How Much Concrete for Footings? Sizes & Amounts
Learn how much concrete for footings you need. Get footing sizes, depths, the volume formula, a worked example, and a sizing table from a working contractor.
By The Concrete Calculator Team ·
How Much Concrete for Footings? Sizes & Amounts
To find how much concrete for footings you need, multiply length in feet by width and depth in feet, then divide by 27 to get cubic yards. A 20 ft strip footing at 16 inches wide and 8 inches deep works out to about 0.66 cubic yards, or roughly 0.72 yards once you add waste. Below I walk through footing types, the formula, and a sizing table so you can order the right amount.
Footing types and how they differ
Not all footings pour the same, and the type drives your quantity.
Strip (continuous) footings run in a long trench under a wall or a row of loads: the perimeter of a house, a garage stem wall, a retaining wall. You size these by linear foot, so a 40 ft wall eats a predictable amount per foot.
Spread (pad) footings are isolated squares or rectangles that carry a single concentrated load, like a steel column, a porch post, or a beam pocket. You calculate these one pad at a time. A common pad might be 24 by 24 inches by 12 inches deep.
Deck and fence post footings are usually round holes filled with concrete, often with a cardboard tube form. A deck post footing might be a 12 in diameter tube going 42 inches deep to clear frost; a fence post hole is narrower and shallower. Round footings use different volume math (pi times radius squared times depth), so I keep those separate from strip and pad numbers. If you are mixing footing types on one job, run each through the concrete footing calculator rather than eyeballing it.
The footing volume formula
For strip and pad footings, the formula is simple:
volume in cubic feet = length_ft times (width_in divided by 12) times (depth_in divided by 12)
Then divide cubic feet by 27 to get cubic yards, since there are 27 cubic feet in a yard.
Worked example: 20 ft strip footing
Say I have a 20 ft strip footing, 16 inches wide and 8 inches deep.
- Width: 16 divided by 12 equals 1.333 ft
- Depth: 8 divided by 12 equals 0.667 ft
- Volume: 20 times 1.333 times 0.667 equals 17.78 cubic feet
- Cubic yards: 17.78 divided by 27 equals 0.66 cubic yards
So the raw number is about 0.66 cubic yards. Add a 10 percent waste allowance and you are ordering roughly 0.72 cubic yards. For a bigger project with multiple footings, total everything up first, then apply the waste factor once at the end. It is the same approach I cover in how much concrete do I need.
Common footing sizes per linear foot
Here is a quick reference for the strip footing sizes I pour most often. The last column is cubic yards per linear foot, so just multiply by your total run length.
| Footing size (W x D) | Cubic feet per ft | Cubic yards per ft |
|---|---|---|
| 12 in x 6 in | 0.50 | 0.019 |
| 16 in x 8 in | 0.89 | 0.033 |
| 20 in x 10 in | 1.39 | 0.051 |
| 24 in x 12 in | 2.00 | 0.074 |
Example: a 30 ft run at 20 by 10 inches is 30 times 0.051, about 1.53 cubic yards before waste.
Typical sizes, depth, and the frost line
Footing width and depth come from the load above and the local building code, so always check with your inspector. As a rule of thumb, a residential footing is twice as wide as the wall it supports and at least as deep as the wall is thick, with 8 inches a common minimum.
Depth is really about the frost line. Your footing has to sit below the depth where the ground freezes, or frost heave will crack and lift your work over the winters. In warm regions that might be 12 inches; up north it can be 42 to 48 inches or more. This is why deck and fence footings look absurdly deep for the load they carry. They are chasing the frost line, not the weight.
Rebar in footings
Most footings get rebar for tensile strength and crack control. A typical residential strip footing runs two or three lengths of number 4 (half inch) rebar horizontally, held on chairs so the steel sits inside the concrete rather than on the dirt. Pads often get a small mat. Overlap your splices by about 24 inches and keep steel roughly 3 inches clear of the earth. To size the bars and total footage, the rebar calculator saves a lot of guesswork.
Why add a 10 percent waste allowance
I never order the exact calculated number. Trenches are never dug perfectly straight, the bottom is uneven, some concrete sticks to the truck and chute, and a little spills. An over dug trench soaks up extra volume fast. Ten percent is my standard cushion. Running short means a cold joint and a second delivery charge, a far worse problem than a little leftover.
Ready-mix vs bags for footings
For most real footing runs, order ready-mix from a truck. Once you pass roughly one cubic yard, bags stop making sense: one yard is around 40 to 45 eighty pound bags, a brutal amount of hand mixing and a real risk of cold joints.
Bags are fine for small isolated work: a few deck footings, a fence line, a single pad. Use the concrete bag calculator to get the count right. Whether you go truck or bag, aim for a 3000 to 4000 psi mix. If you mix your own, follow a proper concrete mix ratio so the strength is actually there.
FAQ
How deep should a concrete footing be? At minimum 8 inches thick, but the bottom must sit below your local frost line. That can range from 12 inches in warm climates to 48 inches or more up north. Always confirm with your building department.
How wide should a footing be? A common guideline is twice the width of the wall it supports, with 16 to 24 inches being typical for residential strip footings. Heavier loads need wider footings.
How many bags of concrete for a footing? Divide your total cubic feet by the yield per bag. An 80 lb bag yields about 0.6 cubic feet, so the 17.78 cubic foot example above needs about 30 bags. That is a lot, which is why most footings go ready-mix.
Do footings need rebar? Almost always yes. Two or three runs of number 4 rebar in a strip footing add tensile strength and control cracking. Check your local code for exact requirements.
How much extra concrete should I order? Add about 10 percent to your calculated volume to cover over dug trenches, spillage, and uneven bottoms.
Get your number in seconds
Plug your footing length, width, and depth into the concrete footing calculator and it will give you cubic yards with the waste allowance already built in, so you can order with confidence.