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.
At the defaults — 50 ft of 16 × 8 in strip — the order is 2 yd³, about $320; the line-by-line math is in the pour math section below.
Types of footing, and what they sit on
A footing is the widened base that spreads a building’s weight onto the soil. Residential work uses a handful of shapes — pick by what stands on it:
Strip (continuous) footing
A long trench of concrete under a foundation wall, running the whole perimeter and under bearing walls. It’s the default under block and poured stem walls — the calculator’s strip mode is this one.
Pad (spread) footing
A square or rectangular block of concrete under a single point load — a post, a column, a deck pier. The pad mode multiplies identical pads by the count field; round pier footings under tube forms belong to the sonotube calculator instead.
Stepped footing
On a slope, the strip footing steps down in level runs so each section still bears on undisturbed soil at frost depth. Estimate it as separate strip runs at each level and add the volumes.
Thickened-edge (monolithic) footing
The footing and slab poured as one — a shed or garage slab with a deepened perimeter. Size the flat part with the slab calculator and the thickened edge as a strip footing, then add.
What’s underneath
Every footing bears on undisturbed or properly compacted soil — never loose backfill. Soft trench bottoms get dug to firm ground and made up in concrete, which is why the waste field exists. A gravel leveling course is common under slabs but not under the footing itself.
How big should a footing be?
Size comes from code and soil, not preference. Three dimensions, three rules that 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. A rule of thumb: footing width ≈ twice the wall thickness.
- Thickness: 6 in is the practical minimum; 8–12 in is typical under foundation walls, and it should be at least as thick as the footing projects past the wall on each side.
- Bad soil widens everything: when the soil report drops below 2,000 psf, the footing grows — that’s an engineered number, not a guess.
How much concrete for common footing runs?
Table 1 runs common strip lengths through the engine at two trench sizes — read your run, read your section, and see how fast a wider trench climbs.
| 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 |
How to measure footings for concrete
Measure the hole you’ll actually fill, then let the waste factor cover the trench’s sins. Dimension fields take feet-inches and metric where they apply.
- Total the strip run — add every wall of the perimeter and the interior bearing lines into one length. Corners overlap slightly; ignore it, the waste covers it.
- Measure trench width and depth in inches at several points and use the largest consistent figure — hand-dug trenches are always bigger at the bottom than the string says.
- Switch to pad mode for point footings — deck piers, post pads, column bases — and set the count to how many identical ones you’re pouring.
- Set waste to 10% on machine-dug trenches, 15% on hand-dug or crumbly ground where the sides slough.
- Enter the plant’s delivered price — with the short-load fee if the order is under about 4 yd³ — or the bag price for small pier jobs.
Worked example: concrete for a 50 ft footing
The default, worked line by line — 50 ft of 16 × 8 in strip, 10% waste, $160 ready-mix against $7.25 bags:
yards = 44.4 ÷ 27 = 1.65 yd³
waste = 44.4 × 1.10 = 48.9 ft³ = 1.81 yd³
order = next ¼ yd³ up = 2 yd³
ready-mix = 2 × $160 = $320
bags = 48.9 ÷ 0.60 = 81.5 → 82 bags ≈ $595
The footing is where the pour-in-one-go rule bites hardest: a strip that runs dry mid-trench leaves a cold joint at the worst possible place, structurally. That’s the whole reason the order rounds up to 2 yd³ instead of trusting 1.81 — and why a full perimeter is a ready-mix job, not 82 bags of hand mixing.
What this footing estimate doesn’t include
The calculator prices the concrete in the trench. The rest of the footing job:
- Excavation. Machine trenching or hand digging to frost depth, and hauling the spoil.
- Rebar. The two continuous bars, dowels or keyway that tie the footing to the wall above.
- Forming. Trench footings often need none, but stepped and above-grade footings take form boards and stakes.
- The wall or slab on top. Block, poured stem wall or slab — a separate order and a separate calculator.
- Drainage. Perimeter drain tile and gravel where the code or the water table calls for it.
- Inspection. The footing is an inspected, photographed pour almost everywhere — open trench, before concrete.
- Labor and pump. Placing and finishing, plus a pump truck where the chute can’t reach the trench.
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.
What is the difference between a footing and a foundation?
The footing is the widened concrete base in the ground; the foundation is the wall, pier or slab that stands on it. The footing spreads the building load out over enough soil to carry it — a wide foot under a narrow leg. This calculator sizes the footing pour; the wall above is a separate order.
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.
Can you pour the footing and wall at the same time?
For a monolithic (thickened-edge) slab, yes — footing and slab go in one pour, common for garages and sheds on stable, non-freezing ground. For a stem-wall foundation the footing is poured first and the wall built on it after it cures, with a keyway or dowels tying the two. Which one you use is a design and frost-depth decision.
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.
Sources and standards
The depth, width and reinforcement rules here come from the codes inspectors check the open trench against:
- IRC R403.1: footing requirements — the 12 in bearing depth, minimum dimensions, and undisturbed-soil rule.
- IRC Table R403.1(1): the minimum footing widths by wall type and soil bearing capacity.
- IRC R403.1.4: frost-depth protection — why the local frost line usually governs the depth.
- ACI 332 / published bag yields: residential concrete practice and the 0.60 ft³ per 80 lb bag figure the bag counts use.
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