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 10, 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. 1,500 lb on the truck; a half-ton pickup carries about 1,500 lb
- 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 of 3/4 in crushed stone under each post so the end grain drains; dig that much deeper than the concrete depth. Frost depth is set by the local building department
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 three times the post width and put one third of the post in the ground, at least 24 in and below the frost line. A 6 ft privacy fence on 10 ft 4x4s gets a 10 to 12 in hole with 36 in of concrete. The building department has the local frost depth. Keep the sides straight or belled at the bottom, because a hole wider at the top is a plug that frost lifts.
Put 6 in of 3/4 in crushed stone in the bottom, set the post on it so the end grain drains, and dig that much deeper. Bring the concrete to grade and trowel a crown that sheds water away from the post; concrete stopped below grade and covered with soil holds water against the wood, where posts rot. Call 811 first; the locate is free and a cut gas or fiber line is billed to you.
Setting posts with fast-setting concrete
Set corner, end and gate posts first, then the line posts to a string. Fast-setting mix needs no mixing: brace the post plumb, pour the dry mix in, add about a gallon of water per 50 lb bag and let it soak. It sets in 20 to 40 minutes, so work one hole at a time and recheck plumb; rails go on after about 4 hours. Standard 60 and 80 lb mix is wet-mixed first; use it for a solid pier. In the Northeast and upper Midwest many crews skip concrete and tamp crushed stone in lifts; a rotted post then comes out without breaking a plug.
Deck and porch posts sit on footings sized from IRC Table R507.3.1 by tributary area and soil bearing. That is usually a 16 to 24 in pad below frost line with the post on a galvanized standoff base. Fences under 7 ft need no permit (IRC R105.2); a deck footing is inspected before the pour, so count bags here after approval.
What this calculator leaves out
The result is concrete and drainage gravel. It leaves out posts, 2x4 braces, string line and the auger, which typically rents for $75 to $120 a day in 2026 and stalls on roots. Thirty 50 lb bags weigh 1,500 lb, a half-ton pickup's payload; a pallet is about 63 bags and delivery typically runs $75 to $100.
Code limits that apply
- IRC R403.1.4.1
Deck and porch footings bottom below the local frost depth
If you miss it: A pier stopped above frost depth lifts the deck corner the first winter and the ledger bolts work loose at the house
- IRC Table R507.3.1
Deck footing width and thickness set by tributary area, soil bearing and snow load
If you miss it: An 8 in post hole under a deck post punches into soft soil and the deck settles until the door binds
- IRC R317.1.2
Posts in the ground or embedded in concrete of ground-contact treated wood
If you miss it: Above-ground UC3B 4x4s rot off at grade in 5 to 8 years and the fence leans with the first hard wind
- AWC DCA 6
Deck post set on the footing with a post base connector and restrained against lateral movement
If you miss it: A post buried in the pier rots at the concrete line and the deck racks; the inspector fails the framing check
- IRC R105.2
No building permit for a fence up to 7 ft high; anything taller, and every deck, is permitted
If you miss it: An 8 ft fence built without a permit draws a stop-work order and comes down to the neighbor's complaint
- IRC R109.1.1
Foundation inspection with the holes dug and any rebar set, before concrete goes in
If you miss it: Pour the deck piers ahead of the inspector and they get dug out or cored to prove depth
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Digging without calling 811: the locate is free, marks take 2 to 3 days, and a cut gas or fiber line is billed to you.
- Concrete stopped 3 in below grade and buried in soil holds water against the post; bring it to grade and crown it so water sheds.
- A post bottom encased in concrete sits in a cup of water and rots first; set it on 6 in of crushed stone instead.
- A hole wider at the top than the bottom is a plug that frost lifts; dig straight sides below frost depth, or bell the bottom.
- Wet-mixing fast-setting mix in a wheelbarrow for ten holes: it sets in 20 minutes and the second half is a rock; pour it dry.
- Above-ground-rated (UC3B) 4x4s rot at grade in 5 to 8 years; posts in the ground need the ground-contact UC4B stamp.
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 with one third of the post in the ground, 24 in at minimum. A 6 ft fence on 10 ft posts gets a 10 to 12 in hole 36 in deep.
- 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 is 0 on the Gulf Coast, 12 to 20 in in the mid-South, 36 to 48 in across the northern tier and 60 in in Minnesota.
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. Pooled across ten holes it is 27 bags of 50 lb, which is why fence crews figure 2 to 3 per post.
How deep should a fence post hole be?
One third of the post length and at least 24 in, plus 6 in more for gravel. A 6 ft privacy fence on 10 ft posts goes 36 in. In cold climates the bottom of the concrete sits below the frost line, which the building department specifies.
Should I put gravel at the bottom of a post hole?
Yes. Six inches of 3/4 in crushed stone 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, never under it.
Should fence posts be set in concrete or gravel?
Concrete for gate posts, corners and any 6 ft privacy fence that catches wind. Tamped crushed stone alone is common in frost country because it drains, does not heave and lets a rotted post come out without breaking a plug. Either way the post needs the UC4B ground-contact stamp.
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)
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.