Bags per post, before the supply run.
A round hole minus the post that sits in it, times the number of posts, plus a little for holes that never come out perfectly round. Then divided by what one bag yields.
Bags, posts and pickets all go on one fence quote. Tell Redline the run and the post count and it drafts the lines at your prices.
Try Redline freeHow the math works
The hole
A cylinder: π x radius x radius x depth. A 10 in hole with 36 in of concrete is 3.1416 x 5 x 5 x 36 = 2,827 cubic inches.
Minus the post
A 4x4 is really 3.5 x 3.5, so the post takes 3.5 x 3.5 x 36 = 441 cubic inches. That leaves 2,386.
Into cubic feet
A cubic foot is 1,728 cubic inches: 2,386 ÷ 1,728 = 1.38 cu ft per hole.
Posts, extra, bags
20 posts is 27.6 cu ft, and 10% extra makes 30.4. At 0.6 cu ft per bag that's 50.6, so 51 bags.
Per post, with 36 in of concrete
10% extra, 0.6 cu ft per bag. The last column is a 20-post job.
| Hole and post | Concrete per hole | Bags per post | Bags for 20 posts |
|---|---|---|---|
| 8 in hole, 4x4 | 0.79 cu ft | 1.5 | 30 |
| 10 in hole, 4x4 | 1.38 cu ft | 2.5 | 51 |
| 12 in hole, 4x4 | 2.10 cu ft | 3.9 | 78 |
| 12 in hole, 6x6 | 1.73 cu ft | 3.2 | 64 |
Bags per post is rounded to one decimal for reading. The job total is worked from the exact figure and rounded up once.
What to watch for
- Depth is your call. Frost line, soil and local code decide how deep posts go. This tool takes the depth you give it and does the arithmetic.
- Gravel in the bottom? Enter only the depth of concrete, not the full hole.
- Gate posts often get bigger holes. Run them separately with their own diameter and add the bags.
- Round posts. Use a square width with about the same area (a 4 in round post is close to 3.5 in square), or 0 to fill the whole hole for a pier.
- Yield changes by product. Fast-setting and standard mixes in the same bag weight can yield differently. Read the bag.
Questions contractors ask
How many bags of concrete per fence post?
It depends on the hole more than the post. In the example, a 10 in hole with 36 in of concrete around a 4x4 takes about 2.5 bags at 0.6 cu ft per bag with 10% extra. A 12 in hole takes closer to 4. Put in your own hole size and the yield from your bag.
Why subtract the post?
Because it fills part of the hole. A 4x4 in a 10 in hole is about 15% of the volume, which adds up over a long fence.
Bags or a truck?
Look at the cubic yards line. 51 bags is about 1.13 yards. Whether a truck makes sense depends on access, your supplier's minimums and how many holes the crew can fill before it sets up.
How does Redline handle the fence quote?
Tell the chat "quote the Brennans: 180 ft of 6 ft cedar privacy, 25 posts in concrete, one 4 ft gate, 50% deposit" and it drafts an itemized quote at your prices. When they say yes, move the job to Accepted and book the post-setting day by chat. Deposit and balance invoices with card pay links are on Solo and up. See fence estimates.
More for fence and deck crews
Keep reading
- Fence calculatorRun length, spacing and gates into posts, rails and pickets.
- Concrete calculatorSlab size and thickness into cubic yards to order, or bags.
- Deck board calculatorDeck size and board specs into boards to order, joists and screws.
- Fence estimate templateFootage, posts, pool gate, removal, permit and a material deposit.