Concrete Footing Calculator
Estimate the concrete needed for strip, pad, and square column footings — in cubic feet, cubic yards, and bags — for one footing or a whole set.
Reviewed by the ToolNestr Editorial Team — July 2026
Tip: a 16-inch-wide strip footing is 1.33 ft; a 2-foot square pad footing is 2 ft × 2 ft.
How the concrete footing calculator works
A footing is the wide concrete base that spreads a wall or column load out onto the soil. Whether it is a long strip footing under a foundation wall, a square pad under a post, or a stepped footing on a slope, the concrete in it is a rectangular block: length times width times thickness.
The calculator keeps length and width in feet and thickness in inches, since that matches how plans are drawn. It divides the thickness by 12 to convert to feet, multiplies the three dimensions for the volume of one footing, then multiplies by the quantity for identical footings. Dividing cubic feet by 27 gives cubic yards, and dividing by the bag yield (0.6 cubic feet for 80 lb, 0.45 for 60 lb) gives the bag count.
This is standard rectangular-volume geometry as used for ACI 318 foundation work. The footing size itself — how wide and thick it must be for your load and soil — is a structural decision from your plans and local code; this tool only turns those dimensions into a material quantity.
Enter dimensions
Length and width in feet, thickness in inches, straight off your foundation plan.
Set the quantity
Use quantity for identical pad footings; enter the full run length for one continuous strip.
Order with extra
Add 5-10% because trench bottoms are rarely flat and over-dig fills with concrete.
The formula explained
Volume of one footing
V = L × W × (T⁄12). A 40 ft strip footing 1.33 ft wide and 8 in thick: 40 × 1.33 × (8⁄12) = 35.5 ft³.
Total and cubic yards
CY = (V × qty) ÷ 27. For the single strip: 35.5 ÷ 27 = 1.31 cubic yards.
Bag count
Bags = ft³ ÷ yield. In 80 lb bags: 35.5 ÷ 0.6 = 60 bags — at this size, ready-mix is usually cheaper.
Worked example
Four square pad footings for deck posts, each 2 ft × 2 ft × 12 in thick.
Common footing types
Strip (wall) footings
A continuous footing under a foundation wall. Enter the full wall-run length and the trench width; keep it as one footing at quantity 1.
Pad (column) footings
Square or rectangular pads that carry point loads from posts and columns. Enter one pad's size and set quantity to how many you are pouring.
Stepped footings
On sloped ground, footings step down in stages. Estimate each level as its own rectangular footing and add the volumes together.
Round pier footings
For round tube or bell piers, use the Sonotube calculator instead, which handles the cylinder volume of the tube directly.
Tips for pouring footings
Dig to firm, level bearing soil
Footings must sit on undisturbed, load-bearing soil below the frost line. Over-excavated or soft spots should be filled with compacted gravel or extra concrete, not loose dirt.
Account for the trench, not just the plan
A hand-dug trench is often wider than the drawn footing, and that extra width fills with concrete. Measure the actual trench for the truest volume, then add waste on top.
Keep rebar off the ground
Reinforcing steel does its job only when embedded in concrete, so support it on chairs a few inches above the trench bottom. This does not change the concrete volume but is critical to the footing's strength.
Real-world context
A full house perimeter shows the scale strip footings usually reach: a 150 ft foundation run, 18 inches wide, 10 inches thick needs 150 × 1.5 × (10/12) = 187.5 cubic feet, about 6.9 cubic yards — well past the point where ready-mix by the truck is clearly cheaper and faster than mixing over 300 bags by hand.
A set of deck or porch post pads shows the opposite case: six 2 ft by 2 ft pads at 12 inches thick need 6 × (2×2×1) = 24 cubic feet, about 0.89 cubic yards — small enough that most suppliers would charge a short-load fee, making 40 bags of mix the more economical choice.
Stepped footings on a sloped lot combine both scales in one job: a foundation that steps down in three 20 ft sections at 8, 10, and 12 inches thick (same 16-inch width) needs each section estimated separately and summed — 20×1.33×(8/12) + 20×1.33×(10/12) + 20×1.33×(12/12) = 17.7 + 22.2 + 26.6 = 66.5 cubic feet total, a case where entering one average thickness would meaningfully misstate the order.
Common misconceptions
"A wider footing is always a safer bet if I'm not sure of the exact size." Not really a safety margin. Footing width and thickness are sized to the actual load and the soil's bearing capacity in your engineer's plans — oversizing wastes concrete without adding meaningful capacity, and undersizing is a code and structural problem this calculator can't catch, since it only converts dimensions you provide into a volume.
"The footing volume is the same as the trench volume I dug." Often not. Hand-dug or machine-dug trenches are almost always a bit wider and rougher than the exact drawn footing dimensions, and that extra space fills with concrete too — measuring the actual trench, not just the plan number, gives a more accurate order.
Related calculators
Frequently asked questions
How do I calculate concrete for a footing?
Multiply the footing length by its width to get the area, then multiply by the thickness to get the volume. Keep length and width in feet and thickness in inches, then divide the thickness by 12 to convert to feet. Divide the cubic-foot result by 27 for cubic yards.
What size footing do I need?
Footing size depends on the load and the soil bearing capacity, set by your local code and engineer. A common residential strip footing is 16 to 24 inches wide and 8 to 12 inches thick, but always confirm the required dimensions for your project before you pour.
How thick should a footing be?
Residential footings are typically 8 to 12 inches thick, and codes often require the thickness to be at least the footing projection beyond the wall. Thicker footings spread heavier loads. Your structural drawings give the exact figure.
Should footings be poured with rebar?
Most footings carry two or more continuous reinforcing bars near the bottom to resist bending and to bridge soft spots in the soil. This calculator sizes the concrete only; add the rebar separately based on your plans.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.