Concrete Footing Calculator

The IRC sets minimum footing widths based on building type and soil bearing capacity. These minimums apply even when the calculation produces a smaller number — the code floor governs.

Building Type	1,500 PSF Soil	2,000 PSF Soil	3,000 PSF Soil	4,000+ PSF Soil
1-storey wood frame	12 in	12 in	12 in	12 in
2-storey wood frame	15 in	12 in	12 in	12 in
3-storey wood frame	23 in	17 in	12 in	12 in
1-storey brick veneer	12 in	12 in	12 in	12 in
2-storey brick veneer	21 in	16 in	12 in	12 in

Values from IRC 2021 Table R403.1. These are the minimums — your calculated width may be larger if loads exceed the IRC presumptive values.

On weaker soils (1,500 PSF clay), multi-storey buildings push footing widths well beyond the 12-inch minimum. A two-storey wood frame home on clay needs 15-inch footings, and adding brick veneer jumps that to 21 inches because of the added dead load.

The two footing types serve different structural roles and suit different situations.

Continuous (strip) footings run the full length of a wall, distributing the wall's weight over a long, narrow concrete pad. They are the default for foundation walls, garage walls, and any bearing wall that transfers load to the ground. A strip footing is typically 12-24 inches wide and 8-10 inches thick, poured in a trench with rebar running lengthwise. The key advantage is uniform load distribution — every linear foot of wall bears on its own section of footing, so there is no load concentration. The trench is dug with a backhoe or excavator, which makes long runs efficient.

Isolated (pad) footings support a single column or post. They are square or rectangular pads, wider than the column they carry, designed to spread the point load over enough soil area. Pad footings are common for interior columns in basements, steel column bases, and porch columns where a full strip footing would be wasteful. Each pad is sized independently based on the load it carries.

For most residential additions and garages, continuous footings are the correct choice. Use isolated pads when supporting freestanding columns that do not connect to a wall. The pier footing size calculator handles round and square pier footings for deck posts and light columns, where Sonotube forms replace box forms.

Three dimensions define a footing: width (calculated from load and soil), depth below grade (set by frost line), and thickness (the vertical dimension of the concrete itself). Each serves a different purpose.

Width handles bearing pressure — the footing must spread the load over enough soil area that the pressure stays below the bearing capacity. Depth protects against frost heave — the bottom of the footing must sit below the frost penetration line so that freezing soil cannot lift it. Thickness provides structural rigidity — a footing that is too thin cracks under the bending forces created when soil pressure pushes up while the wall pushes down.

IRC R403.1.1 sets the minimum footing thickness at 6 inches, and requires that thickness to be at least equal to the projection of the footing beyond the wall face. In practice, most residential footings are 8-10 inches thick. Thicker footings (12+ inches) are used for heavy loads, multi-storey buildings, or when the footing must project significantly beyond a narrow foundation wall.

Reinforcement is not always required by code for residential footings, but it is good practice. Two continuous #4 rebar bars running the length of a strip footing prevent cracking from differential settlement. Pad footings benefit from a grid of #4 bars at 12-inch spacing in both directions. If the footing also supports a masonry wall, the concrete mix ratio calculator helps you proportion the concrete for the required compressive strength — typically 2,500 PSI minimum for residential footings.

A well-planned footing pour avoids the two most common failures: undersized footings that settle and poorly placed footings that miss the frost line.

1. Call 811 before any excavation. Buried gas, electric, water, and communication lines are invisible until you hit them with a backhoe. The 811 service marks all utility locations within 48-72 hours at no cost. This is not optional — it is the law in all 50 states.

2. Survey the footing layout. Use batter boards and string lines to mark the exact footing centreline. Check diagonals to confirm the corners are square. For additions, measure from the existing foundation to maintain alignment. Record the layout dimensions for your building permit application.

3. Excavate the trench to full depth. Dig to at least 6 inches below the local frost line. The bottom of the trench must be undisturbed soil — never backfill and pour on top of loose dirt. If you over-dig, fill with compacted gravel, not soil. Use a laser level or transit to check the trench bottom is level along its length.

4. Set form boards and install rebar. For strip footings, form boards (2x8 or 2x10) line both sides of the trench, staked and braced plumb. Place #4 rebar on chairs or bricks to keep it 3 inches from the bottom and sides. Lap rebar splices at least 24 bar diameters (24 inches for #4 bars). For pad footings, build a box form to the calculated dimensions.

5. Pour and finish. Fill the forms with concrete (3,000 PSI minimum for footings per ACI 332), vibrate or rod to remove air pockets, and screed the top level. Insert anchor bolts or rebar dowels for the foundation wall while the concrete is still wet — their positions must match the wall layout exactly.

6. Cure for at least 7 days. Keep the surface moist by covering with plastic or applying curing compound. Concrete reaches about 70% of its design strength at 7 days and full strength at 28 days. Do not backfill or load the footing before the 7-day mark. The concrete curing time calculator estimates cure schedules for different temperatures and mix types.

Footing failures are expensive to fix because the footing is buried under everything else. Getting it right the first time saves thousands in future repair costs.

Pouring on frozen ground. When soil at the bottom of the trench is frozen, it expands in place and later settles as it thaws, creating a void beneath the footing. The result is uneven settlement — cracks in the foundation wall, doors that stick, and floors that slope. Always pour on unfrozen, firm soil. If the ground freezes before you can pour, thaw it with insulated blankets before placing concrete.

Ignoring the water table. A high water table pushes hydrostatic pressure against the bottom of the footing, reducing its effective bearing capacity. If you hit standing water while digging, stop and consult a geotechnical engineer. You may need a deeper, wider footing, or a drainage system (French drain) to lower the water level around the foundation.

Skipping rebar in strip footings. An unreinforced strip footing that spans a soft spot in the soil has no tensile resistance — it cracks and the wall above follows. Two #4 rebar bars running continuously (with proper lap splices) cost under $1 per linear foot and prevent most crack-related failures.

Setting anchor bolts after the concrete hardens. Drilling into cured concrete and epoxying anchor bolts is never as strong as wet-setting them during the pour. Mark bolt locations on the form boards before the pour so you can place them accurately while the concrete is workable. Standard spacing is 6 feet on centre, with a bolt within 12 inches of each corner and each side of door openings.