Concrete Footing Calculator
Work out the concrete for a continuous (strip) footing under a wall, or for square pad footings under posts and piers. Enter the run length and the footing's width and thickness in inches; for several pads, set the number of footings.
Safety: Footings must reach below your local frost line; check the required depth with your building department or local code.
How to use the concrete footing calculator
- Keep Footing selected. Enter the total length of the footing run in feet; for a rectangle of walls, add up all four sides.
- Enter the footing width and depth (thickness of the concrete) in inches, from your plans. The example is 16 × 8 in.
- For square pad footings, enter the pad's side length as the length (in feet, e.g. 2 ft for a 24 in pad), the same side as the width in inches, and the number of pads under Number of identical footings.
- Check the cubic yards and bags, and add the volume of any piers or tubes on top with the sonotube calculator.
Footing volume formula
A footing is a long rectangular block, so the volume is length × width × thickness. On footings, width and thickness are usually given in inches, so both are divided by 12:
Volume (ft³) = length (ft) × width (in) ÷ 12 × thickness (in) ÷ 12
Divide by 27 for cubic yards and add the waste allowance. Trenches are rarely perfectly straight-sided, which is why 10% is a sensible default for footings dug without forms. Bags are worked out from the yields on the QUIKRETE and Sakrete data sheets (0.60 ft³ per 80 lb bag) and rounded up.
Worked example: 60 ft of 16 × 8 in footing
This is the footing loaded above, a typical run for a small garage or addition.
- Cross-section: 16 ÷ 12 × 8 ÷ 12 = 0.889 ft² per foot of run.
- Volume: 60 × 0.889 = 53.3 ft³, or 1.98 yd³ before waste.
- Add 10%: 58.67 ft³, which is 2.17 yd³.
- Bags: 58.67 ÷ 0.60 = 97.8, rounded up to 98 bags of 80 lb mix.
At about 2 cubic yards, this is a job for ready-mix. A truck can usually chute straight into a trench, which saves wheeling 98 bags' worth of concrete around the site.
Footing size and depth rules
The International Residential Code (R403.1) sets the basics for house footings: concrete footings must be at least 6 in thick, and their minimum width comes from a table based on the soil's bearing capacity and the number of stories they support. Footings must also sit on undisturbed or properly compacted soil, at least 12 in below the ground surface and below the local frost line, so frost heave can't lift them.
Deck footings follow the same idea. The American Wood Council's DCA 6 guide sizes square and round deck footings by the area each post supports and the soil type. Your plans, the code table and your local building department decide the actual size; this calculator only turns that size into concrete.
Concrete for strip footings, by footing size
| Footing (width × thickness) | ft³ per linear ft | yd³ per 10 ft | yd³ per 50 ft | yd³ per 100 ft | 80 lb bags per 10 ft (+10%) |
|---|---|---|---|---|---|
| 12 × 6 in | 0.5 | 0.19 | 0.93 | 1.85 | 10 |
| 16 × 8 in | 0.89 | 0.33 | 1.65 | 3.29 | 17 |
| 18 × 8 in | 1 | 0.37 | 1.85 | 3.7 | 19 |
| 20 × 10 in | 1.39 | 0.51 | 2.57 | 5.14 | 26 |
| 24 × 10 in | 1.67 | 0.62 | 3.09 | 6.17 | 31 |
| 24 × 12 in | 2 | 0.74 | 3.7 | 7.41 | 37 |
No waste in the cubic-yard columns. The IRC's minimum footing thickness is 6 in; the required width depends on your soil and the number of stories (IRC Table R403.1).
Square pad footings: concrete per pad and for several pads
| Square pad (side × thickness) | ft³ each | 80 lb bags each (+10%) | Bags for 4 pads | Bags for 6 pads | Bags for 9 pads |
|---|---|---|---|---|---|
| 16 × 16 × 8 in | 1.19 | 3 | 9 | 14 | 20 |
| 18 × 18 × 8 in | 1.5 | 3 | 11 | 17 | 25 |
| 20 × 20 × 10 in | 2.31 | 5 | 17 | 26 | 39 |
| 24 × 24 × 10 in | 3.33 | 7 | 25 | 37 | 55 |
| 24 × 24 × 12 in | 4 | 8 | 30 | 44 | 66 |
| 30 × 30 × 12 in | 6.25 | 12 | 46 | 69 | 104 |
| 36 × 36 × 12 in | 9 | 17 | 66 | 99 | 149 |
Bag totals for several pads are worked out on the combined volume, so they can be lower than the per-pad count × the number of pads.
Footing tips
- Dig to the plan, then measure. Trenches tend to come out wider than drawn, especially in loose soil. Measure the real width and depth in a few places before you order, and update the calculator.
- Forms vs earth trench. A footing poured straight into a clean, firm trench uses more concrete than a formed one. Keep the waste allowance at 10% or more for earth-formed footings.
- Mind the corners. When you add up the sides of a rectangle, use outside dimensions on two sides and inside dimensions on the other two, or the corners get counted twice.
- Footing plus wall or piers. Work out the footing first, then add the wall with the Wall shape on the concrete calculator, or the block with the concrete block calculator.
- Get it inspected. Many towns inspect footing trenches and rebar before the pour. Book it before you book the truck.
Frequently asked questions
How much concrete do I need for a footing?
Multiply length (ft) × width (in) ÷ 12 × thickness (in) ÷ 12 to get cubic feet, then divide by 27 for cubic yards. A 16 × 8 in footing holds about 0.89 ft³ per foot of run, so 60 ft needs about 2.2 yd³ including 10% waste.
How thick should a concrete footing be?
The IRC requires concrete footings to be at least 6 in thick. Many plans call for 8–12 in, depending on the load and the width of the footing. Follow your plans and local code.
How deep should a footing be?
Below the local frost line and at least 12 in below undisturbed ground (IRC R403.1.4). Frost depths range from a few inches in the South to 4 ft or more in the northern US, so ask your building department for the local figure.
How many bags of concrete for a deck footing?
A 24 × 24 in pad, 10 in thick, holds 3.33 ft³: 7 bags of 80 lb mix with 10% waste. Add the pier or tube above it. The pad footing table above lists other sizes and totals for 4, 6 and 9 pads.
How do I calculate several pad footings at once?
Enter one pad (side length in feet as the length, the same side in inches as the width, and its thickness) and set Number of identical footings. The calculator multiplies the volume and works out bags on the combined total, which can save a bag or two compared with rounding each pad separately.
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