Find out how many bags of concrete you need for your fence posts. Enter your hole and post size, and get bag counts for 40, 50, 60, and 80 lb bags.
0cu ft
| Bag size | Total bags | Per post | Price/bag | Cost |
|---|
Each hole is a cylinder. The calculator finds its volume, subtracts the space the post takes up, and subtracts the gravel base. What remains is concrete.
Concrete per post = (hole area − post area) × (hole depth − gravel depth)
That volume is multiplied by the number of posts, increased by your spill allowance, and divided by how much one bag yields. Bag counts are always rounded up.
| Bag size | Yield (cu ft) |
|---|---|
| 40 lb | 0.30 |
| 50 lb | 0.375 |
| 60 lb | 0.45 |
| 80 lb | 0.60 |
Yields are typical for common brands. Check the label on your bag, since mixes vary slightly.
A 4×4 post in a 10 inch hole that is 30 inches deep with a 4 inch gravel base usually takes about 2 to 3 bags of 50 lb concrete. Use the calculator above for your exact sizes.
Most builders do. Gravel lets water drain from the bottom of the hole, which helps wood posts last longer. Enter the gravel depth above and the calculator reduces the concrete accordingly.
A common rule is to bury one third of the post's length, and to reach below the frost line in cold climates. A 6 foot fence usually uses an 8 foot post with about 2 feet or more in the ground. Check local codes.
Fast-setting mixes can hold in about 20 to 40 minutes, and many manufacturers say you can build after a few hours. Standard mixes often need 24 to 48 hours. Follow the instructions on your bag.
Not always. Some builders use compacted gravel for light fences. Concrete is the better choice for gate posts, corner posts, tall fences, and loose soil.