Running out of concrete halfway through is a bad day. Tell this calculator your post size, hole size and bag size, and it shows how many bags every hole needs and how many you should buy in total.
Your shopping list
How to use the Fence Post Concrete Calculator
- Enter how many posts you are setting.
- Choose your post size. A 4×4 is really 3.5 in wide.
- Enter the hole width and depth. A hole about 3 times the post width is a good rule.
- Enter the gravel at the bottom of the hole and pick your bag size.
- Read the bags per hole and the total to buy.
The math behind it
Hole volume = 3.14 x (hole width / 2)^2 x (depth – gravel)
Concrete per hole = hole volume – post volume
Bags = total concrete / bag yield, rounded up, plus extra
Bag yields: 40 lb = 0.30, 50 lb = 0.375, 60 lb = 0.45, 80 lb = 0.60 cubic feet
A worked example
What we typed in
- Number of posts: 20
- Post size: 6×6 post (5.5 in)
- Hole width: 12 in
- Hole depth: 36 in
- Gravel at the bottom: 4 in
- Bag size: 80 lb bag
- Extra for mistakes and bad boards: 10 %
What we got
- Bags to buy: 57
- Bags per hole: 2.56
- Concrete in total (cubic feet): 30.68
- Gravel in total (cubic feet): 5.24
Tips from the job site
- Add 4 to 6 in of gravel at the bottom so water drains away from the post.
- Quick-set mix works well for fences. Follow the water amount on the bag.
- Slope the top of the concrete away from the post so rain runs off.
Common questions
How many bags of concrete for one fence post?
For a 4×4 post in a 10 in wide, 30 in deep hole, you need about 2 bags of 60 lb.
Should I use gravel in the hole?
Yes. Gravel helps water drain and keeps the bottom of the post from rotting.
Is bag size important?
Yes. A bigger bag holds more, so you need fewer. The tool uses the real yield for each size.
Why add extra?
Spills, uneven holes and wide holes eat concrete. 10 percent keeps you from a second trip.
Last checked: (worked example and recalculation).