated arithmetic, not reported field measurements. Published installation guidance has a stated application; local requirements and the actual product instructions take precedence.
Sizing and quantity examples illustrate their stated assumptions. They do not establish site-specific wind resistance, soil capacity or construction approval.