Post hole calculator
Fence post concrete
calculator
Enter your hole size and post size and get the concrete for one hole and for the whole run — in bags and in cubic yards, with the post itself taken out of the volume and a gravel base allowed for at the bottom.
Enter measurements ↓Optional
What will it cost?
Enter what you actually pay per cubic yard and the quantities above are priced up. The field starts at a typical US price so you have something to work from — change it to your own.
Typical US range $125–$195 per cubic yard, checked 2026-08-29. Prices move — use your own supplier's figure.
More in this series
Put these figures to work
Sizing one part of a pour is easier when the slab itself is settled. Start with the main estimate, then come back for the detail.
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- Concrete Footing Calculator Piers, bell footings, strip footings, form tubes and yards
- Concrete Steps Calculator Stepped volume, landing slab, bags and cubic yards
- Self-Leveling Compound Calculator Underlayment bags, average depth, primer and coverage
- Concrete Removal Cost Calculator Debris weight, dumpsters and demolition cost
- Concrete Bag Yields and Mix Ratios Bag yields, mix proportions and strength classes in one table
Based on your result
What you'll need
Quantities update after every calculation. Buy a bag or two over — a hole that came out wide is discovered with the mixer already running.
- Ready-mix concrete
- Bags to fill the holes around each post
- Calculate above
- Gravel
- Drainage base under each post
- —
- Posts
- Pressure-treated timber or round stock
- —
- Digger and a level
- Auger or clamshell digger, post level, tamper
- As needed
The maths
How this post hole concrete calculator works
A hole is a cylinder. The gravel base comes off the bottom, the post takes up the middle, and what is left is concrete. Multiply by the number of posts, pad it for spillage, and convert to bags or yards.
gravel = same cylinder × gravel depth
post = width² × pour depth (square)
concrete = pour cylinder − post
ordered = concrete × posts × waste
- 3×
- post width, hole diameter
- ⅓
- of post height, buried
- 3.5 in
- a 4×4 actually measures
- 0.60 ft³
- per 80 lb bag
- 3–6 in
- gravel under the post
- 27 ft³
- = 1 cubic yard
-
01
Dig three times the post width
A 4×4 wants a 10 to 12 inch hole. Narrower and there is not enough concrete around the post to hold it against wind.
-
02
Take the gravel off the bottom
A few inches of gravel lets water drain away from the end grain, and that depth is not concrete.
-
03
Subtract the post
The post occupies the middle of the hole. On a 4×4 in a 10-inch hole that is about 16% of the volume.
-
04
Multiply by the run and pad it
Spilled concrete and holes that came out wider than planned are exactly why the waste allowance exists.
Good to know
Post hole questions, answered
For a 4×4 in a 10-inch hole 30 inches deep, about two 80 lb bags, or three 60 lb bags. Wider or deeper holes climb quickly: the same post in a 12-inch hole needs three 80 lb bags.
A third of the post’s above-ground height is the usual rule, so 24 inches for a 6-foot fence, and never shallower than the local frost line. In cold regions the frost line governs and can push holes past 42 inches.
Three times the post width. A 4×4 wants 10 to 12 inches, a 6×6 wants 16 to 18. The concrete collar, not the post, is what resists the wind load.
Yes for wood posts. Three to six inches of gravel under the post lets water drain instead of sitting against the end grain. The calculator removes that depth from the concrete.
Setting a post with dry mix and adding water on top works for light fences and is much faster. It is weaker and less predictable than a wet mix, so gates, corner posts and anything tall should be poured properly.
On one hole, barely. Over a 40-post fence a 4×4 in a 10-inch hole displaces about 8.5 cubic feet, which is roughly 14 eighty-pound bags you would otherwise have bought for nothing.
Crown the concrete an inch above grade and slope it away from the post so water runs off. That last inch is included in the depth you enter.