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Concrete Slab Calculator
Cubic yards to order, the bagged alternative, and reinforcement from your slab dimensions — with the math shown, not hidden.
Field measurements
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Printed from build-your-house.com/calculators/concrete-slab — estimate only.
Methodology
How this takeoff is figured
SHEET TO-03
Slab volume. Length × width × (thickness ÷ 12) gives cubic feet; divide by 27 for cubic yards. A 24 × 24 ft slab at 4" is 192 cubic feet — about 7.1 yards before margin.
Thickened edge. The standard monolithic detail for garages and sheds: a 12" wide × 12" deep footing around the perimeter, poured in the same shot as the slab. On that 24 × 24, the 96-ft perimeter adds 96 cubic feet — roughly 3.5 yards more.
Order margin. The order quantity carries 10% on top of the placed volume and rounds up to the quarter yard. Short-loading a pour is the expensive mistake: if the truck comes up a half yard short, you either pay a second delivery — with its own short-load fee — or cold-joint the slab while you wait. Finishing crews would rather waste half a yard than stop a pour, and so should you.
Bags vs. ready-mix. An 80-lb bag yields 0.60 cubic feet, and the bagged alternative appears in the notes under the schedule so you can compare. Bags stop making sense above roughly 2 yd³ — that is 90-plus bags to haul, mix, and place before the first batch stiffens.
Under and in the slab. Every run includes 6-mil poly vapor barrier at the slab footprint plus 15% for laps and turnups. Mesh is figured as 5×10 flat sheets with a one-square overlap (10%); rebar as #4 both ways at 24" on-center with a 5% lap allowance.
What's not here. Forms, gravel base, labor, and pump or finishing costs. This sheet sizes slabs, garages, and outbuildings — house foundations need engineered footings from your plans, not a calculator. The foundation phase guide covers what happens before and after the pour.
Assumptions
Volume, margin, and materials
ADJUST ABOVE
- Slab volumeL × W × t/12 ÷ 27
- Thickened edge12 × 12" × perimeter
- Order margin+10%, rounded up to ¼ yd
- 80-lb bag yield0.60 cu ft
- Vapor barrier6-mil poly + 15% laps
- Wire mesh5×10 sheets + 10% overlap
- Rebar#4 both ways, 24" OC + 5%
- Ready-mix price$155–$195/yd, Aug 2026
- Short loadsFee under ~4 yd³ — ask the plant
Questions
Owner-builders ask
FAQ
How many yards of concrete for a 24×24 slab?
At 4" thick, 24 × 24 ft is 576 sq ft × 4/12 = 192 cubic feet. A thickened edge adds a 12 × 12" footing around the 96-ft perimeter — 96 more cubic feet — for 288 total, or 10.7 cubic yards placed. Add the 10% order margin and round up to the quarter yard, and you order 11.75 yd³. Skip the thickened edge and the same slab orders at 8 yards even.
Should I use bags or ready-mix concrete?
An 80-lb bag yields 0.60 cubic feet, so a single cubic yard is 45 bags — mixed one batch at a time. Bags stop making sense around 2 yards (90+ bags); past that, mixing time alone risks cold joints between batches. Ready-mix runs $155–$195 per yard delivered (August 2026), though loads under about 4 yards usually carry a short-load fee. The sheet shows the bagged alternative in the notes so you can compare both ways.
How thick should a garage slab be?
4 inches is the typical garage slab for cars and light trucks. Go 5 or 6 inches if the slab will carry heavy trucks, an RV, or a vehicle lift. Your code office and your soil have the final say — expansive or poorly compacted soil can demand more thickness or reinforcement regardless of what parks on it, so confirm with the permit desk before you form.
Do I need rebar or wire mesh in a slab?
For a residential slab on grade, reinforcement is crack control, not structure — it holds cracks tight rather than preventing them. Mesh or rebar both work if placed mid-slab on chairs, not left on the ground to be pulled up during the pour. Choose rebar (figured here as #4 both ways at 24" on-center) when the slab carries point loads like a lift post or heavy racking. Plain or fiber mix with neither is also common for sheds — ask your inspector what local practice expects.
What is a short-load fee?
A ready-mix truck carries about 10 yards, and the plant pays for the truck and driver whether it hauls 10 or 2. Loads under roughly 4 cubic yards usually carry a short-load fee to cover the trip; the exact charge varies by plant, so ask when you order. If your pour lands just under the cutoff, it can be cheaper to add a footing or pad to the same pour, or split a truck with a neighbor.