Calculate footing dimensions, depth, concrete volume, and reinforcement for columns and walls.
Select your footing type (square spread, rectangular, continuous wall, or combined), enter the column load, soil bearing capacity, embedment depth, and footing thickness. Hit Calculate and instantly get required footing dimensions, concrete volume, and a rebar estimate. Switch between Imperial and Metric units using the unit toggle.
Undersized footings are one of the most common and dangerous structural failures in residential and commercial construction. A 10,000 sq ft warehouse with 80-kip column loads on soil with 2 ksf bearing capacity needs footings of at least 6.5 ft × 6.5 ft — a detail that's easy to get wrong without proper calculation.
Contractors, structural engineers, and DIYers all need to know footing sizes before ordering concrete. Overpaying for oversized footings wastes money; undersizing causes settlement, cracking, or catastrophic failure. A single family home's exterior corner post carrying 30 kips on soft soil (1 ksf) needs a 5.5-ft spread footing — far larger than many homeowners realize.
This tool is used for deck posts, load-bearing walls, garage columns, pole barns, and full basement foundations. It covers the most critical early-stage sizing question before detailed engineering drawings are prepared.
The required footing area comes from the basic bearing pressure formula:
A = P / qnet
The footing self-weight is estimated as: Area × Thickness × Concrete unit weight. Since this depends on area (unknown), a quick iterative approach is used — the calculator adds an estimated self-weight (~5–8% of column load) and solves for area. Side length for a square footing is √A, rounded up to the nearest inch. Concrete volume = Area × Thickness. Rebar count is estimated using ACI 318 minimum steel ratio (0.0018 × b × d) for temperature/shrinkage.
Soil bearing capacity is the maximum load per unit area that soil can safely support without shearing or settling excessively. For preliminary design, use 1.5–2.0 ksf for soft clay, 2–3 ksf for medium sand, 4–6 ksf for dense gravel, and 8–12+ ksf for bedrock. A geotechnical engineer can perform soil borings and lab tests to determine the actual value — required for engineered structures.
Most residential building codes require a minimum 12-inch diameter or 12×12 inch square footing for deck posts. However, the actual required size depends on the tributary area of the deck and soil conditions. A 10×10 ft deck section carrying 50 psf (live + dead load) exerts 5 kips on the post, requiring roughly a 1.7 ft × 1.7 ft footing on 2 ksf soil — so 24×24 inches is a safe standard deck post footing size.
ACI 318 requires spread footings to be at least 6 inches thick above the bottom reinforcement. As a practical rule, footing thickness = (footing width − column width) / 2, ensuring a 45° stress angle through the concrete. For most residential footings (12–24 in wide), 12 inches is common. Larger commercial footings may need 18–30 inches of thickness.
For simple residential projects like deck posts and small sheds, prescriptive code tables may suffice. However, any footing supporting a structural element of a building — including load-bearing walls, columns, or additions — should be designed by a licensed structural engineer in most jurisdictions. Using a calculator is great for preliminary planning and budgeting, but engineering review is required before construction.