Free tool · ASTM-based
Fence post depth calculator
Enter your fence height, post type, and frost line — get the professional set depth, hole diameter, post length, and concrete bag count per post. The math follows the ASTM specifications in our post depth guide.
Set depth
30"
dig 36" incl. 6" gravel base
Hole diameter
8"
post dimension + 4 in minimum
Post length
10 ft
next standard size that reaches depth
Concrete per post: ~0.7 cu ft
2 × 80 lb bags · 2 × 60 lb bags · 2 × 50 lb bags
Based on the ASTM-backed formula professional installers use — depth = (½ × fence height) − 6", 24" minimum, 48" maximum — with concrete required for chain link per ASTM F567. Loose or sandy soil: go one size deeper and wider.
The rules behind the math
- Depth: (½ × fence height) − 6", minimum 24", maximum 48" — deeper of that or 6" below your frost line.
- Hole diameter: wood posts need the post dimension + 4"; steel posts set in concrete need 3× the post dimension (ASTM).
- Drainage: dig 6" deeper than the set depth and fill with ¾" clean gravel before pouring; crown the concrete above grade.
- Gate posts: 36–42" deep with a 14–16" hole on a standard 6-foot wood fence — see the gate installation guide.
Full specifications, soil tables, and regional frost depths: How deep should fence posts be?
Frequently asked questions
How deep should a fence post be?
Professional installers follow the ASTM-based formula: set depth = (half the fence height) minus 6 inches, with a 24-inch minimum and 48-inch maximum. For a 6-foot fence that's 30 inches. In cold climates the footing must also reach at least 6 inches below the local frost line, whichever is deeper.
How many bags of concrete do I need per fence post?
A standard 4×4 wood post in a 8-inch hole set 30 inches deep takes roughly 1 cubic foot of concrete — about two 80 lb bags, or three 60 lb bags. The calculator computes the exact annulus volume for your post size, hole diameter, and depth.
Do gate posts need deeper footings?
Yes. Gate posts carry cantilevered load, so the American Fence Association recommends 36–42 inch depth with a wider 14–16 inch hole for a standard 6-foot wood fence — noticeably more than the 30 × 10 inch line-post spec.
Do chain link posts need concrete?
Yes — ASTM F567 requires concrete for all chain link fence posts. Tamped earth is not a compliant substitute. For wood fences, concrete is strongly recommended in loose soils, wet climates, and for any gate or corner post.