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Fence Post Hole Concrete Calculator
Type in your post count and hole size and get the exact bag count to grab at the store — hole volume minus the post, plus your waste margin, rounded up to whole bags.
Hole & post dimensions
Every field updates the estimate instantly — no pressing Calculate.
Bags needed by bag size
Same concrete, different sacks — buy whatever your store stocks.
How it works
Three numbers do all the work: hole volume, post displacement, bag yield.
A post hole is a cylinder: π r² × depth. At 9 in across and 30 in deep that is about 1.10 ft³ of space per hole.
The post already fills part of the hole, so its submerged volume comes out: a dressed 4×4 (3.5 in) eats about 0.21 ft³ of that 1.10 ft³.
Multiply by your waste margin, divide by the yield printed on the bag (50 lb ≈ 0.375 ft³) and round up — you can never buy a partial bag.
Bag yields and hole rules of thumb
| Bag | Mixed yield | Bags per ft³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 3.4 |
| 50 lb | 0.375 ft³ | 2.7 |
| 60 lb | 0.45 ft³ | 2.3 |
| 80 lb | 0.60 ft³ | 1.7 |
Yields are for standard concrete mix; fast-setting and high-early-strength mixes yield slightly less.
- 3× the post width is the classic rule of thumb — a 4×4 wants roughly a 9–12 in hole.
- Put 4–6 in of gravel under the post so water drains away from the wood.
- Crown the concrete slightly above grade and slope it away so water sheds off the post.
- Below the frost line (often 32–48 in in cold states) so frost cannot lift the post.
- One 50 lb bag takes about 1.5 qt of water — mix to a stiff, scoopable consistency.
- Tamp the concrete with a stick to release air pockets after pouring.
- Check the post for plumb before the mix stiffens; you have about 20–30 minutes.
- Fast-setting mixes let you hang rails in as little as 4 hours.
FAQ
Does the calculator subtract the post?
Yes. Each hole starts as a cylinder (π r² × depth) and the submerged post volume is removed, so you only buy concrete that actually fits around the post.
Why does it tell me to buy 27 bags when the label math says 26?
The count is always rounded up. Concrete is sold in whole bags and running one bag short mid-pour means a cold joint, so the estimate errs on the side of a spare bag.
Should I use 4×4 nominal or actual dimensions?
Actual. A nominal 4×4 is dressed to 3.5×3.5 in, and the calculator uses those true dimensions so the displaced volume is not overestimated.
What if I am pouring a concrete footing instead?
Set the post size to the smallest option and skip the subtraction mentally — or just add your waste margin. Footing holes are usually wider and deeper than post holes, so expect more bags.
Is any of this sent to a server?
No. All math runs in your browser as you type — the page never uploads your dimensions and works with no connection after it loads.
Does hole depth matter in cold climates?
A lot. Holes shallower than the local frost line can be pushed up by freezing ground. The calculator flags anything under 32 in so you can dig deeper before you pour.
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