Concrete Footing Calculator

How it works

A strip footing is a long rectangular box of concrete, so its volume is width × depth × length. Width and depth are entered in inches and converted to feet; the length is the total of every run of footing in feet, which gives cubic feet.

Each square pad footing is side × side × depth, and the pads are multiplied by their count and added to the strip. The total is converted to cubic yards (27 cu ft to the yard), the unit ready-mix is sold in.

The waste allowance is added on top of the measured volume. Footings poured straight into an earth trench lose more than formed work, because the trench sides and bottom are never perfectly cut. The bag count divides the volume with waste by the yield of one bag of pre-mixed concrete and rounds up to whole bags.

Formula

strip (cu ft)  = (width ÷ 12) × (depth ÷ 12) × length
pads (cu ft)   = count × (side ÷ 12)² × (pad depth ÷ 12)
total (cu yd)  = (strip + pads) ÷ 27
to order       = total cu yd × (1 + waste ÷ 100)
bags           = ⌈ (strip + pads) × (1 + waste ÷ 100) ÷ yield per bag ⌉
yield per bag: 40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft (QUIKRETE No. 1101, rev. Oct 2022)

Example

A 24 in wide, 12 in deep strip footing 50 ft long is 2 × 1 × 50 = 100 cu ft. Two 24 in square pads 12 in deep add 2 × 2 × 2 × 1 = 8 cu ft, for 108 cu ft in all, which is exactly 4 cu yd.

With a 10% waste allowance the amount to order is 4.4 cu yd. In 80 lb bags that is 108 × 1.1 = 118.8 cu ft ÷ 0.60 = 198 bags exactly.

Assumptions and limitations

• The footing has a rectangular cross-section of constant width and depth along its whole length, and pads are square with vertical sides. Stepped footings, sloped trench bottoms and over-dig are not modelled; the waste allowance is the only margin for them.
• Corners are counted once: the total length is the centre-line length of the footing, not the sum of outside edges.
• Bag yields are the approximate figures on the QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022): 0.30 cu ft per 40 lb bag, 0.45 per 60 lb and 0.60 per 80 lb. Other brands and mixes yield differently and the bag label governs.
• The waste allowance is a typical figure (5–10% is common for earth-formed footings) and is editable.
• Rebar, forms, the gravel base and any anchor bolts are not included.
• This is a material estimate, not engineering. Footing width and depth depend on the load, the soil and the frost depth; they are set by the applicable building code (for houses, the International Residential Code as adopted locally), the building official or a licensed engineer. The result says nothing about whether a footing is adequate or code compliant.

Frequently asked questions