Concrete Footing Calculator
Pouring a continuous strip footing under a foundation wall, or isolated spread footings under posts and columns? Enter the footing length, width, and thickness to see the concrete it takes — in cubic yards for ready-mix and in bags for small pours — with a waste factor built in. Footing depth is a code question: keep the bottom below your local frost line.
Total volume required
1.22Cubic Yards
Cubic feet
33.00
Cubic meters
0.93
Bags (80 lb)
55
Bags by size
40 lb
110
50 lb
88
60 lb
74
80 lb
55
Estimate includes your waste factor. Figures are estimates, not a quote.
How to use this calculator
- 1Measure the footing length in feet, the footing width in feet (an 18 in footing is 1.5 ft wide), and the thickness in inches.
- 2For several identical pad/spread footings, set Quantity to the number of footings.
- 3Keep a 10% waste factor for over-excavation and an uneven trench bottom; raise it for soft or caving soil.
- 4Read cubic yards to order ready-mix, or the bag count for a small footing you mix on site.
Formula & rounding
A footing is a rectangular prism: length × width × thickness, all converted to feet (thickness in inches ÷ 12). Cubic feet ÷ 27 gives cubic yards, and the quantity multiplies the volume for repeated spread footings. Bag counts round up. Footing WIDTH and DEPTH are set by soil bearing capacity, load, and local frost code — the calculator sizes the concrete, not the structural design.
20 ft strip footing, 18 in wide, 12 in thick
- Width = 18 in = 1.5 ft; thickness = 12 in = 1.0 ft
- Volume = 20 × 1.5 × 1.0 = 30 cubic feet
- With 10% waste: 30 × 1.10 = 33 cubic feet
- Cubic yards = 33 ÷ 27 = 1.22 cubic yards
= ≈ 1.22 cubic yards for the strip footing (order ready-mix near this size)
Four spread footings, 2 ft × 2 ft × 12 in, under deck posts
- One footing = 2 × 2 × (12 ÷ 12) = 4 cubic feet
- Four footings = 4 × 4 = 16 cubic feet
- With 10% waste: 16 × 1.10 = 17.6 cubic feet
- 80 lb bags = 17.6 ÷ 0.60, rounded up = 30 bags
= ≈ 30 × 80 lb bags for four 2 ft square footings
Footing size & frost-depth guide
Footing width follows the load and the soil; footing DEPTH must reach below the local frost line where frost protection applies. These are planning references only — your engineered design and local building department govern.
| Footing type | Typical width | Frost / depth note |
|---|---|---|
| Strip footing under a wall | 16–24 in (1.3–2.0 ft) | Bottom below local frost line; depth set by code |
| Spread footing under a post/column | 18–36 in square | Sized for point load; verify bearing capacity |
| Light deck/porch footing | 12–24 in square | Below frost line; check IRC deck provisions |
| Thickened-edge / monolithic | Edge 12–16 in deep | Designed with the slab, not a separate pour |
Code reference: 2024 IRC R403.1 / R403.1.4. Soil, load, and adopted local frost depth override this planning table.
Frequently asked questions
How much concrete do I need for a footing?
Multiply footing length × width × thickness in feet for cubic feet, then divide by 27 for cubic yards. A 20 ft strip footing 18 in wide and 12 in thick is about 1.2 cubic yards with waste. Enter your dimensions for the exact amount.
How deep should a concrete footing be?
Deep enough that the bottom sits below your local frost line and on soil with adequate bearing capacity. Frost depth varies from about 12 inches in warm regions to 48 inches or more in cold climates. Your local building department sets the required depth.
How wide should a concrete footing be?
Width depends on the load above and the soil below. Residential strip footings are often 16–24 inches wide, and spread footings under posts are commonly 18–36 inches square, but the design width should come from code tables or an engineer for anything structural.
Do concrete footings need rebar?
Many footings are reinforced, but whether and how is a structural and code decision, not a volume one. This calculator estimates only the concrete; confirm reinforcement with your plans, local code, or an engineer.
What is the difference between a footing and a slab?
A footing is a thick, narrow element that carries load down to firm soil below the frost line; a slab is a wide, thin surface. Use this footing calculator for trenched or spread footings and the slab calculator for flatwork.
Assumptions & sources
- Volume formula
- length × width × thickness (all in feet); cubic feet ÷ 27 = cubic yards, × quantity for repeated footings.
- Bag yields
- ≈0.60 cu ft per 80 lb bag (0.45 / 0.375 / 0.30 for 60 / 50 / 40 lb); rounded up.
- Waste factor
- Default 10% for over-excavation and uneven trench bottoms; raise for soft or caving soil.
- Frost line
- 2024 IRC R403.1 / R403.1.4 require footing depth below the local frost line where frost protection applies — confirm the adopted local frost depth.
- Scope
- Concrete volume only — footing width, depth, and reinforcement are structural/code decisions this page does not make.
See the methodology & sources for how these values, formulas, and rounding are chosen.
Helpful concrete guides
- How to calculate concrete — covers the length × width × thickness volume math and waste factor used for footings.
This is an estimate, not a quote. Concrete quantities, bag yields, block coverage, and prices vary with product, brand, mix, region, supplier, tax, delivery, and on-site conditions. Always confirm with your supplier and round up for safety. For structural or code-related work, consult a qualified professional or your local building authority.
Frost line: Footing and post-hole depth is often governed by local code. Check your local frost line and building code — footing depth must meet local requirements. This tool does not determine code compliance.