Concrete per Post Hole
Enter the number of posts, hole size, and post size.
Concrete
Post Hole Concrete Calculator
Quick answer: concrete per post = π × (hole diameter ÷ 2)² × depth − post area × depth. A 10 in × 36 in hole around a 4x4 post holds 1.38 ft³ of concrete — about 4 bags of 50 lb fast-setting mix or 2.3 bags of 80 lb mix per post.
Work out how many bags of concrete to buy for fence posts, mailbox posts, and deck or pergola posts set in round holes. The post's own volume is subtracted from every hole, and you can leave room for a gravel drainage base.
Post set in concrete
How to calculate concrete for post holes
A post hole is a cylinder. The concrete fills that cylinder minus the part of it occupied by the post, so the math is the cylinder volume less a square (or round) column, multiplied by the number of holes.
[π × (D ÷ 2)² − post area] × depth × posts
= cubic feet of concrete (all dimensions in feet), then ÷ bag yield for bags
- Convert the hole diameter to feet and halve it for the radius. A 10 in hole has a radius of 5 in = 0.417 ft.
- Find the hole volume: 3.1416 × radius² × depth. Use the depth of concrete only — subtract any gravel you put in the bottom.
- Subtract the post. Use actual lumber sizes: a 4x4 is 3.5 × 3.5 in (12.25 in²), a 6x6 is 5.5 × 5.5 in (30.25 in²). Divide square inches by 144 for square feet and multiply by the concrete depth.
- Multiply by the number of posts and add 5–10% for holes that are wider at the top or belled at the bottom.
- Divide by the bag yield and round up. Buy whole bags per post if you will mix one hole at a time.
Worked example
A deck has 6 posts, each a 6x6 set in a 16 in diameter hole 48 in deep, with no gravel base.
- Hole: 3.1416 × (0.667 ft)² × 4 ft = 5.585 ft³
- Post: 30.25 ÷ 144 × 4 ft = 0.840 ft³, leaving 4.745 ft³ of concrete per hole
- Six holes: 28.47 ft³; with 10% waste = 31.32 ft³ (1.16 yd³)
- Bags: 31.32 ÷ 0.60 = 53 bags of 80 lb mix, or 31.32 ÷ 0.375 = 84 bags of 50 lb fast-setting
At more than a cubic yard, a short load of ready-mix or a rented mixer may be easier than mixing 50+ bags by hand.
Bags of concrete per post hole
Net concrete per hole with the post subtracted, no gravel base, no waste. Round up per hole when you mix hole by hole.
| Post · hole (dia × depth) | ft³ per hole | 50 lb fast-setting | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 4x4 · 8 in × 24 in | 0.53 | 1.4 | 1.2 | 0.9 |
| 4x4 · 10 in × 24 in | 0.92 | 2.5 | 2.0 | 1.5 |
| 4x4 · 10 in × 36 in | 1.38 | 3.7 | 3.1 | 2.3 |
| 4x4 · 12 in × 36 in | 2.10 | 5.6 | 4.7 | 3.5 |
| 6x6 · 12 in × 36 in | 1.73 | 4.6 | 3.8 | 2.9 |
| 6x6 · 16 in × 48 in | 4.75 | 12.7 | 10.5 | 7.9 |
| 6x6 · 18 in × 48 in | 6.23 | 16.6 | 13.8 | 10.4 |
Post hole sizing and setting tips
- Size the hole from the post. A hole about 3 times the post width (10–12 in for a 4x4, 16–18 in for a 6x6) leaves enough concrete around the post to resist lateral loads from wind on a fence.
- Bury enough of the post. Plan on one third to one half of the above-ground height: a 6 ft fence post typically goes 24–36 in deep. Deck and porch posts carry vertical loads and their footing size and depth are set by your local building department.
- Go below the frost line. Frost depth ranges from a few inches in the Deep South to 48 in or more in northern states. A post set above it can heave out of the ground over a few winters.
- Holes are rarely straight. Post-hole diggers and augers leave flared tops and loose spoil. Measure a couple of finished holes and use the real diameter, or keep 10% waste.
- Slope the top away from the post. Finish the concrete slightly above grade and crown it so rainwater runs off instead of pooling against the wood.
- Fast-setting mix is poured dry. With most fast-setting products you add water to the hole after placing the dry mix, so you need no wheelbarrow — but follow the bag's water quantity, not a guess.
Post hole concrete FAQ
How many bags of concrete do I need per fence post?
For a 4x4 post in a 10 in wide, 36 in deep hole, the concrete volume is about 1.38 cubic feet after subtracting the post. That is about 4 bags of 50 lb fast-setting concrete (0.375 cu ft each), 3.1 bags of 60 lb mix, or 2.3 bags of 80 lb mix per post.
How big should a post hole be?
A common rule is a hole diameter about 3 times the post width: roughly 10 to 12 in for a 4x4 and 16 to 18 in for a 6x6. Depth is usually one third to one half of the post's height above ground, and the bottom should be below your local frost line.
How much concrete is in a 50 lb bag of fast-setting concrete?
A 50 lb bag of fast-setting concrete yields about 0.375 cubic feet. That means about 72 bags per cubic yard, compared with 45 bags of 80 lb mix (0.60 cu ft each) or 60 bags of 60 lb mix (0.45 cu ft each).
Should I put gravel in the bottom of a post hole?
Many builders put about 6 in of gravel in the bottom of the hole so water can drain away from the end of the post. If you do, enter that depth in the gravel base field; the calculator removes it from the concrete column, which reduces the bags needed.
Do you subtract the post when calculating concrete for a post hole?
Yes. A 4x4 post (3.5 × 3.5 in actual) takes up about 0.085 cu ft per foot of depth, and a 6x6 (5.5 × 5.5 in) about 0.21 cu ft per foot. In a typical hole that is 15 to 25 percent of the hole volume, so ignoring it over-orders concrete.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.