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Concrete Footing Calculator
Use this free concrete footing calculator to find out how much concrete a footing needs before you dig. Pick a strip, wall, square pad, or round pad footing, enter the dimensions, and it works out the volume in cubic feet and cubic yards, how many 60-pound and 80-pound bags that means, and an estimated cost at a price you can change. Add a waste allowance so you never run short mid-pour.
Concrete prices are estimates and vary by region and supplier — edit the price to match your local quote.
| Volume | 40 ft³ (1.48 yd³) |
| 60-lb bags needed | 89 bags |
| 80-lb bags needed | 67 bags |
| Estimated ready-mix cost | $222.22 |
| Estimated bagged cost (80-lb) | $335.00 |
How it works
A footing is the wide concrete base at the bottom of a foundation. Its job is simple: take the weight of the wall or column above it and spread that weight over a large enough area of soil that the building does not sink or crack. The calculator finds the concrete volume from the footing's shape, then converts that volume into the two units suppliers actually use — cubic feet for bagged concrete and cubic yards for ready-mix trucks.
There are three common footing shapes. A strip footing is a long, continuous pour that runs under a load-bearing wall; a wall footing is the same shape, sized to the wall it carries. A pad footing is a single block — square, rectangular, or round — that supports one point load such as a porch column, a deck post, or a steel column in a garage. The calculator handles all of them: strip and wall footings use length, width, and depth, square pads use two side lengths and depth, and round pads use diameter and depth.
Bagged concrete is sold by weight, and each bag makes a fixed volume of concrete once mixed with water. A 60-pound bag yields about 0.45 cubic feet and an 80-pound bag yields about 0.60 cubic feet. The calculator divides the footing volume by those yields and rounds up, because a partial bag still has to be bought whole. For small footings — a single deck post pad, for example — bags are convenient. For anything over about half a cubic yard, a ready-mix truck is usually cheaper and far less work, and the calculator's cost estimate uses your price per cubic yard for that comparison.
Depth and width are not free choices: they come from your soil and your local building code. A footing must bear on undisturbed soil below the frost line, which in cold climates means the bottom of the footing sits 24 to 48 inches below grade, while in warm climates 12 inches below grade is often enough. The footing must also be wide enough that the soil pressure stays within what the soil can carry — soft clay needs a wider footing than firm gravel for the same wall. Your permit drawings or a local engineer set these numbers; the calculator turns them into a concrete order.
Always add a waste allowance when ordering. Trenches are never perfectly straight, the bottom is never perfectly level, and some concrete is lost to the ground and the forms. Five to ten percent is the usual allowance, and a short load — running out with one wheelbarrow left to pour — costs far more in delay and delivery fees than the extra concrete. The calculator lets you set the waste percentage and adds it to the volume, the bag count, and the cost, so the number you order is the number you actually need.
Worked example
A builder is pouring a strip footing for a garden wall: 20 feet long, 2 feet wide, and 1 foot deep, with no waste allowance. How much concrete is needed, and how many bags would it take?
Step 1: Volume in cubic feet — 20 × 2 × 1 = 40 ft³.
Step 2: Convert to cubic yards — 40 ÷ 27 = 1.48 yd³.
Step 3: 60-pound bags — 40 ÷ 0.45 = 88.9, rounded up to 89 bags.
Step 4: 80-pound bags — 40 ÷ 0.60 = 66.7, rounded up to 67 bags.
Step 5: Ready-mix cost at $150 per cubic yard — 1.48 × $150 = $222.22.
Enter 20, 2, and 1 in the calculator with "Strip footing" selected and a 0 percent waste allowance, and it gives exactly the same result: 40.00 cubic feet, 1.48 cubic yards, 89 sixty-pound bags or 67 eighty-pound bags, and about $222.22 of ready-mix concrete. Add a 10 percent waste allowance and the volume rises to 44 cubic feet, or 1.63 cubic yards — a useful reminder of how cheap insurance that extra concrete is. At $5 per 80-pound bag, the bagged route would cost about $335, which shows why a truck wins once the volume grows.
FAQ
How many cubic yards of concrete do I need for a footing?
Multiply length by width by depth in feet to get cubic feet, then divide by 27 to get cubic yards. A strip footing 20 feet long, 2 feet wide, and 1 foot deep is 40 cubic feet, which is 1.48 cubic yards. Add about 10 percent extra for waste and uneven ground if you are ordering ready-mix concrete.
How many bags of concrete do I need for a footing?
Divide the footing volume in cubic feet by the yield of one bag: a 60-pound bag makes about 0.45 cubic feet and an 80-pound bag makes about 0.60 cubic feet. A 40 cubic foot footing needs 89 sixty-pound bags or 67 eighty-pound bags. Bags make sense for small footings; a ready-mix truck is usually cheaper for anything over about half a cubic yard.
How deep should a concrete footing be?
A footing must sit below the frost line in cold climates and on undisturbed soil, which usually means at least 12 inches below grade and often 24 to 48 inches deep in total including the footing itself. Local building codes set the exact minimum depth and width for your area, so always check the code and your permit drawings before digging.
What is the difference between a strip footing and a pad footing?
A strip footing is a long, continuous pour that runs under a wall, spreading its weight along a line. A pad footing is a single square or round block of concrete that supports one point load, such as a column or a post. Wall footings are strip footings sized to the wall they carry, while pad footings are sized to the load of a single post.
Should I add extra concrete for waste?
Yes, ordering 5 to 10 percent extra is standard practice. Trenches are rarely perfectly straight, the ground absorbs some concrete, and a short load costs far more in time and delivery fees than a little extra mix. Enter your waste allowance in the calculator and it adds it to the volume, bag count, and cost.