Post Hole Concrete Calculator
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Results
3 50 lb bags per hole, 30 in total
| Bag size | Yield per bag | Bags | Weight |
|---|---|---|---|
| 40 lb | 0.300 ft³ | 34 | 1,360 lb |
| 50 lb | 0.375 ft³ | 27 | 1,350 lb |
| 60 lb | 0.45 ft³ | 23 | 1,380 lb |
| 80 lb | 0.60 ft³ | 17 | 1,360 lb |
How much concrete per fence post?
Concrete per hole with 4 inches (10 cm) of gravel at the bottom, post size subtracted:
| Post | Hole (diameter × depth) | Concrete | 50 lb bags | 80 lb bags |
|---|---|---|---|---|
| 4×4 | 8" × 24" | 0.44 ft³ | 2 | 1 |
| 4×4 | 10" × 30" | 1 ft³ | 3 | 2 |
| 4×4 | 12" × 36" | 1.87 ft³ | 5 | 4 |
| 6×6 | 12" × 36" | 1.53 ft³ | 5 | 3 |
| 6×6 | 14" × 42" | 2.72 ft³ | 8 | 5 |
| 6×6 | 16" × 48" | 4.35 ft³ | 12 | 8 |
How the calculation works
Gravel per hole = π × (hole diameter ÷ 2)² × gravel depth
Post area uses actual, not nominal, lumber sizes: a 4×4 is 3.5 × 3.5 inches and a 6×6 is 5.5 × 5.5 inches. For example, 12 fence posts in 10 × 30 inch holes need 1 ft³ each, 11.97 ft³ in total, or 36 bags of 50 lb mix if you mix each hole separately.
How to size a post hole
- Width: about three times the post width. That is 10 to 12 inches for a 4×4 and 14 to 18 inches for a 6×6.
- Depth: one third to one half of the post's height above ground, so 2 to 3 feet for a typical 6-foot fence. Gates and corners benefit from going deeper.
- Frost line: in cold climates the bottom of the hole should sit below the local frost depth to stop frost heave. Your building department can tell you that depth.
- Drainage: 3 to 6 inches of gravel under a wooden post lets water drain away from the end grain.
Setting a post in concrete
- Dig the hole and add the gravel. Tamp it firm.
- Stand the post on the gravel and brace it plumb in two directions.
- Fill with concrete to just below ground level, following the mixing method on the bag.
- Slope the top of the concrete away from the post so rain runs off.
- Wait for the setting time on the bag before hanging rails or gates.
Frequently asked questions
How much concrete do I need per fence post?
A 10-inch-diameter hole 30 inches deep, with a 4×4 post and 4 inches of gravel at the bottom, takes about 1 cubic foot of concrete. That is two 80 lb bags, or three 60 lb or 50 lb bags, per post.
How deep should a fence post hole be?
A common rule is to bury one third to one half of the post's above-ground height, so 2 to 3 feet for a 6-foot fence. In cold climates the bottom of the hole should be below the local frost line; your building department can tell you that depth.
How wide should a post hole be?
About three times the width of the post is a good guide: 10 to 12 inches for a 4×4 (3.5 inches actual) and 14 to 18 inches for a 6×6 (5.5 inches actual).
Should I put gravel at the bottom of a post hole?
Yes, 3 to 6 inches of gravel at the bottom lets water drain away from the end of the post, which helps wooden posts last longer. This calculator includes the gravel layer and reduces the concrete height to match.
Do I need to mix fast-setting concrete?
Many fast-setting post mixes are poured dry into the hole and then soaked with water, while regular concrete mix is mixed before pouring. Always follow the instructions on the bag you buy, because methods differ between products.