How it works
The calculator finds the length of footing, multiplies it by the cross-section (width × depth) and adds your extra allowance. For a rectangle measured on its outside edges, the center line is used so that the four corners are counted once.
Rectangle: Footing length = 2 × (Outside length + Outside width) − 4 × Width
Volume = Footing length × Width × Depth
Concrete to order = Volume × (1 + Extra ÷ 100)
Bags = ⌈ Concrete to order ÷ Yield per bag ⌉
⌈ ⌉ means rounding up to the next whole bag. The volume itself is not rounded.
Units are converted internally, so you can enter the length in feet and the width and depth in inches; the result is shown in cubic yards and cubic feet (or cubic meters and liters). The default yield of 0.60 cu ft is QUIKRETE’s figure for an 80 lb bag of its Concrete Mix – change it to match your bag. This tool estimates material quantity only; footing sizes and reinforcement come from your plans, local code or an engineer.
Worked example
Inputs
- Measure by
- Length of all runs
- Total footing length
- 120 ft
- Footing width
- 16 in
- Footing depth (thickness)
- 8 in
- Yield per bag
- 0.6 cu ft
- Extra concrete
- 0 %
Result
- Concrete to order
- 3.95 cu yd (106.7 cu ft)
- Bags
- 178
- Footing length (center line)
- 120 ft
- Volume before extra
- 3.95 cu yd (106.7 cu ft)
Common mistakes
- Adding up the outside length of every side of a rectangle and using that as the run. Each corner is then counted twice; use the rectangle option or measure along the center line.
- Entering the trench depth below ground instead of the footing thickness. Only the concrete itself counts.
- Ordering the exact calculated volume. Soil trenches are wider and deeper in places than planned, so the real pour is usually larger.
- Forgetting piers, pads under columns or a thickened slab edge poured at the same time. Calculate them separately and add them.
- Taking footing width, depth, frost depth or reinforcement from a calculator. Those come from your plans, local code or a qualified professional.
Assumptions and limits
- The footing is a continuous strip with a rectangular cross-section of the same width and depth along its whole length. Stepped footings, keyways and pads are not modeled.
- In rectangle mode the footing runs around the outside of the rectangle, and the four corners are counted once.
- The volume of rebar, dowels or embedded items is not subtracted.
- Bags are rounded up to whole bags after the extra allowance. Without a yield, only the volume is shown.
- This is a material quantity only, not a structural design.
Frequently asked questions
How do I calculate the concrete for a footing?
Multiply the length by the width and the depth. 120 ft of footing 16 in wide and 8 in deep is 120 × 1.333 × 0.667 ≈ 106.7 cu ft, or about 3.95 cu yd (divide cubic feet by 27).
How do I measure the length of a footing around a building?
Measure along the center line of the footing, or use the rectangle option with the outside dimensions. For a 40 × 24 ft rectangle with a 16 in footing, the center line is 2 × (40 + 24) − 4 × 1.333 ≈ 122.7 ft.
How wide and deep should my footing be?
That depends on the load, the soil, frost depth and local code. This calculator doesn’t recommend sizes – use your plans or ask your building department or an engineer.
Bags or ready-mix for a footing?
Footings for a building quickly reach several cubic yards, which is a lot of bags. Compare the bag count with a ready-mix quote, including any short-load charge.
Sources
- QUIKRETE Concrete Mix (No. 1101) Product Data Sheet — The QUIKRETE Companies. Yield per bag: 40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³, 90 lb ≈ 0.675 ft³.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 4, 2026