Concrete footing calculator

How to calculate concrete for a concrete footing

A rectangular structural footing carrying a wall, deck post or column down below the frost line. The volume formula is Length × Width × Depth, worked in feet and then converted. There are 27 cubic feet in a cubic yard, which is the unit every ready-mix plant quotes in. The default project on this page comes to 33.0 cubic feet, or 1.22 cubic yards.

The conversion that catches people is thickness. Slabs and footings are specified in inches and ordered in cubic yards, so a 4-inch slab is 4 ÷ 12 = 0.333 feet deep, not 4, an error that overstates the order twelve-fold. Working the other way, one cubic yard covers 81 square feet at 4 inches thick, 65 at 5 inches and 54 at 6 inches.

On top of the geometric volume the calculator adds a 10% waste allowance. That is not padding: subgrade is never perfectly level, forms bow outward under the weight of wet concrete, and some is always lost to the wheelbarrow. Ordering the exact calculated volume reliably leaves you short, and being short is the expensive direction. A partial second load carries the $75 short-load fee and leaves a cold joint where it meets concrete that has already begun to set.

How many bags of concrete for a concrete footing

Bag yields are the manufacturers' published figures: a 40 lb bag of QUIKRETE or Sakrete concrete mix yields 0.3 cubic feet, a 60 lb bag 0.45, and an 80 lb bag 0.6. That is 90, 60 or 45 bags to a cubic yard. For this project the concrete calculator returns 55 eighty-pound bags, 74 sixty-pound or 110 forty-pound.

All three counts are shown at once because the decision at the store is rarely about price. Larger bags are marginally cheaper per cubic foot and considerably heavier to move, 55 eighty-pound bags is 2.2 tons to load, carry and cut open, and 60 lb bags are what many people can actually handle repeatedly.

At 1.22 cubic yards this is comfortably inside DIY territory, a rented mixer and a helper will get through it in a morning.

Bags or ready-mix for a concrete footing?

Ready-mix is cheaper per yard almost every time: $155 delivered against $293 a cubic yard bought in 80 lb bags. What bags buy is scheduling, no delivery window, no minimum load, and the freedom to stop and start. That is worth real money below about 2 cubic yards and worth nothing above it.

For this project specifically, bags come to $358 and ready-mix about $264, including the $75 short-load fee that applies under 5 yards. There is no finishing labor line on this shape. The concrete is buried, so nobody is troweling a surface.

A truck also arrives uniformly mixed, which matters more than it sounds. Concrete mixed in batches at slightly different water ratios cures to different strengths, and a pour that stretches over several hours develops cold joints where each batch meets the last.

Footings, piers and Sonotube column forms

A cardboard column form, Sonotube is the brand name most people search for, though the yard may stock another, turns a round hole into a clean pier. The volume is straightforward: 0.35 cubic feet per foot of height at 8 inches across, 0.55 at 10 inches, 0.79 at 12 and 1.40 at 16. Switch the shape selector above to column and the Sonotube concrete calculator sizes any diameter and depth, including the quantity.

Volume rises with the square of the diameter, which is the part people get wrong when they upsize on site. Going from a 10-inch form to a 12-inch one adds 44% more concrete, not 20%, on eight piers four feet deep that is roughly 7.7 extra cubic feet, or about 13 more bags than you bought.

Whatever the form, the bottom of the footing has to bear below the frost line and on undisturbed soil, never on backfill. A bell or a spread pad at the base distributes the load across weaker soils, and most jurisdictions want to inspect the open hole before any concrete goes into it.

How we calculated these estimates

Every figure on this page comes from the same open formula the calculator above uses. Volume is pure geometry, the shape's formula worked in feet, plus a 10% waste allowance. Prices start from 2026 national baselines: $155 per cubic yard delivered, a $75 short-load fee under 5 yards, retail bag prices against the yields QUIKRETE and Sakrete publish, and $3.50 per square foot for placing and finishing flatwork. Ready-mix and labor are then scaled by the construction earnings multiplier the Bureau of Labor Statistics publishes for your state; bagged concrete is not, because it is priced nationally. Nothing is hidden behind a lead form and no supplier pays to appear here. What the result is not is a quote: excavation, base preparation, forms, reinforcement, permits and demolition are excluded, and a plant will price your delivery distance and mix specification on top.