Concrete Footing Calculator
Footings transfer structural loads into the ground and must extend below the frost line. This calculator computes the concrete volume needed for rectangular strip footings or spread footings. Enter the footing dimensions in feet, the number of footings, and a waste percentage. The formula is: Volume = Length x Width x Depth x Count / 27, where dividing by 27 converts cubic feet to cubic yards. Ready-mix concrete is ordered by the cubic yard. Per IRC Section R403, residential footings must be placed on undisturbed natural soil or engineered fill, and their depth and width depend on the load and soil bearing capacity.
Footing volume formula
Volume (ft3) = Length x Width x Depth x Count
Volume (yd3) = Volume (ft3) / 27
Order quantity = Volume (yd3) x (1 + waste% / 100)
Dividing cubic feet by 27 converts to cubic yards, the unit used for ordering ready-mix concrete. Always add waste to account for imperfect trench walls and any overpour at the top of the footing.
Footing design considerations
- Footings must bear on soil or rock with adequate bearing capacity. Soft or organic soils may require geotechnical engineering.
- Per IRC R403.1.4, footings must extend at least 12 inches below undisturbed soil surface.
- Frost protection depth varies by location; check ASCE 7 maps or local amendments for your jurisdiction.
- Reinforcing steel (rebar) in footings is required in seismic design categories C through F and many local codes.
- Concrete for footings should be at least 2,500 psi compressive strength per IRC R402.
Concrete footing calculator: frequently asked questions
How do I calculate concrete for footings?
Multiply the footing length (ft) by its width (ft) by its depth (ft) to get cubic feet, then divide by 27 to convert to cubic yards. For multiple footings, multiply by the number of footings.
What is the minimum footing size for a residential structure?
Per IRC Section R403, footings must be at least 12 inches wide and 6 inches deep for a 1-story structure, increasing with additional stories and load. Always check local building code requirements.
How deep should footings go below frost line?
Footings must extend below the local frost depth to prevent frost heave. The frost depth varies by location, from 0 inches in southern states to 72 inches or more in northern states. Check ASCE 7 frost depth maps or local codes.
Should I add a waste factor for footings?
Yes. Add 5 to 10 percent for waste, irregular trench walls, and over-pour. Trench walls are rarely perfectly vertical, so actual volume is often slightly higher than calculated.
Can I use this for a deck footing or fence post footing?
Yes. For cylindrical tube footings (like Sonotube), compute the volume as pi x radius squared x depth. This calculator handles rectangular footings. For round footings, use radius squared x 3.1416 x depth / 27.
Official sources
- International Code Council: International Residential Code (IRC) R403 Footings.
- American Concrete Institute: ACI 318 Building Code Requirements for Structural Concrete.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.