Concrete Footing Calculator

1. Enter the footing length in feet. For a continuous footing this is the total run; for a pad or pier it is one footing length.
2. Enter the width and depth in inches, from your plans or local code.
3. Set the number of footings for identical pads or piers, and a 5 to 10 percent waste allowance.
4. Pick a bag size if you want a bag count, then read the cubic yards to order.

Here is what each result line means:

A concrete footing calculator turns the length, width and depth of a footing into the amount of concrete to order. A footing is the widened base that spreads a load from a wall, column, post or deck into the soil, so getting its volume right is the first step in any foundation job.

It handles the two common cases: a continuous strip footing that runs under a wall, and isolated pad or pier footings under columns, posts and deck supports. Concrete is sold by the cubic yard for ready-mix or by the bag for small pours, so the tool converts feet and inches into both and adds a waste allowance for an uneven trench. To get the mix proportions right use the concrete mix calculator, and to size the binder in the mix use the cement calculator.

A footing is a rectangular prism, so its volume is length times width times depth. Because concrete is priced in cubic yards and one cubic yard is 27 cubic feet, the tool works in feet, finds cubic feet, then divides by 27.

1. Convert the width and depth from inches to feet by dividing by 12 (16 inches is 1.333 feet).
2. Multiply length by width by depth for the volume of one footing in cubic feet.
3. Multiply by the number of footings, then add the waste allowance.
4. Divide by 27 for cubic yards, multiply by 0.0283168 for cubic meters, and divide cubic feet by the bag yield for a bag count.

Mixing water for the pour is a separate question; size it with the water cement ratio calculator.

Suppose you are pouring a continuous strip footing 40 feet long, 16 inches wide and 8 inches deep, with 10 percent waste.

Calculation: width is 16 / 12 = 1.333 ft and depth is 8 / 12 = 0.667 ft. Volume = 40 x 1.333 x 0.667 = 35.56 cu ft. In cubic yards that is 35.56 / 27 = 1.32 cu yd. Adding 10 percent waste gives 39.1 cu ft, or 1.45 cu yd, about 66 eighty-pound bags and roughly 2.93 short tons.

The lesson: at nearly a cubic and a half yard, this footing is well past the point where ready-mix beats mixing 66 bags by hand.

Strip Footings Under Walls

Residential strip footings are commonly 16 to 24 inches wide and 8 to 12 inches deep, often twice the width of the wall they carry. The exact size depends on the load and the soil bearing capacity.

Pad Footings Under Columns and Posts

Isolated pads spread a point load from a column or post. They are sized to the load and soil, and are usually square, from about 2 by 2 feet upward.

Pier Footings and Frost Depth

Deck piers and post footings often have to reach below the local frost line so the ground does not heave them in winter. That depth can be several feet in cold regions.

These are common figures, not a specification. The required width, depth, frost depth and reinforcement come from your plans, local building code or a structural engineer.

Width and Depth

Both multiply the volume directly, so an over-wide or over-deep trench uses far more concrete than planned. Form the trench to the design size rather than digging loose.

An Uneven or Caving Trench

A footing is poured against the soil, so a ragged, collapsing or over-excavated trench soaks up concrete below the design line. This is the main reason real footings use more than the raw geometry.

Number of Footings

Pad and pier jobs multiply one footing by the count, so a small error per footing is multiplied across the whole job. Confirm the single-footing size first.

Waste and Spillage

Concrete is lost to the barrow, the trench walls and finishing. A 5 to 10 percent allowance covers it; add more for a rough trench or a long barrow run.

The two footing types carry load differently, which changes how you measure and pour them.

For the delivered price of either pour, the concrete cost calculator in the related tools prices it by the yard.

Ordering Ready-mix for a Foundation

Size the footing order in cubic yards and add waste before you call the supplier, so you can pour the whole trench in one go without a cold joint.

Pricing a Bag Job for a Few Piers

For a handful of pier or post footings, the bag count tells you how many bags to buy and whether hand mixing is realistic.

Planning Access and Labour

The weight and the volume per footing help you plan barrow loads, access down a slope and how many hands you need on the day.

1. Mixing Up Feet and Inches

Entering width or depth in feet instead of inches shrinks the result twelvefold. Keep length in feet and width and depth in inches, as the fields are labelled.

2. Forgetting the Quantity on Pads

Entering one pad footing but leaving the count at 1 under-orders a whole job. Set the number of footings for identical pads and piers.

3. Ignoring an Over-dug Trench

A footing poured against loose or caving soil holds more than the design rectangle. A waste allowance covers a tidy trench; a rough one needs more.

4. Using the Wrong Depth for Frost

A pier that stops above the frost line can heave. Use the depth your code calls for, which this tool then turns into volume.

5. Treating the Estimate as a Structural Spec

Volume is not bearing. Footing size, reinforcement and depth for a load-bearing foundation come from a code or an engineer, not from this tool.

The arithmetic is exact; the usefulness depends on the dimensions, the quantity and the waste allowance you enter.

Multiply length by width by depth, all in feet, then divide by 27 for cubic yards and add 5 to 10 percent waste. A 40 foot strip footing 16 inches wide and 8 inches deep is about 1.32 cubic yards, or 1.45 with waste. Enter your sizes above for the exact figure.

Enter the size of one pad in feet and inches, then set the number of footings to the count. The tool multiplies one footing by the quantity, adds waste and gives both the total and the volume per footing, so you can plan each small pour.

Residential strip footings are commonly 8 to 12 inches deep, but pier and post footings often reach below the local frost line, which can be several feet in cold regions. The required depth comes from your local code or engineer; this tool turns that depth into volume.

Strip footings are often 16 to 24 inches wide, frequently about twice the width of the wall they carry, but the exact width depends on the load and the soil bearing capacity. Use the width on your plans; the calculator converts it to concrete volume.

It depends on the size and bag. A 40 foot strip footing 16 inches by 8 inches is roughly 66 eighty-pound bags with waste, which is why ready-mix usually wins at that size. For a few small piers, the bag count shows whether hand mixing is realistic.

Yes, a 5 to 10 percent waste allowance. A footing is poured against the soil, so an uneven or over-dug trench soaks up extra concrete, and running short mid-pour risks a cold joint in the foundation.

For a few small pier or post footings, bags can be practical and cheaper. For a continuous strip footing past about a cubic yard, ready-mix is usually easier and lets you pour the whole run in one go without a cold joint.

No. It estimates the concrete volume once you enter the size. The required width, depth, reinforcement and bearing for a load-bearing footing come from your plans, local code or a structural engineer.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and ordering).
• American Cement Association (American Cement Association; concrete and foundations basics).