Free Post Hole Concrete Calculator
Use this post hole concrete calculator to see how many bags of concrete you need for your fence or deck posts.
Each post hole is a cylinder. The concrete needed is the volume of the hole minus the part the post takes up. Enter the hole diameter, depth and number of posts, and the post size to subtract it.
A common rule of thumb is a hole about three times the width of the post, buried about a third of the post's length. Check local code for frost depth.
How many bags of concrete per post? An example
One 10 in hole, 24 in deep, with a 4×4 post: 0.5 sq ft × 24 in ÷ 12 = 0.9 cu ft, plus 5% extra = 1 cu ft. Divide by 27 to get 0.04 cubic yards, which weighs about 0.07 tons, or 2 × 80 lb bags (0.60 cu ft).
How deep should concrete be?
| Use | Hole depth | Notes |
|---|---|---|
| Fence post, 4×4 (3.5 in actual) | 24 to 36 in | Hole about 10 to 12 in wide. |
| Gate post | 30 to 36 in or more | Gate posts carry more load; go deeper and wider. |
| Deck post, 6×6 (5.5 in actual) | Below frost depth | Hole about 12 to 16 in wide. |
| Mailbox post | 18 to 24 in |
Bags of concrete per cubic yard
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb bag | 0.3 cu ft | 90 |
| 60 lb bag | 0.45 cu ft | 60 |
| 80 lb bag | 0.6 cu ft | 45 |
Buying tips
- Many fence builders put 4 to 6 inches of gravel in the bottom of each hole for drainage. If you do, reduce the hole depth here by that amount.
- Fast-setting post mix is poured dry into the hole and then watered. Check its yield on the bag; it is often a little lower than standard mix.
- Buy a bag or two extra. Hand-dug holes are usually wider at the top than planned.
How much does concrete weigh?
A cubic yard of concrete typically weighs 3,800 to 4,200 lb (1.9 to 2.1 tons). The calculator uses 4,000 lb (2 tons) per cubic yard. Moisture and grading change the weight, so if your supplier sells by the ton, ask for their figure.
Frequently asked questions
How many bags of concrete for a fence post?
For a 10 in hole 24 in deep with a 4×4 post, about 0.9 cu ft of concrete: 2 bags of 80 lb, 3 bags of 60 lb or 4 bags of 40 lb.
How deep should a fence post hole be?
A common rule is a third of the post length; for a 6 ft fence on 8 ft posts that is about 2 to 3 ft. Go below frost depth where the ground freezes.
How wide should a post hole be?
About three times the post width: roughly 10 to 12 in for a 4×4 and 16 to 18 in for a 6×6.
How accurate is this post hole concrete calculator?
The post hole concrete calculator gives the exact volume for the measurements you enter, then adds 5% extra (you can change it). Weights use a typical 2 tons per cubic yard; real loads vary with moisture, so confirm with your supplier if you buy by the ton. Measure carefully: an error of one inch in depth changes a 3 inch layer by a third.
How the post hole concrete calculator works
Volume = area × hole depth. Each area is worked out from its shape (length × width, π × radius², or ½ × base × height), converted to square feet, multiplied by the hole depth in feet, and added up. Cubic feet ÷ 27 = cubic yards. For post holes, the volume of the post is subtracted from each hole.The extra percentage is then added, and bag counts are rounded up to whole bags. See how we calculate for every figure we use.