Length × width × thickness in feet, divided by 27 — that's the cubic yards of concrete your slab contains. A 10 ft × 12 ft slab at 4 inches is 1.48 yd³ exact. What you order is bigger, and the cushion between the two is where flippers run pours short.
- Multiply length × width × thickness, all in feet.
- Divide by 27 (cubic feet in a cubic yard).
- Add 5-10% waste, then round up to your dispatcher's increment.
| Slab size | 4″ thick | 6″ thick |
|---|---|---|
| 8 × 8 ft | 0.79 yd³ | 1.19 yd³ |
| 10 × 10 ft | 1.23 yd³ | 1.85 yd³ |
| 10 × 12 ft | 1.48 yd³ | 2.22 yd³ |
| 12 × 12 ft | 1.78 yd³ | 2.67 yd³ |
| 12 × 16 ft | 2.37 yd³ | 3.56 yd³ |
| 16 × 20 ft | 3.95 yd³ | 5.93 yd³ |
Geometric volume only. Add 5% (level subgrade) to 10% (rough), then round up to your dispatcher's increment.
Run any other dimensions through the concrete yardage calculator — it surfaces the practical-order cushion and flags the residential ready-mix minimum below 3 yd³ where the short-load fee starts to bite.
The math, walked through
Length and width go in as feet — measure the long edge of the form and the short edge. Thickness usually starts in inches (4″, 6″, 12″), so convert it: 4 inches is 4 ÷ 12 = 0.333 ft. Multiply all three and you get volume in cubic feet. For a 10 ft × 12 ft slab at 4″, that's 10 × 12 × 0.333 = 40 ft³.
Divide by 27 to convert cubic feet to cubic yards. A cubic yard is 3 × 3 × 3 ft = 27 ft³, so the 40 ft³ slab 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.
Then add 5% for waste on a level compacted subgrade — 10% if the subgrade is rough or the edge forms could bow under the truck's chute pressure: 1.48 × 1.05 ≈ 1.554 yd³. 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 (most plant-direct suppliers outside the metro, most rural Michigan plants) round to 2.0. Always confirm the dispatcher's minimum and increment before placing the order.
For irregular slabs — L-shapes, curves, a step in the form — split the shape into rectangles, compute each, and sum.
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
Final order: 1.75 yd³ ready-mix on a quarter-yard plant, 2.0 on a half-yard plant — or 67× 80 lb bags. The bagged-vs-ready-mix call gets its own section below; at 1.48 yd³ this slab is in the awkward middle where the answer isn't automatic.
Run your slab numbers
Same engine used on the main calculator page, which also covers footing, post hole, and stairs.
Bagged or call the truck?
The economics flip around 1 cubic yard. Below that, bagged usually wins on cost; above it, ready-mix wins on labor — and above the residential plant's minimum, ready-mix wins on cost too.
- Under 1 yd³ (a small pad, a stoop, a fence-line section) — bagged. 60 lb yields 0.45 ft³ once mixed; 80 lb yields 0.60 ft³ per the Quikrete and Sakrete data sheets, so a yard is 60 of the 60-lb bags or 45 of the 80-lb. Stay bagged through 1 yd³ if labor is free and the supplier is local.
- 1 to 3 yd³ (most one-room slabs, basement pads, small garage corners) — the awkward middle. Bagged is a long Saturday and a back injury looking for a wheelbarrow. Ready-mix triggers the residential minimum (most Chicago-suburb plants are 1 to 3 yd³) plus a short-load fee — NRMCA CIP 31 puts it at $40-60 per yard under threshold. The calculator surfaces the short-load warning when the practical-order lands below 3 yd³.
- 3 yd³ and up — call the truck every time. Labor savings plus chute-direct discharge avoid wheelbarrow loss, and the finish window is more forgiving with a continuous pour.
The shortcut: above 1 yd³ I'd order ready-mix every time the dispatcher's minimum allows it. Bags above 1 yd³ is just a slower way to get to the same number.
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 territory1and the cure window blows out. Above 85°F slump-loss eats your finish window inside ASTM C94's 90-minute discharge limit2. 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.
What I'd do next
- Run your slab through the calculator
Plug in dimensions, get exact yardage + the practical-order cushion.
- Decide bagged or ready-mix
The economics flip around 1 yd³. Bag math + short-load fee math live here.
- Footing math (if your slab needs them)
Strip / spread / pier footings. The bell-out cushion the cylinder math doesn't see.
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.
- 1. ACI 306R-16 — Guide to Cold Weather Concreting — anchor for sub-40°F curing-blanket and surface-temperature practice. ↩
- 2. ASTM C94/C94M — Standard Specification for Ready-Mixed Concrete — 90-minute discharge window from batch to placement. ↩
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.