A continuous footing (also called a strip or wall footing) is the wide band of concrete under a foundation wall, garage wall, block wall or thickened shed edge. Its volume is simply length × width × thickness, but footings are measured in feet, inches and inches, so it's easy to slip. This concrete footing calculator converts everything, adds a trench allowance and gives the result in cubic yards, bags and ready-mix trucks. It also counts rebar sticks for the continuous bar lines, including lap splices.
The wall above the footing can be sized with the concrete block calculator for CMU or the wall framing calculator for wood studs. For isolated piers and deck footings, use the concrete column & post hole calculator.
How to use the concrete footing calculator
- Enter the total footing length. For a rectangular building, that is the perimeter; include interior bearing-wall footings if you are pouring them at the same time.
- Enter the width and thickness from your plans. The International Residential Code sets minimum widths by soil bearing capacity and building load, and a minimum thickness of 6 inches.
- Add 5–15% for trenches that are wider or deeper than planned. Earth-formed trenches usually need more than wood forms.
- Enter the number of continuous rebar lines, the bar size, stick length and lap length from your plans or engineer.
- Optionally add a ready-mix price per cubic yard to estimate the concrete cost.
Formula
Rebar weight uses the standard nominal weights: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft.
Worked example
A 30 × 30 ft garage has 120 ft of footing, 16 inches wide and 8 inches thick, with two lines of #4 bar in 20 ft sticks lapped 24 inches, and 10% extra concrete.
- Volume: 120 × (16 ÷ 12) × (8 ÷ 12) = 120 × 1.333 × 0.667 = 106.7 ft³ = 3.95 yd³.
- With 10%: 106.7 × 1.10 ÷ 27 = 4.35 yd³, one truck, or 196 bags of 80 lb mix.
- Rebar: (120 − 2) ÷ (20 − 2) = 6.6, so 7 sticks per line × 2 lines = 14 sticks of #4, 280 ft and about 187 lb.
Corner bars that bend around each corner are extra; add them from your plans.
Cubic yards per 10 feet of footing
Concrete for every 10 linear feet of footing, no waste. Multiply by your total length ÷ 10.
| Footing width | 6 in deep | 8 in deep | 10 in deep | 12 in deep |
|---|---|---|---|---|
| 12 in | 0.185 | 0.247 | 0.309 | 0.370 |
| 16 in | 0.247 | 0.329 | 0.412 | 0.494 |
| 20 in | 0.309 | 0.412 | 0.514 | 0.617 |
| 24 in | 0.370 | 0.494 | 0.617 | 0.741 |
A 16 × 8 inch footing uses about 0.33 yd³ per 10 ft, so 100 ft of it is about 3.3 yd³ before waste.
Footing size rules of thumb
The IRC's footing tables (Section R403) size the width by the soil's presumptive load-bearing value and the number of stories, from 12 inches upward, and require at least 6 inches of thickness. The bottom of the footing must sit at least 12 inches below undisturbed ground and below the local frost depth. An older rule of thumb, a footing twice as wide as the wall and as thick as the wall is wide, gives 16 × 8 inches under an 8 inch block wall. Always follow your local code and any engineer's drawings, which may require larger footings on weak or expansive soils.
Bags or ready-mix for footings?
Footings are usually poured from a truck: even a small garage footing is 4 to 5 yards, or about 200 bags of mix. Bags make sense for short runs, such as a thickened slab edge under a shed wall or a footing for a short garden wall. The concrete bags per yard calculator converts any volume into bags, and the rebar calculator handles slab grids if you're pouring a floor too.
Caveats and assumptions
- This calculator gives volume only. Footing size, reinforcement and depth must follow your local building code or an engineer's design.
- Stepped footings on sloping sites have vertical sections; add those volumes separately.
- Rebar counts cover straight continuous bars only. Corner bars, dowels into the wall and chairs are extra.
- Ready-mix trucks typically carry up to about 9 to 10 yd³; actual capacity varies by supplier and local weight limits.
Frequently asked questions
How much concrete do I need for a footing?
Multiply length (ft) × width (ft) × thickness (ft) and divide by 27 for cubic yards. A 100 ft footing that is 16 × 8 inches needs 100 × 1.333 × 0.667 ÷ 27 = 3.3 yd³ before waste.
How many bags of concrete for a footing?
Divide the volume in cubic feet by 0.60 for 80 lb bags. A 20 ft footing that is 12 × 8 inches is 13.3 ft³, so 23 bags of 80 lb mix, or 25 with 10% extra.
What is the minimum footing size under the IRC?
The IRC requires footings at least 6 inches thick, with widths from its tables starting at 12 inches depending on soil bearing value and stories. Check the edition your jurisdiction has adopted.
How deep should a footing be?
The bottom must be at least 12 inches below undisturbed ground and below the frost line, which can range from a few inches in warm climates to more than 4 feet in cold ones. Ask your building department.
How much rebar goes in a footing?
Many residential footings use two continuous lines of #4 bar, but requirements vary with seismic zone and design. Each 20 ft stick lapped 24 inches covers 18 ft of footing.
Sources
- ICC — 2021 International Residential Code, Chapter 4: Foundations — Section R403 footing width, thickness and depth
- Portland Cement Association — Concrete technology — foundations and concrete placement
- National Ready Mixed Concrete Association — ordering ready-mixed concrete
- Concrete Reinforcing Steel Institute — rebar sizes and nominal weights
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.