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 |
Frost depth and why footings go so deep
A footing has two jobs: spread the load over enough soil that it does not sink, and sit below the frost line so that freezing ground does not lift it. The second is what drives the depth. Water in soil expands as it freezes and can push a shallow footing upward by an inch or more each winter, and because it never settles back evenly, a deck or fence rises and racks over a few seasons.
Frost depth varies enormously, from essentially zero along the Gulf Coast to 4 feet or more in the northern plains and interior Northeast. Your local building department publishes the figure and it is not negotiable. Where frost is not a factor, footings still need to reach undisturbed soil below any topsoil or fill, which is often deeper than expected on a graded lot.
Diameter handles the load-spreading half. A 10 or 12 inch tube suits typical deck posts and fence posts; heavier loads and softer soils want a wider bearing area, sometimes a belled base or a poured pad footing that is wider than the pier above it. Doubling the diameter quadruples the bearing area, which is why moving from 8 to 12 inches makes a bigger difference than it sounds.
Tube forms, bells, and pouring
Cardboard tube forms are the standard approach: fast, cheap, and they give a clean cylinder that concrete releases from easily. Cut the tube so it stands an inch or two above grade, which keeps the post base out of standing water and lets you level the tops of several piers to a common line. Brace each tube before pouring, because wet concrete will float and shift an unbraced form.
A belled or flared base widens the bottom of the pour to increase bearing without excavating a larger hole all the way up. Plastic footing forms that attach to the base of a tube do this in a repeatable way. In frost-prone ground a bell also resists uplift, since the wider base has to pull through the soil above it to move.
Fill in lifts and rod each one with a stick or rebar to release air pockets, which otherwise leave voids that weaken the pier. Set the post anchor into the wet concrete rather than drilling later, and check it for position and level while the mix is still workable, since you have perhaps 30 to 45 minutes before that becomes impossible.
Anchors instead of buried posts
For a deck, a galvanized post base anchored into the top of the pier is strongly preferable to setting the post directly in concrete. Wood embedded in a concrete collar sits in the water that inevitably collects around it, and even treated lumber eventually rots at exactly the point where the failure is hidden and structural. A standoff base holds the post an inch clear so it can dry.
Fence posts are the common exception, since the concrete collar provides lateral resistance that a bracket cannot easily match. If you set fence posts in concrete, crown the top of the collar so water sheds away from the post rather than pooling against it, and accept that the posts are a wear item that will need replacing before the rest of the fence does.
Match the hardware to the treatment. Modern preservatives corrode plain steel and even electroplated fasteners quickly; use hot-dip galvanized or stainless connectors and the fasteners the connector manufacturer specifies. Mixing metals in a wet exterior connection accelerates corrosion in a way that is not visible until something moves.
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.