Fence Post Concrete Calculator
The cylinder law per hole — 3× post width wide, a third of the post deep, gravel floor — then bags: 40/50/60/80 lb at their honest yields.
Fence Post Concrete Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Round holes around rectangular posts — bell or footer forms differ.
- No wind-load engineering for tall solid panels.
- Yield table is bag-quoted; wet augers change the soil story, not the yield.
In short: The workhorse hole: 10 in wide × 24 in deep around a 3.5-in post with 4 in of gravel — 1,325.8 in³ = 0.7672 ft³ of concrete → 2 bags of 80-lb (fast 2× setting) per hole, or 3 of 50-lb. A 100-ft run at 8-ft bays: 14 posts → 28 bags of 80-lb.
Formula
posts = ⌈run ÷ spacing⌉ + 1
hole: width = 3 × post, depth = max(⅓ post, frost)
V = (πr²h − post²h) ÷ 1728, h = depth − gravel
bags = ⌈V ÷ yield⌉; 40/50/60/80 lb = 0.30/0.375/0.45/0.60 ft³
[('3×', 'hole width vs post width'), ('⅓', 'minimum buried depth vs post height'), ('1,728', 'cubic inches per cubic foot'), ('0.60 ft³', 'an 80-lb bag’s honest yield')]
Worked Example
- Posts = run ÷ spacing, rounded up, plus 1.
- Hole: 3× post width; depth max(⅓ post, frost).
- Subtract the gravel floor from the depth.
- Cylinder minus post, ÷ 1,728, ÷ bag yield.
- Round up per hole; multiply the run.
10×24 hole, 4-in gravel, 3.5-in post: 1,325.8 in³ = 0.7672 ft³ → 2 × 80-lb or 3 × 50-lb. 100 ft at 8 ft: 14 posts → 28 80-lb bags.
Strengths & Limits Of This Model
Where this engine is strong
- Gravel SUBTRACTED, not ignored
- All four bag yields, honestly sourced
- Run-level bag count with truck pounds
Where it stops
- Cylindrical holes only
- No concrete cost line
Practical Use Cases
Wood privacy runs
4x4 posts, 8-ft bays.
Chain-link lines
2⅜-in steel, 10-ft bays.
Gate posts
6x6 and the 12-in hole.
Frost-country builds
Depth past ⅓.
Bag-truck logistics
Pounds per pickup run.
Methodology & Editorial Standards
Posts = ceil(run ÷ spacing) + 1. Net volume = π(diameter/2)² × (depth − gravel) − width² × (depth − gravel), in cubic inches ÷ 1,728. Bags = ceil(ft³ ÷ yield) at the sourced table (40/50/60/80 lb → 0.30/0.375/0.45/0.60 ft³), then posts × bags for the run. Holes under 2× post width refused (3× is the law); gravel ≥ depth refused; bag sizes outside the yield table refused; runs past a mile refused as contractor territory.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Fence Post Concrete Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How much concrete per fence post?
A 4x4 in a 10-in × 24-in hole over 4 in of gravel needs 0.767 ft³ — two 80-lb bags (or three 50s). Gate posts in 12-in holes double that; figure each hole separately.
How deep should fence posts be set?
A third of the above-ground height at minimum — 24 in for a 6-ft fence — and deeper where frost reaches: below the frost line or the ground itself jacks the posts out each winter.
How wide should the hole be?
Three times the post — about 10 in for a 4x4. The collar of concrete is what resists wind load; a post-width hole is a fence that leans.
How many bags of concrete per post hole?
Divide the hole’s cubic feet by the yield: 0.30 ft³ for 40-lb, 0.375 for 50, 0.45 for 60, 0.60 for 80. Round up per hole — a starved footing never shows until the first storm.
Does the gravel count in the concrete math?
It must be SUBTRACTED: 3-4 in of stone at the hole bottom drains water away from the post base, and the concrete fills only what is left above it.
Can I set fence posts without concrete?
Tamped gravel works for temporary fencing; a post holding 6 ft of wind-loaded panel is a footing, and footings are concrete. The bag math on this page is the permanent answer.
Fast-setting or standard mix for posts?
Fast-setting, poured DRY around the post and wetted in place, is the fence-builder standard — no mixer, no wheelbarrow, set in minutes. Standard mix needs forms and pours; both cure to the same strength.
Fence page or this page?
The Fence page counts posts, rails and pickets from your run. This page digs: hole geometry, gravel, and the concrete each post drinks.