How this concrete footing calculator works
The calculator tells you how much concrete you need for footings — as cubic yards of ready-mix or bags of concrete — plus weight and cost. A strip footing is a long box; a pad footing is a small one. Both are volume:
Pads: V = (L ÷ 12) × (W ÷ 12) × (depth ÷ 12) × count · order = yd³ × (1 + waste%) → next ¼ yd³
- Waste: trenches over-excavate more than forms — 10% is the floor, use 15% for hand-dug or crumbly ground.
- Rounding: ready-mix sells in ¼-yard steps, and a footing you can't finish is a cold joint — round up, always.
- Modes: switch to Pads for deck piers and post pads; the count field multiplies identical footings.
Worked example: how much concrete for a 50 ft footing
The default is 50 ft of 16 × 8 in strip footing: 50 × 1.33 × 0.67 = 44.4 ft³, which is 1.65 yd³. Add 10% waste (48.9 ft³ = 1.81 yd³) and round up to the quarter yard: order 2 yd³ — about $320 of ready-mix, versus 82 bags of 80 lb at roughly $595.
How big should a footing be?
Size comes from code and soil, not preference. Three rules cover most residential work:
- Depth: bear at least 12 in below undisturbed ground (IRC R403.1.4) and below the local frost line — frost usually governs, and in cold climates that's 36–48 in.
- Width: IRC Table R403.1(1) starts at 12 in for light-frame walls on decent soil; 16–24 in spreads the load on softer ground or under heavier builds.
- Thickness: 6 in is the practical minimum; 8–12 in is typical under foundation walls.
Footings must sit on undisturbed or properly compacted soil — never on backfill. If the trench bottom is soft, dig to firm ground and make up the difference in concrete, then raise the waste figure here to match.
How much concrete for common footing runs?
| Run length | 16 × 8 in — order | 80 lb bags | 20 × 10 in — order | 80 lb bags |
|---|---|---|---|---|
| 20 ft | 0.75 yd³ | 33 | 1.25 yd³ | 51 |
| 40 ft | 1.5 yd³ | 66 | 2.5 yd³ | 102 |
| 60 ft | 2.25 yd³ | 98 | 3.5 yd³ | 153 |
| 80 ft | 3.0 yd³ | 131 | 4.75 yd³ | 204 |
| 100 ft | 3.75 yd³ | 163 | 5.75 yd³ | 255 |
Concrete footing FAQ
How much concrete do I need for a 50 ft footing?
At 16 in wide and 8 in deep, a 50 ft strip footing is 44.4 ft³ — order 2 yd³ of ready-mix with 10% waste (about $320 at the calculator default), or 82 bags of 80 lb concrete. Wider or deeper trenches climb fast: the same 50 ft at 20 × 10 in needs 3 yd³.
How deep should concrete footings be?
The IRC (R403.1.4) requires footings to bear at least 12 in below undisturbed ground surface — and below the local frost line, which usually governs: that can mean 36–48 in of depth in cold climates. Your building department publishes the local frost depth; dig to it.
How wide should a footing be?
IRC Table R403.1(1) allows 12 in as the minimum for most one- and two-story light-frame builds on 2,000 psf soil. Weaker soil or heavier construction spreads the load with wider footings — 16–24 in is common. When in doubt, the soil report and your inspector decide, not the calculator.
Should I order ready-mix or mix bags for footings?
Same crossover as any pour: about 1 cubic yard, roughly 45 bags at 80 lb. Deck-pier pads often make sense as bags; a full perimeter strip footing almost never does — the default 50 ft job is two tons of hand-mixing as bags.
Do footings need rebar?
Common residential practice is two continuous #4 bars near the bottom of strip footings, lapped at splices and kept 3 in clear of soil. Requirements vary by region and load — seismic and high-wind areas have specific rules, so confirm with local code before the pour.
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