How the post hole calculation works
A post hole is a cylinder. The concrete fills the cylinder above the gravel, minus the space the post takes up.
- Hole area = 3.1416 × (diameter ÷ 2)².
- Concrete area = hole area − post cross-section (3.5 × 3.5 in = 12.25 sq in for a 4 × 4).
- Concrete depth = hole depth − gravel at the bottom.
- Volume = concrete area × concrete depth, converted to cubic feet (÷ 1,728) and multiplied by the number of posts.
cu ft per hole = (3.1416 × r² − post area) × (depth − gravel) ÷ 1,728, all in inches
Worked example
Ten 4 × 4 fence posts in holes 10 in wide and 36 in deep, with 6 in of gravel at the bottom. The hole area is 78.54 sq in; less the post, 66.29 sq in. Over 30 in of concrete that is 1.15 cu ft per hole, and 11.51 cu ft for all ten. That is 20 bags of 80 lb, 26 bags of 60 lb or 31 bags of 50 lb.
In metric, ten 90 × 90 mm posts in holes 300 mm wide and 900 mm deep with 150 mm of gravel need 46.9 L of concrete each, 469 L in total: 51 bags of 20 kg or 41 bags of 25 kg.
Bag yields
| Bag | Typical yield | Bags per cu ft |
|---|---|---|
| 80 lb | 0.60 cu ft | 1.67 |
| 60 lb | 0.45 cu ft | 2.22 |
| 50 lb | 0.375 cu ft | 2.67 |
| 20 kg | about 9.4 L | about 107 per m³ |
| 25 kg | about 11.7 L | about 85 per m³ |
Fast-setting post mixes often come in 50 lb bags and are poured dry into the hole, then soaked with water. Standard mixes are mixed in a barrow first. Either way, follow the bag directions for water and setting time.
Sizing the hole
Common rules of thumb, which are a starting point rather than a code requirement:
- Diameter: about three times the post width. A 4 × 4 (3.5 in) suits a 10 to 12 in hole; a 6 × 6 suits 12 in or more.
- Depth: about one third to one half of the post height above ground, and at least 24 in for most fences.
- Frost: in cold climates, holes for decks and structures usually go below the local frost depth. Fence holes often do too, to stop heaving.
Deck footings are structural and are set by local code: diameter, depth, frost depth and connection details all matter. Check with your building department before you dig deck or porch footings.
Setting the posts
- Dig the holes to depth and add the gravel. Tamp it firm.
- Stand the post on the gravel and brace it plumb in two directions.
- Fill with concrete, mixed or dry-poured as the bag directs, working out air pockets with a stick.
- Check plumb again and leave the braces on until the concrete has set.
- Wait for the time on the bag before hanging rails, gates or panels.
Practical tips
- Call your utility-locating service a few days before you dig. Hitting a gas or power line is dangerous.
- Bell the bottom of the hole slightly wider than the top. It resists frost heave and uplift.
- Gravel at the bottom lets water drain away from the end of a wood post.
- Crown the concrete at the top so water runs away from the post instead of pooling against it.
- Wet concrete is caustic. Wear gloves and eye protection, and wash splashes off skin quickly.
Not sure whether to buy fast-setting or regular mix? See fast-setting vs. regular concrete for fence posts. To plan the fence itself, use the fence calculator.
Frequently asked questions
How much concrete do I need for a 4 × 4 post?
In a 10 in hole with 30 in of concrete above 6 in of gravel, a 4 × 4 post takes about 1.15 cu ft: two 80 lb bags, or three 60 lb bags.
Should I subtract the post from the concrete volume?
Yes. A 4 × 4 takes up 12.25 square inches of the hole, about 16 percent of a 10 in hole. Subtracting it avoids buying extra bags.
How many 80 lb bags of concrete make a cubic yard?
45. A cubic yard is 27 cu ft, and an 80 lb bag yields about 0.60 cu ft.
Do I need gravel at the bottom of a post hole?
It is common practice for wood posts because it lets water drain from the end grain. Some codes and post-setting methods differ, so follow local practice.