A fence or deck is only as steady as its posts, and the usual way to set a wood post is to surround it with concrete. The mistake most people make when estimating the concrete is to fill the whole hole in their calculation and forget that the post itself takes up room. This calculator subtracts the post, deducts a gravel base if you use one and converts the result into bags of three common sizes.
How to use the post hole concrete calculator
- Enter the hole diameter and depth. Dimensions are in inches by default, but you can switch the unit.
- Choose the post shape and size. Remember the actual size: a 4×4 is 3½ inches, a 6×6 is 5½ inches.
- Add the gravel base depth, if any, and the number of posts.
- Add extra if the holes will be irregular, and read the bags.
Post hole concrete formula
Post volume = post width² (square) or π × (post diameter ÷ 2)² (round), times the same depth
Concrete per hole = hole volume − post volume
Bags = ⌈ total cubic feet ÷ yield per bag ⌉ (80 lb: 0.60, 60 lb: 0.45, 40 lb: 0.30 cu ft)
Volumes are computed in cubic inches and divided by 1,728 to get cubic feet. The yields are the figures commonly printed on bagged concrete mix; if your bag says otherwise, divide the cubic feet by that number.
Worked example: one 4×4 post in a 10 × 30 inch hole
- Hole: π × 5² × 30 = 2,356 in³.
- Post: 3.5 × 3.5 × 30 = 367.5 in³.
- Concrete: 2,356 − 367.5 = 1,989 in³ = 1.15 cu ft.
- Bags: 1.15 ÷ 0.60 = 1.92 → 2 80 lb bags (or 3 60 lb, or 4 40 lb bags).
For a 20-post fence in 12 × 42 inch holes, the total is 51.5 cu ft, which means 86 80 lb bags.
Bags per post (square post, no gravel)
| Hole | Post | Concrete (cu ft) | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 8 in × 24 in | 3.5 in | 0.53 | 1 | 2 | 2 |
| 10 in × 30 in | 3.5 in | 1.15 | 2 | 3 | 4 |
| 10 in × 36 in | 3.5 in | 1.38 | 3 | 4 | 5 |
| 12 in × 36 in | 5.5 in | 1.73 | 3 | 4 | 6 |
| 12 in × 42 in | 5.5 in | 2.01 | 4 | 5 | 7 |
| 14 in × 48 in | 5.5 in | 3.44 | 6 | 8 | 12 |
Setting posts well
- Frost depth. In freezing climates, set posts below the frost line so frost heave cannot lift them.
- Keep the post plumb. Brace it in two directions with a level while the concrete sets.
- Slope the top. Crown the concrete above grade, sloping away from the post, so water runs off rather than collecting at the wood.
- Use treated lumber rated for ground contact, and don’t wedge wood posts into concrete without drainage at the bottom.
- Wait before loading. Normal mix needs a day or more; fast-setting mix may allow work after a short time per the label.
Gravel or concrete only?
Some builders set posts in compacted gravel without concrete because water drains away and the posts are easier to replace. Concrete holds posts more rigidly, which is better for gates, tall fences and decks. Your design determines which is right.
The general-purpose Concrete Calculator handles slabs and footings too. For the fence layout see the Fence Calculator and Vinyl Fence Calculator, for decks the Deck Calculator, and for continuous footings the Concrete Footing Calculator.
Frequently asked questions
How many bags of concrete do I need for a fence post?
A 10-inch hole 30 inches deep around a 4×4 post needs about 1.15 cubic feet of concrete, which is 2 80 lb bags (3 60 lb bags or 4 40 lb bags), since the post takes up space.
How wide should a post hole be?
A common rule is three times the width of the post, so a 4×4 post (3½ in) goes in a hole about 10 to 12 inches in diameter.
How deep should fence posts be?
A common guideline is to bury about a third of the post’s length, with at least 2 feet for short fences; you also have to go below the local frost depth in cold climates. Your building department can tell you the frost depth.
Should I put gravel at the bottom of the hole?
Many installers add 4 to 6 inches of compacted gravel under the post for drainage. Enter that depth and the calculator subtracts it from the concrete depth.
Can I use fast-setting concrete?
Yes. Fast-setting mix is popular for posts because it can be poured dry into the hole and watered, but its yield per bag can differ from standard mix. If your bag lists a different yield, divide the cubic feet by that number.