How to measure for your fence post concrete project
- Count posts and measure the hole. Note how many posts your fence requires. For each hole, measure (or plan) the diameter and depth. A common starting point is a hole diameter about three times the post width — so a 4×4 post (3.5 inches actual) gets roughly a 10-inch-diameter hole.
- Select your post size and enter hole dimensions. Pick your post size from the drop-down — the calculator knows the actual cross-section area for each option. Then enter hole diameter in inches and hole depth in inches. If your fence runs through frost-prone soil, set depth to at least the local frost depth (commonly 24–36 inches for a 6-foot fence).
- Read bags per post and total, then add waste. The result shows 50 lb, 60 lb, and 80 lb bag counts per post and in total. Round up and buy at least one extra bag per every 8–10 posts to cover spillage and slightly under-mixed batches. Fast-setting bags can be poured dry into the hole and activated with water — no mixing required.
How the fence post concrete calculator works
The hole is a cylinder, so its volume is π × (diameter ÷ 2)² × depth. The post displaces some of that space, so the calculator subtracts the post’s cross-section area before multiplying by depth. The result in cubic inches is divided by 1,728 to convert to cubic feet, then divided by each bag’s yield (0.375 cu ft for 50 lb, 0.45 for 60 lb, 0.60 for 80 lb) with the result rounded up to whole bags. For a concrete example: a 4×4 post (3.5 in actual, 12.25 sq in cross-section) in a 10-inch hole 24 inches deep has a hole area of π × 5² = 78.54 sq in. Subtract 12.25 for the post: 66.29 sq in. Multiply by 24 inches of depth: 1,590.9 cu in ÷ 1,728 = 0.92 cu ft. At 0.375 cu ft per 50 lb bag, that rounds up to 3 bags — matching the reference table exactly.
Which type are you estimating?
4×4 wood post — standard panel fence
The most common residential choice. Rule of thumb: hole diameter ≈ 3× the actual post width (3.5 in), so 10 inches is the standard bore. Hole depth should be one-third of the above-ground post length or below the local frost line, whichever is deeper — 24 inches is typical for a 6-foot fence. A 10-inch hole at 24 inches deep uses about 0.92 cu ft of concrete, or 3 bags of 50 lb fast-setting mix.
Enter: Post size: 4×4 wood · Hole diameter: 10 in · Hole depth: 24 in
4×4 wood post — frost-country depth
Where frost lines run deep (upper Midwest, mountain West), bumping the hole to 30 inches or more keeps heaving from working the post loose over time. Increase hole depth to 30 inches and the same 10-inch hole jumps to about 1.15 cu ft per post — 4 bags of 50 lb mix.
Enter: Post size: 4×4 wood · Hole diameter: 10 in · Hole depth: 30 in
6×6 wood post — heavy gate or corner post
Corner posts and gate posts take more lateral load than line posts, so builders often upsize to a 6×6 and dig a wider, deeper hole. A 12-inch hole at 36 inches deep around a 6×6 post uses about 1.73 cu ft — 5 bags of 50 lb fast-setting mix. Note this is slightly less than the equivalent hole for a 4×4 because the 6×6’s larger cross-section displaces more of the hole volume.
Enter: Post size: 6×6 wood · Hole diameter: 12 in · Hole depth: 36 in
4×4 post — oversized footing for gate or corner
A 12-inch hole drilled 36 inches deep around a 4×4 post gives a beefy footing that resists racking on high-traffic gates. That hole holds about 2.10 cu ft of concrete — 6 bags of 50 lb fast-setting mix, the largest per-post figure in the reference table.
Enter: Post size: 4×4 wood · Hole diameter: 12 in · Hole depth: 36 in
Metal round post (2⅜ in) — chain-link style
Chain-link terminal and corner posts are typically a 2⅜-inch round steel tube. Their small cross-section (4.43 sq in) barely reduces hole volume, so concrete usage is almost the full cylinder. A 10-inch hole at 24 inches deep with a metal post uses nearly the same concrete as the 4×4 scenario — calculate it here rather than estimating from another post size.
Enter: Post size: Metal 2⅜ in round · Hole diameter: 10 in · Hole depth: 24 in
Tips & ways to save
- Dig the hole diameter about 3 times the post’s actual width — roughly 10 inches for a 4×4, 12 inches for a 6×6 — to give the concrete footing enough bearing area to resist overturning.
- Place 2–4 inches of gravel at the bottom of the hole before setting the post. The gravel improves drainage and slows wood rot at the buried end.
- Crown the top of the concrete slightly above grade and slope it away from the post — a domed cap sheds rain and prevents water from pooling against the wood where rot starts.
- Fast-setting mix (typically 50 lb bags) lets you pour the dry mix around the post and add water without removing the post; it sets in 20–40 minutes so you can move on the same day. Standard 60 or 80 lb bags cost less per bag but require mixing in a wheelbarrow or tub.
- Corner and gate posts bear far more lateral stress than line posts. Consider going one hole size wider (10 in to 12 in) and 6–12 inches deeper, or use a 6×6 post with an oversized footing for anything that swings or anchors a run of fence.
Concrete per fence post hole (50 lb fast-setting bags)
| Post / hole size | Concrete per post | 50 lb bags per post |
|---|---|---|
| 4×4 post · 8 in hole · 24 in deep | 0.53 cu ft | 2 |
| 4×4 post · 10 in hole · 24 in deep | 0.92 cu ft | 3 |
| 4×4 post · 10 in hole · 30 in deep | 1.15 cu ft | 4 |
| 4×4 post · 12 in hole · 36 in deep | 2.10 cu ft | 6 |
| 6×6 post · 12 in hole · 36 in deep | 1.73 cu ft | 5 |
Concrete per post = hole volume − post volume. A 50 lb bag of fast-setting mix yields about 0.375 cu ft (60 lb ≈ 0.45, 80 lb ≈ 0.6). Dig the hole about 3× the post width and one-third to one-half the post height deep, below the frost line.
Frequently asked questions
How many bags of concrete do I need per fence post?
How deep should a fence post hole be?
Do I need gravel at the bottom of the post hole?
What is fast-setting concrete and should I use it for fence posts?
How wide should a fence post hole be?
Do corner and gate posts need more concrete?
Sources
Related calculators
Reviewed by the BackyardCalc editorial team. Figures are computed from the formula above and checked against manufacturer yields.