Concrete for footings and sonotubes

How to measure

Wall footings run under a foundation wall, a garage wall, or a block retaining wall. Measure the total length of every side and add them up. A 20 by 24 foot garage has a run of 88 feet. Width and depth go in inches, because that is how footings are drawn. Many house plans call for something like 16 to 24 inches wide and 8 to 12 inches deep, but your plan and your local code decide the real size.

Square pads sit under posts, columns, and deck beams. Measure one pad and enter how many you have. Round footings are dug holes or cardboard form tubes (often sold as Sonotube). Use the tube diameter and the full height of concrete, which is the hole depth plus any part of the tube that sticks up above the ground.

The formula

• Wall footing: run (ft) x width (in / 12) x depth (in / 12) = cubic feet
• Square pad: length x width x depth, all in inches, divided by 1,728, times the count
• Round: 3.1416 x radius x radius x height, in feet, times the count

Divide cubic feet by 27 to get cubic yards. For example, a 12 inch tube that is 4 feet tall holds about 3.14 cubic feet, so eight of them is about 25 cubic feet, a little under one yard before waste.

Waste factor

Footings are dug in dirt, and trenches are rarely as neat as the drawing. Ten percent is a sensible default. Go to 15 percent if the trench walls are crumbly or you dug wider than planned. Form tubes are more exact, so 5 percent is often enough for sonotubes.

Bags or a truck?

An 80 lb bag makes about 0.60 cubic feet and a 60 lb bag about 0.45, so one yard is about 45 or 60 bags. A few deck piers are a bag job. A full house or garage footing is usually a truck, and plants often add a short-load fee on small orders. The cost guide explains those fees.

Tips before you pour

• Footings need to go below your local frost depth. Ask your building department.
• Rebar takes up so little room that you can ignore it in the math.
• Pour on firm, undisturbed soil, not loose fill.
• Pouring a slab too? Use the slab calculator for the flat part and add the two numbers.