Short answer
A 24x24 garage foundation poured at 6 inches thick requires 10.67 cubic yards of concrete. With a standard 10% waste factor, order 11.74 yards — most ready-mix plants will round that to 12 yards. If you're pricing out bagged concrete for comparison, that works out to 480 eighty-pound bags.
How this calculator works
The concrete slab calculator uses one straightforward formula: multiply the length by the width by the thickness (converted from inches to feet), then divide by 27 to convert cubic feet into cubic yards.
For this specific pour:
- Length: 24 ft
- Width: 24 ft
- Thickness: 6 in ÷ 12 = 0.5 ft
24 × 24 × 0.5 = 288 cubic feet 288 ÷ 27 = 10.67 cubic yards
That's the raw volume. The calculator then applies a 10% waste factor to account for uneven subgrade, spillage, and the small amount of concrete that stays in the chute or gets wasted at the edges. Ten percent is the industry standard for flatwork; steeper or more complex forms sometimes warrant 15%.
What the secondary outputs mean
The calculator also tells you how many bags you'd need if you went the bagged-concrete route:
- 480 bags at 80 lbs (each yields 0.6 cu ft)
- 640 bags at 60 lbs (each yields 0.45 cu ft)
These numbers exist for reference, not as a recommendation. At 10+ cubic yards, bagged concrete is impractical. You'd be mixing bags for the better part of two days, and the first section of the slab would already be setting before you finished the last. Ready-mix is the correct choice for any garage foundation.
Inputs and what changes them
Thickness is the variable that moves the needle most on a fixed footprint. A 24x24 slab at 4 inches (the code minimum for light passenger vehicles) uses 7.11 cubic yards — 3.5 yards less than the 6-inch version. If you're planning to park a diesel truck, use a floor jack regularly, or install a hydraulic lift, stick with 6 inches.
Subgrade prep matters too. The calculator assumes a flat, level form. If your lot has significant slope across 24 feet, the actual pour depth will vary, and you may end up using more concrete than calculated. Always have your subgrade graded and compacted before finalizing your order.
Footings vs. slab: This calculator covers the slab itself. If your design calls for thickened-edge footings around the perimeter (common in colder climates where code requires frost-depth footings), you'll need to calculate those separately using the footing calculator and add that volume to your order.
How ready-mix ordering works
Ready-mix is sold in cubic yards with a typical minimum load of 1 yard and often a short-load fee for orders under 5–7 yards. For a 12-yard order you'll likely get a standard full truck (most mixers hold 8–10 yards), so you may need two trucks scheduled back-to-back. Tell your supplier your pour rate (how fast your crew can spread and screed) so they can time the second truck's arrival correctly — you don't want it sitting and spinning for an hour.
Common mistakes and gotchas
Common mistakes and gotchas
Subgrade is not flat — your "4 inch" slab is really 4 inches in the worst spot. The calculator assumes uniform thickness. In reality, gravel base settles unevenly between when you screed it and when the truck arrives. Pad the order by 10% just for this. If you're pouring on bare dirt over an existing slope, pad it 15%.
The truck driver's "yard" is generous, but only on round numbers. Ready-mix companies typically charge by quarter-yard increments and short-load fees apply under 3 yards. If your calculation comes to 3.6 yards, ordering 4 is rarely a waste — the alternative is a short-load fee that costs more than half a yard of concrete.
Bag math lies on small pours. Premix bag yields are theoretical, assuming you mix to spec. In practice you'll lose half a bag to splash, partial mixes, and one bag that "felt dry." For pours under 1 cubic yard, add 2 extra bags to the calculator's bag count. For under half a yard, add 3.
Sonotubes flare at the bottom. If you're setting tubes in soil and pouring, the bottoms expand outward 1–2 inches as concrete pushes against the wet earth. Add 5% to your sonotube total or accept that your last tube will be a little short.
Cold concrete needs more bags. Below 50°F, hydration slows and a "fast-setting" mix isn't fast anymore. Don't try to pour a footing the day before a freeze unless you're prepared to tarp and heat — you'll either lose the pour or end up adding a second batch of higher-PSI mix on top.
Footings under frost line aren't optional. If you're in a frost zone, the calculator's depth input must be deeper than your local frost line, or the structure will heave. Frost depth varies by region: Florida 0", Pennsylvania 36", Minnesota 48", Maine 60". Check your county's building code before pouring footings, not after.
Recommended materials
For a 24x24 garage slab, the work breaks down into the pour itself, finishing the surface, and reinforcing against cracking. If you're filling a small void or doing test mixes before the truck arrives, bagged mix comes in handy. Once the slab is poured, you'll need the right tools to get a flat, smooth finish before the surface sets up.
- Quikrete 80lb fast-setting concrete mix — useful for filling post holes around the garage perimeter or patching low spots before the main pour
- Concrete float (16-inch finishing tool) — a magnesium float is the standard tool for bringing cream to the surface and closing the slab after screeding
- #4 grade 60 rebar (20-foot length) — most codes require #4 bars on 18-inch centers for a garage slab; 20-foot lengths minimize lap splices on a 24-foot span
FAQ
How many cubic yards of concrete does a 24x24 garage slab need? At 6 inches thick, a 24x24 slab requires 10.67 cubic yards before waste. Add a 10% waste factor and order 11.7 cubic yards — most contractors round up to 12 yards when calling a ready-mix plant.
Should I use 4-inch or 6-inch thickness for a garage floor? Four inches is code-minimum for a passenger-vehicle garage in most jurisdictions. If you plan to park trucks, RVs, or run a lift, go 6 inches. The 24x24 slab at 6 inches adds roughly 3.5 cubic yards over a 4-inch pour.
How many 80 lb bags of concrete does a 24x24 slab take? The calculator shows 480 bags at 80 lbs each for 10.67 cubic yards. With a 10% waste buffer you're looking at 528 bags. At that quantity, ready-mix is almost always cheaper and less labor-intensive.
Is ready-mix or bagged concrete better for a 24x24 garage? Ready-mix is the right call for any pour over about 1 cubic yard. A 24x24 slab at 10+ yards would take a crew all day mixing bags and the concrete would set unevenly. Order ready-mix and get it done in a few hours.
What PSI concrete should I use for a garage floor? 3,000 PSI is the standard minimum for residential garage slabs. If you live in a freeze-thaw climate or expect heavy vehicle loads, specify 4,000 PSI with air entrainment.
Do I need rebar in a 24x24 garage slab? Most building departments require either #4 rebar on 18-inch centers or 6x6 welded wire mesh for a garage floor. Rebar provides better crack control, especially at 6-inch thickness. Check your local code before pouring.
How do I account for waste when ordering concrete? A 10% waste factor is standard for slabs. Multiply your calculated volume by 1.10. For 10.67 yards that means ordering 11.74 yards — call it 12 yards with your ready-mix supplier.
How long does a 24x24 garage slab take to cure? Concrete reaches about 70% of its design strength in 7 days and 95% by 28 days. You can walk on it in 24–48 hours, but don't drive vehicles onto the slab for at least 7 days, ideally 14.
What is a sonotube, and do I need one for a garage foundation? A sonotube is a round cardboard form used to pour cylindrical concrete piers or columns. For a standard garage slab on grade, you don't need sonotubes — those are used when the slab sits on separate footings or piers.
How thick should the gravel base be under a garage slab? Four inches of compacted gravel base (crushed stone or bank-run gravel) is standard under a residential garage slab. It improves drainage and prevents frost heave from cracking the concrete.
