For a 30 ft strip footing 12″ wide × 18″ deep — a typical residential foundation wall footing in the Chicago suburbs — the volume is 1.67 cubic yards exact. With a 5% waste cushion that rounds up to 2.0 yd³as a practical order on a quarter-yard residential dispatch. If you're going bagged instead, that's 76× 80 lb bagsoff the Quikrete data sheet at 0.60 ft³ yield per bag. At 1.67 yd³ the footing sits below the typical 3 yd³ ready-mix minimum, so you're looking at a short-load fee, a phased pour, or bagged.
Run the same math on the concrete yardage calculator for any footing dimension; it surfaces the practical-order cushion, flags the residential ready-mix minimum below 3 yd³, and computes bag counts at manufacturer-spec yields.
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Hi, I'm the SiteworkMath assistant. I answer questions about the calculators and guides on this site — concrete yardage, topsoil, mulch, tile, and decking material math. I'm not a structural engineer or a licensed contractor; I'm a calculator built around the math that James Wu — Chicago-area flipper — uses on his own jobs. For structural decisions (joist sizing, beam spans, footing depth in your soil + climate, anything that needs a permit) talk to a licensed structural engineer or your local building department.
How pros actually do this
A garage footing pour at a Chicago-suburb gut rehab in fall 2020 was the one that sold me on triple-checking footing depth before the truck rolls. Detached two-car garage going in over a previous slab tear-out, perimeter strip footing 24 ft × 24 ft × 12″ wide × 42″ deep. The trench was dug to 38″ when I showed up at 7 AM with the inspector booked for 9. Frost-line spec for that municipality was 42. Four inches of unexcavated clay across 96 linear feet of trench is about 1.3 yd³ of additional concrete you weren't planning to order — and that's before the inspector says “dig deeper” and you bury the trench another 4″ on top of that.
The math was right. The trench was wrong. We pushed the truck back two hours, finished the dig, and had to call dispatch to bump the ticket from 4 yd³ to 5.5. The dispatcher had a slot — that was luck. On a busy Saturday with ready-mix trucks running tight, the truck would've gone to the next pour and we'd have eaten the re-route fee plus a half-day of crew sitting on cold trench.
That's the gap between exact-math and what-it-actually-takes for a footing. The volume calc is the easy part. Frost-line depth, inspector tolerances, and form pressure are what move the order. A 24-inch-deep footing in your calculator assumes a box form. Your forms have to be rated for around 150 lb/sq ft of lateral pressure (ACI 332), not just built — when the truck starts chuting, anything that flexes adds yardage you didn't order.
My default cushion on footings is the same as slabs — 5% on a clean square trench with rigid forms, 10% on rough work or where the form looks like it might bow. But footings have a second cushion most calculators miss: rebar pocket fills. Two #4 bars tied 12″ on center in a 12″ × 18″ strip footing displace some volume but the mix-around-rebar is never as clean as the math assumes. Add a half bag for every 30 linear feet of strip footing and you won't be short.
The four steps
1. Measure the trench, not the wall.
Length is the linear footage of the footing. Width is the trench width — typically 12-24″ for a strip footing, sized to be at least twice the wall thickness per IRC 2021 R403.1.1. Depth is the inside-trench depth from the bearing surface to the top of the footing, NOT the trench depth from grade. If the trench bottoms out 12″ below the planned footing top, that's the depth. Convert inches to feet by dividing by 12 — a 12″ wide trench is 1.0 ft, an 18″ depth is 1.5 ft.
2. Compute volume in cubic feet.
Length × width × depth, all in feet, gives volume in cubic feet. For a 30 ft strip footing 12″ × 18″ the math is 30 × 1 × 1.5 = 45 ft³. For a stepped footing — common where grade changes or where the foundation steps from a basement to a crawlspace — split into rectangles, compute each separately, and sum. The engine doesn't handle the step in a single pass.
3. Convert to cubic yards.
Divide by 27 to get cubic yards. For our 45 ft³ strip footing the answer is 45 ÷ 27 ≈ 1.67 yd³. That's the EXACT volume — what the trench geometrically contains, before any cushion or rounding. Concrete is sold by the cubic yard, so this is the unit that talks to dispatchers.
4. Add the cushion, then round to the dispatch unit.
Multiply by (1 + waste% / 100). At 5% the math is 1.67 × 1.05 ≈ 1.75 yd³. Then round UP to the next dispatch unit your supplier honors. Quarter-yard rounding lands at 2.0 yd³. Below the 3 yd³ residential ready-mix minimum, the order economics flip — you either eat a short-load fee, phase the pour into two trips, or go bagged at 76× 80 lb bags.
The worked example, end to end
Inputs:
- 30 ft long × 12″ wide × 18″ deep strip footing
- 5% waste cushion (clean square trench, rigid forms)
- Bagged-alternative computed at the EXACT volume
Step by step:
- Volume in cubic feet:
30 × 1 × (18 / 12) = 45 ft³ - Cubic yards exact:
45 / 27 ≈ 1.67 yd³ - With 5% cushion:
1.67 × 1.05 ≈ 1.75 yd³ - Practical order, quarter-yard rounding:
ceil(1.75 × 4) / 4 = 2.0 yd³ - Bag count, 80 lb at 0.60 ft³ each:
ceil(1.67 × 45) = 76 bags - Bag count, 60 lb at 0.45 ft³ each:
ceil(1.67 × 60) = 101 bags
So you call the dispatcher with 2.0 yd³ — and you'll likely hear about the short-load fee since the order sits below the 3 yd³ residential minimum on most Chicago-suburb plants. The cleaner play on a single short footing is bagged: 76× 80 lb bags is a long Saturday but no fee, no schedule pressure, no truck waiting on the inspector. For a multi-footing pour where the cumulative ticket clears 3 yd³, ready-mix wins on labor every time.
Run the math yourself
Type the dimensions for your footing. The calculator below is the same one used on the cluster anchor page; it covers slab, footing, post hole, and stairs.
Where this number breaks down
A few footing-specific traps that put the math on the wrong side of the order — or the wrong side of the inspector:
- Forms bowing under hydrostatic head.A footing form holds a tall column of wet concrete — a 24″ deep pour generates roughly 150 lb/sq ft of lateral pressure at the base per ACI 332-20 form-design guidance. 2x lumber forms with stakes every 4 ft can flex visibly under that load, and every quarter-inch of outward bow across a long footing adds measurable volume. If you can deflect the form with hand pressure, you're going to lose mix into the bow before you finish.
- Frost line, missed depth. Chicago and Cook County require footings to extend at least 42″ below finished grade (per municipal code referenced under IRC 2021 R403.1.4). Lower Michigan generally needs 42-48″; the upper peninsula needs 60+″ in the coldest counties. Coming up short on the trench dig means either the inspector fails the inspection or you eat the cost of extra concrete to bring the footing down to spec. Always confirm depth with the local building department before the truck rolls.
- Rebar pocket fills.Two #4 (1/2″) bars tied 12″ on center in a strip footing displace about 0.3 ft³ per 30 linear feet — but the math doesn't account for the mix that tucks unevenly around rebar ties and the void that occasionally forms on the underside of a bar. Add half a bag for every 30 linear feet of strip footing and the bag count won't come up short.
- Water table on a wet trench. Pumping a flooded footing trench can pull subgrade out with the water and change effective depth — what reads as 18″ on the tape becomes 19-20″ in the back corner where the pump suction sat. The 5% cushion covers small variation; visible muck pulls move you to 10%. On high-water-table sites, dewatering is the conversation to have with the foundation contractor BEFORE the pour, not during.
- Stepped footings on grade changes. The engine math assumes one continuous rectangle. Stepped footings — common where the foundation transitions from basement to crawlspace — need to be split into rectangles, computed individually, and summed. A 30 ft footing with one 8″ step in the middle is two rectangles, not one.
- Belled or drilled-pier footings. The cylinder mode in the calculator handles a clean cylindrical hole. Belled footings — where the base is widened to spread the load on weak soil — add 30-50% volume below the cylinder math depending on the bell geometry. Compute the bell separately or talk to a structural engineer about the actual schedule.
Frequently asked
How deep should a residential footing be in Chicago?
42 inches below finished grade for most Chicago and Cook County residential footings. IRC 2021 Section R403.1.4 doesn't fix a number — it requires footings to extend below the frost line as determined by the local building official, and Chicago / Cook County code references 42″ as the minimum depth for non-frost-protected shallow foundations. ACI 332-20 Section 4.2 backs the same rule: footing depth must extend to or below the frost line. Michigan's lower peninsula generally lands at 42-48″; the upper peninsula needs 60+″ in the coldest counties. Always confirm with the local building inspector before pouring — frost line is municipal, not state-wide.
How much concrete do I need for a 30 ft strip footing?
For a 30 ft strip footing 12″ wide and 18″ deep, the math is 30 × 1 × 1.5 = 45 ft³, divided by 27 = 1.67 cubic yards exact. With a 5% waste cushion that rounds up to 2.0 yd³ as a practical order on a quarter-yard residential dispatch. That's below the typical 3 yd³ ready-mix minimum, so you're either looking at a short-load fee, a phased pour, or 76× 80 lb bags. Bag count comes off the Quikrete data sheet at 0.60 ft³ yield per 80 lb bag.
Do I need rebar in a residential footing?
ACI 332-20 Section 4.4 requires reinforcement in footings supporting structural loads — typically two #4 (1/2″) horizontal bars continuous, top and bottom, in a strip footing. IRC 2021 Section R403.1.3 mirrors this for footings supporting braced wall lines in seismic regions. Plain unreinforced footings are allowed under R403.1.1 only for light loads on stable soil with no expansive-clay or freeze-thaw concerns. Chicago-suburb expansive clay (the gray-blue stuff under most Cook County yards) is the wrong soil to skip rebar on. Talk to a structural engineer for the actual schedule — this guide doesn't stamp the plan.
What's the difference between a strip footing and a spread footing?
A strip footing is a continuous concrete beam under a load-bearing wall — typically 12-24″ wide × 8-18″ deep, running the full length of the wall. A spread footing (sometimes called a pad footing or isolated footing) is a discrete pad under a single column or post — typically 24-48″ square × 12-18″ deep. Same volume math (length × width × thickness / 27) for both modes; the calculator's footing mode handles either. Stepped footings on grade changes don't fit the single-rectangle math — split into rectangles, compute each, and sum.
Should I order ready-mix or bags for footings?
Most residential strip footings land between 1 and 4 yd³, which sits right on the ready-mix minimum threshold. Above 3 yd³ ready-mix is almost always cheaper and easier — discharging direct from the truck's chute into the trench beats wheelbarrowing 135+ bags of 80 lb mix. Below 3 yd³ you're paying a short-load fee (NRMCA CIP 31 — typically $40-60 per yard under threshold) and bagged becomes the call. Long strip footings can also be phased — pour half at 2 yd³, return for the second half — to clear the minimum without a fee.
Why does the calculator round up to a quarter-yard?
Most Chicago-suburb residential ready-mix plants — Ozinga, Prairie, Vulcan — honor quarter-yard dispatch tickets even though the truck's discharge unit is internally tracked in half-yards. Quarter-yard rounding matches what the dispatcher will actually write on your ticket. Stricter half-yard plants (more common at rural Michigan suppliers and some plant-direct outlets outside the metro) will round one further step up; if the calculator says 2.0 yd³ at quarter-yard rounding, a half-yard plant might bump you to 2.0 anyway since 2.0 is already a half-yard increment. Always confirm dispatch unit before placing the order.
Related guides
- Concrete yardage for a slab →
- Concrete yardage for steps and stairs →
- Concrete yardage for a circle (post holes) →
- How many bags of concrete make a yard? →
Once the footing math is dialed, the next decision is usually post holes — for fence posts, deck piers, or an isolated column footing that doesn't share geometry with a strip. The math swaps rectangle for cylinder. How to figure concrete yardage for a circle (post holes) covers it.
By James Wu. Volume math is site arithmetic, formulas shown above. Bag yields trace to Quikrete Concrete Mix #1101 data sheet. Footing depth and reinforcement guidance from ACI 332-20 (Residential Concrete Code) and IRC 2021 Chapter 4 (Foundations). Frost-line depths cross-checked against USDA NRCS and NOAA regional frost-depth maps; Chicago / Cook County 42″ figure is referenced under municipal building code per IRC R403.1.4. Ready-mix delivery and short-load guidance from NRMCA CIP 31 — Ordering Ready Mixed Concrete; discharge window from ASTM C94/C94M. Engine logic in lib/sitework/concrete.ts. Not structural-engineering advice — for code questions, footing design, rebar schedules, or frost-depth verification, work with a licensed structural engineer or your local building inspector. Full methodology.