Concrete Footing Calculator

How to Calculate Concrete for a Footing

A footing is a volume problem. Measure each run or pier, convert every dimension to feet, then convert cubic feet to cubic yards:

Cubic feet = Length (ft) × Width (ft) × Depth (ft)
Cubic yards = Cubic feet / 27

Width and depth are usually recorded in inches on a plan. For handwritten math, convert those inches to feet (divide by 12) before multiplying. In the calculator above, do not convert the depth: Rectangular Slab takes length and width in feet, and Thickness in inches. It divides thickness by 12 internally.

Strip (continuous) footings

Treat each straight run as a rectangle. Do not average different widths or depths — split the foundation into sections, calculate each one, then add the cubic yards.

Example: a 40 ft strip, 16 in wide and 8 in deep:

• Hand math — width: 16 / 12 = 1.333 ft; depth: 8 / 12 = 0.667 ft
• Volume: 40 × 1.333 × 0.667 = 35.56 cubic feet
• Cubic yards: 35.56 / 27 = 1.32 cubic yards
• With 10% waste: 1.32 × 1.10 = 1.45 cubic yards
• Calculator fields: Length 40, Width 1.333, Thickness 8 (inches). Do not type 0.667 into Thickness.

That volume is about 59 bags of 80-lb mix. For anything over about 1 cubic yard, ready-mix is usually cheaper and more consistent than bags.

Pier pads and sonotube piers

Round piers use a cylinder: π × radius² × height. In the calculator, open Tube / Column, enter the outer diameter and height, and leave inner diameter empty (0) for a solid pier. Use Round Slab for a shallow circular pad.

Example: twelve 12 in diameter piers, each 4 ft deep:

• One pier: π × 0.5² × 4 = 3.14 cubic feet = 0.116 cubic yards
• Twelve piers: 1.40 cubic yards
• With 10% waste: 1.54 cubic yards

Waste, bags, and ready-mix

Footing trenches are rarely square. Add 5–10% extra for over-excavation, sloughing, and spillage. Running short on a continuous pour creates a cold joint in the footing.

• Under ~1 cubic yard: 80-lb bags (about 45 bags per cubic yard) are practical.
• Over 1–2 cubic yards: order ready-mix. Ask about short-load fees on small truck orders.

Many residential footings use 3000–4000 PSI concrete, but mix, width, depth, frost line, and rebar come from the plans and the authority having jurisdiction — not from this calculator.

What this calculator does not decide

Quantity only. It does not size the footing, choose reinforcement, evaluate soil, or replace a structural engineer, inspector, or building code. Confirm dimensions on the approved drawings before you order.