How to use the footing calculator
Pick the kind of footing. Around a building takes the outside length and width of the footing and works out the concrete in the perimeter ring, counting each corner once. Straight run is for a single wall, fence or retaining wall footing. Pier pads are separate square pads under posts or piers, such as a deck or a pole shed.
Enter where the bottom of the footing will sit and your local frost depth, and the calculator warns you if the footing is too shallow. The section drawing shows the footing, the bars and the frost line to scale.
The formulas
perimeter footing = (L × W − (L − 2w) × (W − 2w)) × thickness
straight run = length × width × thickness
pier pads = count × length × width × thickness
cubic yards = cubic feet × (1 + waste) ÷ 27
Worked example: a 24 × 16 ft garage footing, 16 in wide and 8 in thick
- Area of the ring: 24 × 16 − (24 − 2.67) × (16 − 2.67) = 384 − 284.4 = 99.6 ft².
- Volume: 99.6 × (8 ÷ 12) = 66.4 ft³.
- Add 10%: 73.0 ft³, which is 2.70 yd³.
- Centerline length: 2 × (24 + 16) − 4 × 1.33 = 74.7 ft. Two continuous #4 bars, lapped 20 in at each joint, need 8 bars of 20 ft.
Sizing footings
| Item | Typical requirement |
|---|---|
| Minimum width (IRC) | 12 in |
| Minimum thickness (IRC) | 6 in |
| Depth | Below frost depth, and at least 12 in below undisturbed ground |
| Rule of thumb for light walls | Width about 2× the wall thickness; thickness about equal to the wall thickness |
| Reinforcement | Often 2 continuous #4 bars, 3 in clear of the soil |
Code tables size footings by the soil's bearing capacity and the weight of the building. Weak soil, two stories, heavy masonry or a seismic zone all mean wider or reinforced footings. Treat this calculator as the quantity takeoff, not the design.
Digging and forming
- Dig to undisturbed soil. Footings on loose fill settle. If you over-dig, fill the extra with concrete, not soil.
- Trench pours: in firm soil you can pour straight into a neatly dug trench with no forms. The dirt to remove is shown as "Trench / holes to dig", measured to the bottom of the footing.
- Inspection: most jurisdictions inspect footing trenches and rebar before the pour. Book it before you order concrete.
- Keyway or dowels: if a wall will sit on the footing, set vertical dowels or form a keyway while the concrete is wet.
Common mistakes
- Counting corners twice. Adding up four outside wall lengths overstates a perimeter footing. The calculator uses the ring area instead.
- Bars on the dirt. Rebar needs about 3 in of concrete below it. Use bricks or chairs, not stones.
- Pouring on frozen ground. Never. The ground thaws, the footing settles.
Questions people ask
How big should a concrete footing be?
Under the International Residential Code (R403.1.1) footings must be at least 12 inches wide and 6 inches thick, and wider on weak soil or under heavier buildings; Table R403.1 gives widths by soil bearing and number of stories. A common rule for light walls is a footing twice the wall width and as deep as the wall is wide.
How deep should a footing be?
The bottom must sit below your local frost depth, and at least 12 inches below undisturbed ground under the IRC. Frost depth ranges from zero in parts of the South to 48 inches or more in the northern states; your building department publishes the local figure.
How much concrete is in a footing?
Length × width × thickness, in feet. A 1 ft wide, 8 in thick footing holds 0.67 cubic feet per running foot, so 100 ft of it is 66.7 cubic feet, or 2.47 cubic yards before waste.
How much rebar goes in a footing?
Residential footings commonly get two continuous #4 bars, lapped where they join and supported about 3 inches off the soil. Your plans or local code set the real requirement, especially in seismic areas.
Can I pour footings with bagged concrete?
For pier pads and short runs, yes. A 2 × 2 ft pad 12 in thick needs 4 cubic feet: seven 80 lb bags. For a whole perimeter footing, ready-mix is usually cheaper and gives one continuous pour without cold joints.
Sources and assumptions
- 2021 International Residential Code, R403.1 Footings (minimum 12 in wide, 6 in thick; depth below frost line)
- QUIKRETE Concrete Mix (No. 1101) data sheet: 80 lb bag yields 0.60 ft³
- Rebar: ASTM A615 unit weights; laps taken as 40 bar diameters, a common rule of thumb. Your plans govern.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.