How this calculator works
Each hole is a cylinder. The calculator takes the hole volume above the gravel layer, subtracts the part the post fills, multiplies by the number of posts and adds a waste allowance, because holes are rarely dug to the exact size. It then divides by the bag yield and rounds up to whole bags.
Bag yields are the standard published figures for regular concrete mix: 0.60 cu ft per 80 lb bag, 0.45 per 60 lb and 0.30 per 40 lb; about 12 L per 25 kg. Fast-setting post mixes yield less per bag, so read the bag if you use one. Gravel comes in 0.5 cu ft or 25 kg bags.
concrete per hole = (π × (diameter ÷ 2)² − post area) × (depth − gravel)
bags = holes × concrete per hole × (1 + waste) ÷ bag yield
gravel = holes × π × (diameter ÷ 2)² × gravel depthWorked example
Ten 4×4 posts in 10″ holes 30″ deep, with 6″ of gravel in the bottom, leave 24″ of concrete per hole. The hole holds π × 5² × 24 = 1,885 cu in; the 3½″ post takes 294, leaving 0.92 cu ft per hole. Ten holes need 9.21 cu ft, 10.13 cu ft with 10% waste, which is 16.9 bags of 80 lb, so the list buys 17. The gravel is 2.73 cu ft, or 6 bags of 0.5 cu ft.
Tips before you buy
- Dig below your local frost line, or the post can heave in winter. Your building department can tell you the depth.
- Crown the top of the concrete so rain runs away from the post instead of pooling against the wood.
- Brace each post plumb in two directions before you pour, and leave the braces on until the concrete has set.
- Call your local utility locating service before you dig, even for shallow post holes.
Questions
- How many bags of concrete per fence post?
- A 4×4 post in a 10-inch hole with 24 inches of concrete takes about 0.92 cubic feet, or one and a half to two 80 lb bags. Larger holes, 6×6 posts and gate posts take more, which is why the calculator works from your hole size.
- How big should a fence post hole be?
- A common rule is a hole about three times the width of the post and deep enough to bury about a third of the post height, plus 6 inches for gravel. Gate and corner posts are often set deeper and wider because they carry more load.
- Do I need gravel under a fence post?
- A few inches of gravel at the bottom of the hole lets water drain away from the end grain of a wood post, which helps it last longer. Set the gravel depth to 0 if you prefer to skip it.
- Can I use fast-setting concrete instead?
- Yes. Fast-setting post mixes are poured dry into the hole and then watered, and they set in under an hour. Their yield per bag is usually lower than regular mix, so check the bag and adjust the count.