How to calculate a post holes
Fence and deck posts are set in round holes, and the numbers add up quickly when there are many posts.
- Decide on the hole diameter and depth for each post.
- Subtract the volume of the post itself if you want a tighter number.
- Multiply by the number of holes using the quantity box.
- Order bags for small jobs, or a small ready mix batch if you have many posts.
Worked example
For a typical post holes with diameter 10 in, height 36 in and 8 pieces, the volume is 13.09 cubic feet. Divide by 27 for 0.48 cubic yards. With a 10 percent allowance, plan for 0.53 cubic yards.
In concrete, 0.48 cubic yards weighs about 0.98 US tons at a planning value of 4,050 pounds per cubic yard.
Post Holes sizes and quantities
These common sizes are calculated with the same formula as the tool above, so you can find your number quickly.
| Size | Cubic feet | Cubic yards | With 10 percent extra | US tons |
|---|---|---|---|---|
| 8 inch hole, 30 in deep | 6.98 | 0.26 | 0.28 | 0.52 |
| 10 inch hole, 36 in deep | 13.09 | 0.48 | 0.53 | 0.98 |
| 12 inch hole, 42 in deep | 21.99 | 0.81 | 0.90 | 1.65 |
| 16 inch hole, 48 in deep | 44.68 | 1.65 | 1.82 | 3.35 |
Tips
- A hole about three times the post width is a common rule of thumb.
- Gravel at the bottom helps drainage and is worth including in your count.
- Each 80 pound bag fills only about 0.6 cubic feet, so count the bags.
Common mistakes to avoid
- Counting one hole and forgetting to multiply.
- Using the post outside size as the hole size.
- Underestimating the volume of the corner and gate posts, which need deeper holes.
These figures are planning estimates. Real material varies in moisture, size, and compaction, so confirm weights and order quantities with your supplier.
Frequently asked questions
How many cubic yards do I need for a post holes?
Multiply the dimensions in feet and divide by 27. The example above, diameter 10 in, height 36 in, needs 0.48 cubic yards before waste.
How much extra should I order?
Add 5 to 10 percent for waste. Add more if the ground is uneven or the material is loose.
Can I enter inches and feet together?
Yes. Each box has its own unit selector, so you can mix feet and inches.
How much does post holes material weigh?
0.48 cubic yards of concrete weighs about 0.98 US tons as a planning value.