How to calculate concrete for footings
- Strip footings run continuously under walls. Measure the total length (the perimeter for a building), then the width and depth of the concrete.
- Pad footings are square blocks under posts or columns. Count them and measure their size and depth.
- Convert to cubic feet — length × width × depth, all in feet — then divide by 27 for cubic yards.
- Add 10% because dug trenches are never perfectly square.
Footing formula
Pad footing (ft³) = Number × Side (in)² ÷ 144 × Depth (in) ÷ 12
Cubic yards = Total ft³ ÷ 27
Worked example: a garage foundation
A 24 × 24 ft garage has 96 ft of perimeter footing, 16 in wide and 8 in deep: 96 × 1.333 × 0.667 = 85.3 cu ft. With 10% extra that is 93.9 cu ft, or 3.48 cubic yards — a ready-mix order, since it would take 157 bags of 80 lb mix.
Worked example: deck pads
Nine pad footings 24 × 24 in and 12 in deep hold 9 × 4 × 1 = 36 cu ft. With 10% extra that is 39.6 cu ft, or 1.47 cubic yards (66 bags of 80 lb).
Typical footing sizes
| Footing | Typical size | Notes |
|---|---|---|
| Light-frame house wall | 12–24 in wide × 6–10 in thick | Width depends on soil bearing and number of stories |
| Garage or shed wall | 12–16 in wide × 8 in thick | Often thickened-edge slab instead for small sheds |
| Garden or retaining wall (masonry) | About twice the wall width | Engineered for walls retaining soil |
| Deck or porch pier | 18–24 in square × 8–12 in thick | Size by load and soil; below frost line |
| Column pad | 24–36 in square × 12 in thick | Engineered for heavy loads |
The International Residential Code sets a minimum footing thickness of 6 in and widths from a table based on soil bearing capacity and the number of stories. Footings must also bear below the local frost depth (or be frost-protected). Your building department will tell you the frost depth and any local requirements.
Concrete per 10 ft of strip footing
| Width × depth | Cubic feet per 10 ft | Cubic yards per 10 ft | 80 lb bags per 10 ft |
|---|---|---|---|
| 12 × 6 in | 5.0 | 0.19 | 9 |
| 12 × 8 in | 6.7 | 0.25 | 12 |
| 16 × 8 in | 8.9 | 0.33 | 15 |
| 20 × 8 in | 11.1 | 0.41 | 19 |
| 24 × 10 in | 16.7 | 0.62 | 28 |
| 24 × 12 in | 20.0 | 0.74 | 34 |
Rebar
Continuous footings are commonly reinforced with two horizontal #4 bars, but requirements depend on your code, seismic zone and design. The calculator multiplies footing length by the number of bar runs and adds 10% for lap splices and bends. Keep bars about 3 in clear of the soil using chairs or bricks — concrete cast against earth needs that cover to protect the steel.
Pouring tips
- Dig to firm, undisturbed soil and remove loose material from the trench bottom.
- Get the footing inspected before pouring if your permit requires it.
- Order ready-mix for anything over about 1 cubic yard; mixing dozens of bags by hand is slow and risks cold joints.
- For posts, piers and round holes, use the concrete calculator; for block walls on the footing, the mortar calculator.
Frequently asked questions
How much concrete do I need for a footing?
Multiply length × width × depth in feet and divide by 27 for cubic yards. A 96 ft footing 16 in wide and 8 in deep needs about 3.2 cubic yards before waste.
How many bags of concrete for a footing?
Divide cubic feet by 0.6 for 80 lb bags. Each 10 ft of 16 × 8 in footing takes about 15 bags of 80 lb.
How deep should a footing be?
At least 6 in thick per the IRC, and bearing below your local frost depth — often 12–48 in or more depending on climate.
How wide should a footing be?
Typically 12–24 in for houses, set by code tables using soil bearing capacity and the number of stories.
Do footings need rebar?
Many codes and designs call for reinforcement — commonly two #4 bars in continuous footings — especially in seismic areas. Follow your local code or engineer.
Sources & standards
This calculator's formulas and default values are based on the following codes, standards and published data. Codes are adopted and amended locally, so check with your building department for permitted work. See how we build and check our calculators.
- International Residential Code (IRC) R403.1: footings — minimum width and thickness (Table R403.1(1)), frost protection (R403.1.4) and reinforcement (R403.1.3).
- ACI 332, Code Requirements for Residential Concrete: footing concrete and cover.
- Manufacturers’ published bag yields: about 0.60 ft³ per 80 lb and 0.45 ft³ per 60 lb bag of concrete mix.