Spread (Pad) Footing Calculator — ft, yd³, cost

How to Calculate Concrete for Spread Footings

A spread (pad) footing spreads a concentrated column or post load over enough soil so the contact pressure stays below the allowable soil bearing capacity. Pick a shape, enter the plan dimensions in feet/inches/eighths, set the footing thickness and quantity, then add a pedestal stub if the column starts above grade. The pour quantity below already includes your waste allowance — order ready-mix to the next 0.25 yd³ since short trucks cost more than over-ordering.

How to use it

For isolated column footings the plan size comes from the service load divided by the allowable soil pressure (e.g. a 40 kip load on 2,000 psf soil needs ≈ 20 ft², so a 4′6″ square pad). Use the Reinforcement tab for the bottom mat, the Cost tab to compare ready-mix vs. bagged concrete, and add a concrete pier or grade beam when footings tie together. For continuous wall footings use the strip footing calculator instead, and for slabs over the pads see the slab calculator.

Footing Volume Formulas

Square: V = side² × depth. Rectangular: V = length × width × depth. Circular: V = π × (diameter/2)² × depth. Pedestal volume is added on top, total × number of footings, then × (1 + waste%). Cubic yards = ft³ ÷ 27; weight uses 150 pcf normal-weight concrete. Bag counts use 0.60 ft³ per 80-lb bag (representative; verify product) and 0.45 ft³ per 60-lb bag (representative; verify product) — practical for small pads but ready-mix is cheaper above ~1 yd³. For exact bag math see the concrete bag calculator.

Reinforcement per ACI 318-19

The bottom mat runs both ways with 3″ clear cover when cast against soil (ACI 318-19 §20.5.1.3). Minimum flexural reinforcement is the shrinkage/temperature ratio (0.0018 for Grade 60), and bars are spaced 6″–18″ o.c. Lap splices are taken at 40·d_b; this tool adds your waste/overlap percentage on top of net bar length. Heavier mats and dowels into a pile cap belong in the pile cap calculator.

FAQ

How deep should a spread footing be? Minimum 6″ of effective depth per ACI; 12″ is typical for light residential pads and 18–24″ for commercial columns. The bottom must sit below the local frost line and on undisturbed or compacted soil.

What size footing do I need? Preliminary concentric plan area may be estimated as service load divided by allowable bearing pressure, but final design must also check eccentricity, settlement, shear, flexure and local code requirements. Soil capacity comes from a geotechnical report; a structural engineer should size and reinforce footings for any load-bearing work.

How much extra concrete should I order? 5% for clean machine-dug holes, 10% typical, and up to 15% for hand-formed or over-excavated pads. The pour quantity shown already includes this allowance.

On pour day

Three things estimators forget when pricing isolated pad footings. Short-load fee — most ready-mix plants charge $80–150 if you order under ~3 yd³, so four 4×4 ft × 12″ pads (≈1.6 yd³) often come in cheaper with 80-lb bags than a truck. Pedestal dowel projection — vertical dowels must stick up out of the footing far enough to lap into the column above (typically 12–24″ plus a 40·d_b splice per ACI 318-19 §25.5), so order rebar at footing depth + projection, not just the pad thickness. Frost depth — IRC §R403.1.4 requires the bottom of the footing below the local frost line (36–48″ in northern US states); a 12″ pad poured shallow will heave. Typical 2025 USA prices: $150–220 per yd³ delivered (short-load surcharge below 3 yd³), $5–8 per 80-lb bag, $1.00–1.40 per lf of #4 rebar. Save runs to History and export the diagram as text, CSV, A4 JPG, or PDF — estimates are planning numbers, not a substitute for a structural drawing.