For a 10 ft × 12 ft × 4″ residential interior slab — the most common piece of concrete on a flip — the volume is 1.48 cubic yards exact. With a 5% waste cushion that rounds up to 1.75 yd³as a practical order on a Chicago-suburb supplier that honors quarter-yard residential dispatch. If you go bagged instead, that's 67× 80 lb bags — the count comes straight off the Quikrete data sheet (0.60 ft³ yield per 80 lb bag).
Run the same math on the concrete yardage calculator for any slab dimension; it surfaces the practical-order cushion and flags the residential ready-mix minimum below 3 yd³ where the short-load fee starts to bite.
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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
The first time the math earned its keep was a basement slab pour at a suburban Chicago gut rehab. Old plank-on-grade subfloor was coming out, new 4″ slab going in over compacted aggregate. The tape said 11 ft × 14 ft. The calculator said 1.91 cubic yards exact. I told the dispatcher 2.25 to leave a working cushion.
The truck arrived with 2.25 on the ticket. The south corner of the basement turned out to have a half-inch sag in the old subfloor that nobody had measured — when the screed cleared the rest of the slab, that corner was about three inches short of where it needed to land. We used every cubic foot of the cushion on that corner. The math was right; the order was right BECAUSE it was bigger than the math.
That gap between exact-math and what-to-actually-order is what this guide is about. The exact volume is a starting point, not an order. The practical order is what you tell the dispatcher. The cushion in between is the experience layer — small grade variation, edge forms that bow under the truck's chute, the shovelfuls that fall off the wheelbarrow on the way back from a long pour. None of that shows up in length × width × thickness.
My default is 5% on level compacted subgrade and 10% on rough. Most calculator sites split the difference and recommend “5 to 10%” without a tiebreaker. I'd rather have the tiebreaker pinned: if the subgrade is screeded clean and the forms look square, 5% is plenty. If anything looks like it'll move, go 10%. Below 3 yd³ the cushion direction inverts anyway — ordering ready-mix smaller than the residential minimum costs you a short-load fee, and bagged becomes the cheaper call.
The four steps
1. Measure in feet (or convert).
Length and width go in as feet — measure the long edge of the form and the short edge. Thickness is more often given in inches (4″, 6″, 12″) and you convert: 4 inches is 4 ÷ 12 = 0.333 ft. The whole formula expects feet because cubic feet is what divides cleanly by 27 to give cubic yards.
2. Compute volume in cubic feet.
Length × width × thickness, all in feet, gives volume in cubic feet. For 10 × 12 × 0.333 the answer is 40 ft³. If you have an irregular slab — an L-shape, a curve, a step in the form — split it into rectangles, compute each, and sum.
3. Convert to cubic yards.
Divide by 27 to get cubic yards. A cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³. For our 40 ft³ slab the answer is 40 ÷ 27 ≈ 1.48 yd³. That number is the EXACT volume — what the slab geometrically contains, before any cushion or rounding. Concrete is sold by the cubic yard, so this is the unit that matters.
4. Add the cushion, then round to the dispatch unit.
Multiply by (1 + waste% / 100). At 5% the math is 1.48 × 1.05 ≈ 1.554 yd³. Then round UP to the next dispatch unit your supplier honors. Most Chicago-suburb residential plants — Ozinga, Prairie, Vulcan — take quarter-yard tickets, so 1.554 rounds up to 1.75 yd³. Stricter half-yard dispatchers round to 2.0. Always confirm the dispatcher's minimum and dispatch unit before placing the order.
The worked example, end to end
Inputs:
- 10 ft long × 12 ft wide × 4″ thick
- 5% waste cushion (level compacted subgrade)
- Bagged-alternative computed at the EXACT volume
Step by step:
- Volume in cubic feet:
10 × 12 × (4 / 12) = 40 ft³ - Cubic yards exact:
40 / 27 ≈ 1.48 yd³ - With 5% cushion:
1.48 × 1.05 ≈ 1.554 yd³ - Practical order, quarter-yard rounding:
ceil(1.554 × 4) / 4 = 1.75 yd³ - Bag count, 80 lb at 0.60 ft³ each:
ceil(1.48 × 27 / 0.60) = ceil(66.6) = 67 bags - Bag count, 60 lb at 0.45 ft³ each:
ceil(1.48 × 27 / 0.45) = 89 bags
So you call the dispatcher with 1.75 yd³ (or 2.0 on a half-yard plant) — OR pick up 67× 80 lb bags from the supplier of choice. At 1.48 yd³ the slab is below the typical residential ready-mix minimum of 3 yd³, so bagged is usually the cheaper call here even though ready-mix wins on labor.
Run the math yourself
Type the dimensions for your slab. 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 traps that put the math on the wrong side of the order:
- Edge forms that bow.The math assumes the slab is exactly the dimensions of the form. A 2x6 edge form bowing outward a quarter-inch over a 12-foot run adds about 0.05 yd³ of unaccounted volume, and that's after the form ALREADY absorbed the truck's lateral pressure. If you can deflect the form with hand pressure, you're going to lose mix into the bow before you finish.
- Subgrade variation. A 4″ slab on a subgrade that varies between 3.5″ and 4.5″ deep effectively pours 4-5″ in places. On a 120 sq ft slab that variation alone can pull a quarter-yard of mix. The 5% cushion covers small variation; the 10% bump covers obvious dips that you can see when you stand at one corner and look across.
- Wheelbarrow loss on long runs.Bagged slabs poured by hand from a mixer to a basement form lose a couple shovelfuls per trip — across 67 bags that's several cubic feet you didn't plan for. Ready-mix discharged direct from the truck's chute mostly avoids it; the cushion shifts in response.
- Half-yard dispatchers.Quarter-yard rounding assumes Ozinga / Prairie / similar Chicago-suburb practice. Stricter dispatchers — including most plant-direct suppliers outside the metro and most rural Michigan plants — only honor half-yard tickets. If the math says 1.75 and the dispatcher says “we round up to 2,” order 2.
- Pour temperature.Below 40°F you're into ACI 306 cold-weather territory and the cure window blows out. Above 85°F slump-loss eats your finish window inside ASTM C94's 90-minute discharge limit. The volume math doesn't change with temperature, but the pour does — a slab math-checked at 70°F can fail at 35°F if you didn't plan curing blankets.
Frequently asked
How do I calculate cubic yards for a 10×12 concrete slab?
Multiply length × width × thickness, all in feet, then divide by 27. For a 10 ft × 12 ft × 4″ slab the math is 10 × 12 × (4/12) = 40 ft³, divided by 27 = 1.48 cubic yards exact. Add a 5% waste cushion to land at 1.554 yd³, then round up to the next dispatch increment your supplier honors. Most Chicago-suburb residential plants take quarter-yard tickets, so the practical order is 1.75 yd³. Stricter half-yard plants would round to 2.0.
How thick should a residential concrete slab be?
IRC 2021 Section R506.1 sets the minimum at 3.5 inches for a slab on grade inside a dwelling. 4 inches is the contractor minimum on most flips — it adds about 0.7 ft³ over a 10×12 footprint and the small upcharge keeps you out of an inspector debate over edge thickness. For driveways and garage floors carrying vehicle weight, 5-6 inches is the practical floor. Heavier sustained loads (RV pads, equipment) need a structural review per ACI 332.
How much waste should I add when ordering concrete for a slab?
5% on level compacted subgrade. Bump to 10% on rough subgrade or where the edge forms can bow under the truck's chute. The 5-10% range is what you'll see across InchCalculator, OmniCalculator, and supplier guidance — but those sources don't separate level from rough, so the choice gets pinned at write-time. Don't double-cushion at both the cubic-yard layer AND the bag-count layer when going bagged; buy bags for the EXACT yardage and keep one or two extra by hand if the subgrade looks dicey.
How many 80 lb bags of concrete do I need for a yard?
45 bags. The number comes off the Quikrete Concrete Mix #1101 data sheet — an 80 lb bag yields 0.60 cubic feet once mixed with water, and a cubic yard is 27 ft³, so 27 / 0.60 = 45 bags per yard. Sakrete High-Strength matches the same yield and meets ASTM C387. For a 1.48 yd³ slab the bag count is Math.ceil(1.48 × 45) = 67 bags. That's also the cutover where most flippers should switch to ready-mix — 67 bags is a long Saturday with a wheelbarrow, and at $6-7/bag the $400-450 spend lines up with a residential ready-mix delivery anyway.
Can I pour a concrete slab in cold weather?
Below 40°F the pour falls under ACI 306R-16 (Cold Weather Concreting). You need curing blankets to keep the surface above 50°F until the concrete reaches roughly 500 psi, which typically takes 24-48 hours, and the standard 28-day cure window stretches longer. Above 85°F the pour falls under ACI 305 — slump-loss accelerates inside the 90-minute ASTM C94 discharge window and you have less time to finish. Chicago shoulder seasons are the trap because the day-night swing can put both ends of the day inside the risk window.
Should I use ready-mix or bagged concrete for a slab?
Above 1 cubic yard, ready-mix is almost always the cheaper and easier call — 1 yd³ is around 67 bags of 80 lb, which is a serious mix-and-pour day and a lot of bag handling. The catch is the residential delivery minimum: most Chicago-suburb plants set it between 1 and 3 yd³, and ordering under triggers a short-load fee of $40-60 per yard under threshold (NRMCA CIP 31). For a 1.48 yd³ slab below the typical minimum, the practical answer is either bagged, a phased pour split across two days, or finding a smaller-minimum supplier. The calculator surfaces the warning when the practical-order amount falls under 3 yd³.
Related guides
- Concrete yardage for footings →
- Concrete yardage for steps and stairs →
- Concrete yardage for a circle (post holes) →
- How many bags of concrete make a yard? →
Once the slab math is dialed, the next decision is usually footings — strip footings under a foundation wall or spread footings under a column. The math swaps thickness for depth and adds frost-line considerations. How to figure concrete yardage for footings covers it.
By James Wu. Volume math is site arithmetic, formulas shown above. Bag yields trace to Quikrete Concrete Mix #1101 data sheet and Sakrete High-Strength Concrete Mix (ASTM C387). Ready-mix delivery and short-load guidance from NRMCA CIP 31 — Ordering Ready Mixed Concrete; discharge window from ASTM C94/C94M; cold-weather practice from ACI 306R-16. Slab thickness minimum from IRC 2021 Section R506.1. Engine logic in lib/sitework/concrete.ts. Not structural-engineering advice — for code questions, footing design, or rebar layout, work with a licensed structural engineer or your local building inspector. Full methodology.