Enter the hole or tube diameter, how deep it is and how many holes you have. If a post sits in the concrete, pick its size and the calculator takes it out of the volume.
Extra for waste and cost
Footing size and depth depend on your frost line and local code. Check them before you dig.
What this calculator is for
Use it for fence post holes, deck and pergola footings, mailbox posts and round piers poured in cardboard tube forms, often called sonotubes. Each hole is a cylinder of concrete, so the calculator works out the volume of that cylinder, takes away the post, and turns the result into bags.
Footing formula
- Volume of one hole = 3.1416 × radius × radius × depth
- Take away the post: post width × post width × depth
- Bags = cubic feet ÷ the yield of one bag
Example: four fence posts (4×4) in 10 inch holes, 36 inches deep.
- Hole area: 3.1416 × 5 × 5 = 78.5 square inches
- Take away the post (3.5 × 3.5 in): 78.5 − 12.25 = 66.3 square inches
- 66.3 × 36 = 2,386 cubic inches, or 1.38 cubic feet per hole
- With 10 percent extra: 1.52 cubic feet, about 2.5 bags of 80 lb per hole
- For four holes: 11 bags of 80 lb
Bags of concrete per hole
For a solid footing with no post, using 80 lb bags (0.60 cubic feet each) and no extra:
| Hole diameter | 24 in deep | 36 in deep | 48 in deep |
|---|---|---|---|
| 8 in | 1.2 bags | 1.7 bags | 2.3 bags |
| 10 in | 1.8 bags | 2.7 bags | 3.6 bags |
| 12 in | 2.6 bags | 3.9 bags | 5.2 bags |
You can’t buy part of a bag, so add up all the holes first, then round up. That’s what the calculator does.
How big and deep should the holes be?
- Width: a common rule of thumb for fence posts is a hole about three times the width of the post, so roughly 10 to 12 inches for a 4×4.
- Depth: for fence posts, about a third of the post’s length is often buried. In cold areas, footings must go below the frost line, which ranges from a few inches in the South to several feet in the North.
- Decks and structures: footing size depends on the load and the soil, so follow your plans and local code.
Tips for setting posts and piers
- Put a few inches of gravel in the bottom of fence post holes so water can drain away from the post.
- Slope the top of the concrete away from the post, so rain runs off instead of pooling against the wood.
- Brace each post plumb before the concrete sets, and check it again after a few minutes.
- For tube forms, cut the tube to length, set it plumb and backfill around it before you pour.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10 inch hole, 3 feet deep, plan on about 2.5 bags of 80 lb concrete per post, including 10 percent extra.
How deep should a fence post hole be?
A common rule is about a third of the post’s length, plus a few inches of gravel. In cold climates, go below the frost line, and check local rules.
Do I need a sonotube?
Tube forms are useful when the concrete has to rise above the ground, such as deck piers, or when the soil is too loose to hold a clean hole. Fence posts set in firm ground usually don’t need one.
Can I pour dry concrete into a post hole?
Some fast-setting mixes are designed to be poured dry and then soaked with water. Regular concrete mix should be mixed first. Always follow the instructions on the bag.
How much concrete does a sonotube hold?
A 10 inch tube holds about 0.55 cubic feet per foot of length, and a 12 inch tube about 0.79 cubic feet per foot. Enter the diameter and length above for an exact amount.
Results are estimates for planning. Bag yields are typical for premixed concrete. See How We Calculate for the formulas and values we use.