Concrete Footing Calculator
A concrete footing calculator multiplies run × width × depth ÷ 27. A 20 ft footing, 2 ft wide and 12 in deep, takes 1.48 yd³, or 1.63 yd³ with 10% extra.
How much concrete does a footing need?
A concrete footing calculator treats a strip footing as a long, flat box: cubic yards = length × width × (depth in inches ÷ 12) ÷ 27. A 20 ft run of footing, 2 ft wide and 12 in deep, is 40 cubic feet, which is 1.48 cubic yards. Add 10% for an uneven trench and order 1.63 yd³. At $150 a yard that is $222.22 without the buffer, or $244.44 with it.
Ready-mix is ordered in cubic yards, often in quarter-yard steps, and a short load or a second truck costs extra. Getting the volume right before the truck is booked saves money.
The formula, step by step
- Depth in feet = footing depth in inches ÷ 12.
- Cubic feet = length × width × depth in feet.
- Cubic yards = cubic feet ÷ 27.
- With waste = cubic yards × 1.10, if the waste box is ticked.
- Cost = cubic yards × price per yard.
Worked examples
- A single 20 ft footing, 24 in wide and 12 in deep (calculator defaults)
- Depth: 12 ÷ 12 = 1 ft
- Volume: 20 × 2 × 1 = 40 ft³
- Cubic yards: 40 ÷ 27 = 1.48 yd³, or $222.22 at $150
- With 10% waste: 1.63 yd³, $244.44
- Perimeter footing for a 24 × 30 ft garage, 24 in wide and 12 in deep
- Outside perimeter: 2 × (24 + 30) = 108 ft
- Centerline: 108 − (4 × 2) = 100 ft, because each corner square would otherwise be counted twice
- Volume: 100 × 2 × 1 = 200 ft³
- Cubic yards: 200 ÷ 27 = 7.41 yd³, $1,111.11
- With 10% waste: 8.15 yd³, $1,222.22
Enter 100 as the length, 2 as the width, and 12 as the depth. Measuring the outside perimeter instead would add 8 ft of footing that does not exist, about 0.6 yd³ of extra concrete.
Concrete per 10 ft of footing
| Footing size (width × depth) | Yd³ per 10 ft | 80 lb bags per 10 ft |
|---|---|---|
| 12 × 6 in | 0.185 | 8.3 |
| 16 × 8 in | 0.329 | 14.8 |
| 20 × 8 in | 0.412 | 18.5 |
| 20 × 10 in | 0.514 | 23.1 |
| 24 × 12 in | 0.741 | 33.3 |
Bag counts use 0.6 cu ft per 80 lb bag and do not include waste. Many US codes set a minimum footing width of about 12 in and require the bottom of the footing to sit below the local frost line. Required sizes rise with the load and fall with better soil, so check local code for your project.
How to measure a footing
- Run: add every straight section along its centerline. Interior footings under bearing walls count too.
- Width: measure the trench or the inside of the forms, not the plan width, if the trench came out wider.
- Depth: measure the footing’s own thickness, from the trench bottom to the planned top of concrete. Take readings every few feet and average them, because trench bottoms wander.
- Pads and piers: square column pads are separate boxes. Run each one with its own length, width, and depth, or use the concrete slab calculator for a pad.
Common mistakes
- Counting corners twice. Measure the centerline, as in example 2.
- Confusing depth below grade with footing thickness. A footing 36 in below grade might be only 10 in thick. Enter the 10.
- Ordering the exact net volume. Earth trenches swallow concrete. A cold joint from a short load is worse than a little leftover.
- Forgetting the wall. A stem wall or block foundation on top is a separate pour or material. Size it with the concrete wall calculator.
Planning the rest of the foundation
Use the footing size calculator for a first look at how wide a footing must be for its load and soil. Count the bars with the rebar calculator. For small jobs, the concrete bag calculator converts the yardage into 60 or 80 lb bags. Footings carry the whole structure, so have the dimensions and reinforcement confirmed by your building department or a licensed structural engineer before you pour.
Frequently asked questions
What do the length, width, and depth fields mean for a footing?
Length is the total run of footing in feet, measured along its centerline. Width is how wide the footing is across, in feet. Depth is the footing's own thickness in inches, not how far below grade it sits and not the height of the wall on top. A 24 in × 12 in footing is entered as width 2 and depth 12.
How do I add up the length of a perimeter footing?
Use the centerline, not the outside edge, or the corners get counted twice. For a 24 × 30 ft building with a 2 ft wide footing, the outside perimeter is 108 ft; subtract 4 × 2 ft for the corners to get 100 ft of centerline. That footing at 12 in deep is 7.41 yd³.
How do I know what size my footing needs to be?
Footing width and depth are set by the load, the soil's bearing capacity, the frost depth, and local code, not by this calculator. The footing size calculator gives a first estimate from load and soil bearing, but confirm the final size with your building department or a structural engineer.
Should I add extra concrete for waste on a footing pour?
Yes. Trenches are rarely dug to an exact width, and earth-formed footings spread into any over-dig. The 10% toggle turns 1.48 yd³ into 1.63 yd³. On rough or sandy trenches, some crews order 15% extra, because running out mid-pour leaves a cold joint.
How many 80 lb bags are in a footing?
One 80 lb bag makes about 0.6 cu ft, so a cubic yard takes 27 ÷ 0.6 = 45 bags. The default 1.48 yd³ footing is 40 cu ft, which is about 67 bags. Past roughly 1 yd³, ready-mix delivery is usually easier than mixing that many bags.
Estimates only. Verify quantities with your supplier before purchasing.