Concrete Footing Calculator
Bags of concrete for post footings and tube forms.
Bottom of the footing must be below your local frost line.
You need
16 bags
Assumes 0.60 cu ft yield per 80 lb bag. Buy one spare per four footings.
- 60 lb bags
- 21 bags
- Volume per footing
- 2.36 cu ft
- Total volume
- 9.4 cu ft
- Total in cubic yards
- 0.35 cu yd
Estimates only. Verify quantities before ordering.
How this is calculated
Volume per footing = pi x (diameter / 2)^2 x depth, with the diameter converted to feet. Total = volume x number of footings. Bag counts assume an 80 lb bag yields 0.60 cu ft and a 60 lb bag yields 0.45 cu ft.
Pick the tube diameter, enter the depth and how many footings you are pouring, and this calculator returns the volume and bag counts. Cylindrical footings are how decks, pergolas, and pole barns meet the ground: a cardboard tube form in an augered hole, filled with concrete around or under a post.
Depth is set by your frost line, not preference: the bottom of the footing must sit below it so freezing soil cannot heave the structure. That ranges from a nominal 12 inches in the warm South to 4 feet or more in the northern states, and your local building department will tell you the number. Diameter follows the load; a common rule is a footing about three times the post width, so a 4x4 post gets a 10 or 12 inch tube.
Concrete for common footing setups
Rows assume 80 lb bags at 0.60 cu ft yield. Depth must reach below your frost line.
| Setup | 80 lb bags | Total volume |
|---|---|---|
| 4 x 10 in tubes, 3 ft deep | 11 bags | 6.5 cu ft |
| 4 x 12 in tubes, 3 ft deep | 16 bags | 9.4 cu ft |
| 6 x 12 in tubes, 3.5 ft deep | 28 bags | 16.5 cu ft |
| 8 x 12 in tubes, 4 ft deep | 42 bags | 25.1 cu ft |
| 9 x 16 in tubes, 4 ft deep | 84 bags | 50.3 cu ft |
Common questions
How many bags of concrete per fence or deck post?
A 12 inch tube filled 3 feet deep holds about 2.4 cubic feet, which is four 80 lb bags. A 10 inch tube at the same depth takes about three bags. Fence posts in unformed 10 inch holes 2.5 feet deep typically take two 60 lb or one and a half 80 lb bags.
How deep do deck footings need to be?
Below the local frost line, with 12 inches as an absolute minimum even in frost-free areas. Frost depth varies from about a foot in the Gulf states to 4 feet or more along the northern border, so call your building department; deck footings are also usually inspected before the pour.
What diameter tube form should I use?
Roughly three times the post dimension: 10 to 12 inches for a 4x4, 14 to 16 inches for a 6x6. Heavier loads or weak soils justify a wider footing or a flared base, which spreads the load over more soil area. The tube must also satisfy your area's minimum bearing requirements.
Should the post sit in the concrete or on top of it?
For decks, best practice is a post base bracket anchored to the top of the cured footing, keeping the wood out of ground contact entirely. Burying a post in concrete is normal for fences, but it traps moisture against the wood, so use ground-contact treated posts and crown the concrete to shed water.