This footing calculator finds the concrete for continuous strip footings and square pier pads — cubic yards for the truck or bags for the mixer — from trench dimensions.
Trench reality: earth-formed footings overbreak — walls crumble and bottoms run uneven — so the 10% line is the order quantity, not a pessimism tax. Footing width/depth and frost depth are code-and-soil decisions (bearing capacity rules width; frost line rules depth) — this calculator prices a cross-section you’ve already determined.
Volume = Length × Width × Depth — Then Add the Trench Tax
Footing arithmetic is a simple prism; footing estimating is the prism plus humility about earth-formed trenches. Slabs and walls run through our Concrete Calculator; rebar for the footing lives on the Rebar Calculator. Part of our Construction Calculators collection.
Worked Examples (Verified)
- 40 ft strip, 12″ × 12″ (defaults): 40 ft³ = 1.48 yd³ neat → 1.63 yd³ with overbreak → a 2-yard order in practice (74 bags if you’re mixing — reconsider)
- Deck pier pads: six 24″ × 24″ × 12″ pads = 24 ft³ = 0.89 yd³ (55 bags with allowance) — the scale where a rented mixer earns its keep
- Width drives cost quadratically in spirit: doubling width 12″ → 24″ doubles volume; going deeper 12″ → 18″ adds half — bearing-driven width changes are the expensive specification edits
- Stepped footings on slopes: total length along the slope’s steps (sum of horizontal runs) — enter the sum; the vertical steps’ concrete is inside the step boxes if you measure generously
Sizing Reminders (Code-Territory, Handle Accordingly)
- Residential minimums commonly start at 12″ wide × 12″ deep footings for light frame on decent soil, projecting ≥ 2″ beyond the wall each side — your frost depth and soil report govern.
- Footings bear on undisturbed soil or engineered fill — never on loose trench spoil; clean the bottom before the pour.
- Rebar (typically 2 continuous #4 bars in strips) is inspected before concrete in most jurisdictions — schedule the inspection before the truck.
- Keyways/dowels tie wall to footing per local practice and seismic category.
Related Tools
- Concrete Calculator — slabs, walls and general volumes
- Excavation Calculator — the hole before the footing
- Construction Calculators — the full collection
Frequently Asked Questions
How do I calculate concrete for a footing?
Multiply total length × width × depth (in feet) for cubic feet; divide by 27 for cubic yards; add ~10% for trench overbreak. A 40-ft strip footing 12″ wide × 12″ deep: 40 ft³ = 1.48 yd³, ordered as ~1.63 yd³.
How wide should a footing be?
It depends on the load and soil bearing capacity — the footing spreads the wall’s load so soil pressure stays within its allowance. Common residential minimums are 12–16″ wide for one-story light frame on average soil, wider for heavier loads or weaker soils; local code tables and any geotechnical report govern.
How deep do footings need to be?
Below the local frost line (so frost heave can’t lift them) and onto bearing soil — depth below grade ranges from ~12″ in frost-free regions to 48″+ in cold climates. Frost depth maps and your building department set the number; the calculator prices whatever depth you enter.
Bags or ready-mix for footings?
Beyond about ½ yd³ (≈ 23 ft³, 40 bags), ready-mix wins on labor and consistency — the defaults here (74 bags with allowance) sit firmly in truck territory. Small pad sets are the bag-friendly case: six deck pads ≈ 55 bags or one rented mixer and a morning.
Do footings need rebar?
Most codes require continuous longitudinal reinforcement in strip footings (commonly two #4 bars) and prescriptive details for pads/piers vary. This calculator prices concrete volume only; the Rebar Calculator estimates the steel — and your inspection happens before the pour, not after.
Is this footing calculator free?
Yes — free, instant, no sign-up.