Post Hole Concrete Calculator
Calculate concrete for fence posts, deck piers and mailbox posts. Enter hole diameter, depth and post count to get bags and cost.
Concrete bags needed
12
- Volume per hole
- 0.49 cu ft
- Total volume (10 holes)
- 5.4 cu ft
- Total volume
- 0.2 cu yd
- 60 lb bags
- 12
- Estimated material cost
- $78.00
Includes a 10% waste allowance. Measure hole depth below the post, not the post height.
Fence posts, deck piers and mailbox posts all need a concrete collar that is wider than the post itself. This calculator uses the actual hole diameter and depth, not the post size, so you buy the right number of bags.
A common rule is a hole diameter two to three times the post width, with a depth of one-third to one-half the post height plus a gravel base. Local frost depth overrides that rule in cold climates.
How the post hole calculator works
Volume per hole = π × (diameter ÷ 2)² × depth, with diameter and depth converted to feet. Multiply by the number of holes, add the waste allowance, then divide by the yield of your bag size (0.45 cu ft for 60 lb, 0.60 cu ft for 80 lb).
Worked example
A 6-inch hole that is 30 inches deep holds about 0.49 cu ft. Ten holes are 4.9 cu ft; with 10% waste that is 5.4 cu ft, or about 12 bags of 60 lb concrete.
How to set a fence post
- Call 811 or your local utility locator before digging. Hitting a gas or power line is a real risk.
- Dig the hole two to three times the post width and deep enough to sit below the frost line.
- Add 4–6 inches of gravel at the bottom and compact it. The post should not sit in standing water.
- Set the post plumb and brace it in two directions before pouring concrete.
- Slope the concrete slightly away from the post so water runs off instead of pooling at the base.
Frost depth and drainage
In cold climates, the bottom of the hole must be below the local frost line. If the soil freezes under the footing, frost heave can lift the post and rack the fence. Your building department can tell you the required depth.
Good drainage matters as much as depth. A gravel base, a rounded hole and concrete that slopes away from the post all help keep water out of the wood and reduce rot.
Common post hole mistakes
- Digging a hole barely wider than the post, which leaves almost no concrete collar.
- Skipping the gravel base and setting the post directly on clay.
- Dumping dry mix into the hole and hoping rain will cure it evenly.
- Removing the braces too early, before the concrete has set.
- Building the concrete above grade into a mound that traps water against the post.
Frequently asked questions
How deep should a fence post hole be?
A common rule is one-third of the post height above ground plus 6 inches, and always below the local frost line. For a 6-foot fence that usually means a 30–36 inch hole. Check your local code before digging.
What size hole do I need for a 4 × 4 post?
An 8–10 inch diameter hole is typical. The concrete collar should be two to three times the post width so it resists frost heave and lateral loads.
How many bags of concrete do I need per fence post?
A 6-inch hole, 30 inches deep, needs about one 60 lb bag per post. An 8-inch hole at the same depth needs about two bags. Always round up and keep one spare bag for low spots.
Should I use fast-setting or regular concrete?
Fast-setting mixes are convenient for fence posts and can be poured without premixing in many products. Use regular concrete for structural deck piers and anything that needs full design strength.
Do I need gravel at the bottom of the hole?
A 4–6 inch gravel base improves drainage and helps keep the post from sitting in water. It does not replace concrete for structural posts; it sits below the concrete collar.
Do I need to brace the post while the concrete sets?
Yes. Posts move while the mix is green, so brace them plumb and check them again after the initial set. Fast-setting mixes usually allow bracing to come off within a few hours; regular concrete needs about 24 hours.
Assumptions and accuracy
- Dimensions are converted to feet before the calculation; 1 ft = 12 in = 0.3048 m.
- Waste allowances are estimates. Add more for irregular shapes, many cuts and uneven subgrade.
- Bag yields, coverage rates and material densities vary by product. Check the label or ask your supplier.
- Local building codes and site conditions always take precedence over a general-purpose calculator.
- Read our full methodology