Post Hole Concrete Calculator
Calculate how many bags of fast-setting concrete you need per fence or deck post from the hole diameter, depth and post size, plus total bags and cubic yards.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- 50 lb bags per post
- 3 bags
- 1.01 cu ft per hole including 10% waste, at 0.375 cu ft per bag
- Total bags
- 30 bags
- 10 posts x 3 bags; 27 bags if partial bags are shared between holes
- Concrete per hole
- 0.92 cu ft
- 10 in hole 24 in deep, minus the post, before waste
- Total concrete
- 0.38 cu yd
- 10.1 cu ft for 10 posts including waste
- Gravel for drainage
- 2.7 cu ft
- 6 in to 8 in of gravel at the bottom of each hole; dig that much deeper than the concrete depth. Local code sets post depth below the frost line.
Post Hole Concrete formula
Hole area (sq in) = 3.1416 x (Diameter / 2)² - post area (4x4 = 12.25, 6x6 = 30.25, round 4 in = 12.57)
Concrete per hole (cu ft) = Hole area x Depth (in) / 1,728
Bags per post = round up (Concrete per hole x (1 + waste %) / bag yield) (50 lb = 0.375, 60 lb = 0.45, 80 lb = 0.60 cu ft)
Total bags = Bags per post x Number of postsWorked example
Eight 6x6 deck posts in 12 in holes 30 in deep, using 80 lb bags: the hole is 3.1416 x 6² = 113.1 sq in, minus 30.25 sq in for the post = 82.85 sq in. Times 30 in deep = 2,485.5 cu in, or 2,485.5 / 1,728 = 1.44 cu ft per hole. With 10% waste that is 1.58 cu ft, and 1.58 / 0.60 = 2.6, so 3 bags of 80 lb per post and 24 bags in all.
Total concrete is 8 x 1.58 = 12.7 cu ft, or 0.47 cubic yards. Each hole also gets about 6 in of gravel first: 113.1 x 6 / 1,728 = 0.39 cu ft per hole.
Quick reference
| Hole size | Concrete per hole | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 8 in x 24 in deep | 0.53 cu ft | 2 | 2 | 1 |
| 8 in x 30 in deep | 0.66 cu ft | 2 | 2 | 2 |
| 10 in x 24 in deep | 0.92 cu ft | 3 | 3 | 2 |
| 10 in x 30 in deep | 1.15 cu ft | 4 | 3 | 3 |
| 12 in x 24 in deep | 1.40 cu ft | 5 | 4 | 3 |
| 12 in x 30 in deep | 1.75 cu ft | 6 | 5 | 4 |
How to size a post hole
Dig the hole about three times the width of the post and deep enough to put a third of the post's length in the ground, or to reach below the local frost line, whichever is deeper. A 6 ft privacy fence on 8 ft posts gets a 10 to 12 in hole with 24 in of concrete. Keep the sides straight or slightly wider at the bottom; a hole that tapers inward at the bottom can heave with frost.
Put 6 to 8 in of gravel in the bottom before the post goes in so water drains away from the end grain, and dig that much deeper to keep the concrete depth. Crown the top of the concrete so it slopes away from the post rather than holding water against it.
Setting posts with fast-setting concrete
Fast-setting mix needs no mixing: brace the post plumb, pour the dry mix into the hole to about 3 in below grade, then add about a gallon of water per 50 lb bag and let it soak in. It sets in 20 to 40 minutes, so work one hole at a time and check plumb again before it firms up. Standard mix must be mixed in a tub or wheelbarrow first and gives a stronger, wetter fill for structural piers.
Deck and porch posts carry real loads. Their footing size and depth come from the building code and the inspector, not from a fence rule of thumb, so treat this calculator as a planning estimate for those and have the plan checked before you dig.
Key facts
- A 4x4 post is actually 3.5 x 3.5 in and a 6x6 is 5.5 x 5.5 in; use the actual size when subtracting the post from the hole.
- The rule of thumb is a hole three times the post width and one third of the post length deep: a 10 to 12 in hole about 24 in deep for a 6 ft fence on 8 ft posts.
- A 50 lb bag of fast-setting concrete yields 0.375 cu ft, a 60 lb bag 0.45 cu ft and an 80 lb bag 0.60 cu ft.
- Fast-setting mix is poured dry into the hole and watered in place; it sets in 20 to 40 minutes and can take a load in about 4 hours.
- Frost depth ranges from a few inches in the South to 4 ft or more in the northern states, and post holes should reach below it.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a 4x4 in a 10 in hole 24 in deep, about 0.92 cu ft of concrete per hole. That is 3 bags of 50 lb fast-setting mix or 2 bags of 80 lb mix per post with 10% waste.
How deep should a fence post hole be?
About one third of the post length and at least 24 in for a 6 ft fence, plus 6 in more for gravel. In cold climates the bottom of the concrete should sit below the frost line, which local code specifies.
Should I put gravel at the bottom of a post hole?
Yes. Six to eight inches of gravel lets water drain away from the bottom of the post, which is where wood posts rot first. Set the post on the gravel and pour the concrete around it.
Do I have to mix fast-setting concrete before pouring it in the hole?
No. Pour the dry mix around the braced post and add about one gallon of water per 50 lb bag. It sets in 20 to 40 minutes; regular concrete mix should be mixed with water first.
Sources and references
- QUIKRETE Fast-Setting Concrete Mix product page (Quikrete)
- QUIKRETE Fast-Setting Concrete Mix No. 1004 data sheet (Quikrete)
- Setting posts in concrete project guide (Quikrete)
- DCA 6 Prescriptive Residential Wood Deck Construction Guide (2015 IRC) (American Wood Council)
Next steps for this project
The usual order for a concrete & masonry project. See all concrete & masonry calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.