Pouring a slab and not sure how much concrete to buy? The math is simpler than it looks. Here's the exact method the pros use, a worked example, and a free tool to do it for you.

The formula in one line

Concrete volume is just length × width × thickness. The only trick is units: length and width are in feet, but thickness is almost always in inches. Convert the thickness to feet by dividing by 12, multiply everything together for cubic feet, then divide by 27 to get the cubic yards a ready-mix supplier quotes.

In short: cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27.

Worked example: a 10 × 20 ft slab, 4 inches thick

Say you're pouring a 10 by 20 foot patio at the standard 4-inch thickness:

Area: 10 × 20 = 200 square feet
Thickness in feet: 4 ÷ 12 = 0.333 ft
Volume: 200 × 0.333 = 66.7 cubic feet
Cubic yards: 66.7 ÷ 27 = 2.47 cubic yards

Add a 10% waste allowance and you'd order about 2.7 cubic yards of ready-mix.

Skip the math

Concrete Calculator

Enter your slab size and get the cubic yards and exact number of 60 lb and 80 lb bags instantly.

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How many bags is that?

For smaller jobs you'll buy bags of pre-mix instead of ordering a truck. Each bag lists its yield: an 80 lb bag makes about 0.60 cubic feet, and a 60 lb bag about 0.45 cubic feet. Divide your cubic feet by that number:

66.7 ÷ 0.60 = 112 bags of 80 lb
66.7 ÷ 0.45 = 149 bags of 60 lb

That's a lot of bags — which is exactly why anything over about one cubic yard is usually cheaper and far easier to order as ready-mix delivered by truck.

Why you should always add waste

Real ground is never perfectly level and forms are never perfectly tight. If you order the exact volume and come up short mid-pour, you get a cold joint — a weak seam where fresh concrete meets partly-set concrete. Adding 5–10% is cheap insurance; lean toward 10% on rough or over-excavated ground.

What thickness should the slab be?

Patios and walkways are typically 4 inches. Driveways and anything carrying vehicles are usually 5–6 inches. Always follow your local building code and any engineer's specification — this guide covers the concrete volume only, not the gravel base, rebar or forms you'll also need.

Don't forget the base under the slab

The slab sits on a compacted gravel base, usually 4 inches of crushed stone. It is calculated exactly like the concrete: for the same 10 × 20 foot patio, 4 inches of base is another 2.47 cubic yards, or about 3.5 tons of crushed stone at 1.4 tons per yard. Our gravel calculator does the conversion. Compact the base in layers; a slab poured on loose fill settles and cracks.

Forms and reinforcement

Forms: for a 4-inch slab, 2x4 lumber on edge works as the form, braced with stakes every 3 to 4 feet. A 10 × 20 slab has a 60-foot perimeter, so you need 60 feet of form board plus stakes.
Reinforcement: patios and walkways commonly use welded wire mesh or a grid of #3 or #4 rebar at 18 to 24 inches. Driveways and anything under a vehicle should follow local code. The rebar calculator counts the bars for a grid.
Control joints: concrete cracks as it shrinks, so you decide where. A common rule is to space joints no more than 2 to 3 times the slab thickness in feet (8 to 12 feet for a 4-inch slab), cut to a quarter of the thickness.

Ordering ready-mix

A ready-mix truck typically carries up to about 9 or 10 cubic yards, but many plants charge a short-load fee on small orders, often anything under a few yards. Ask about the minimum before you decide between a truck and bags. Also check access: the chute on the back of the truck reaches roughly 15 to 20 feet, so a backyard slab may need wheelbarrows or a pump truck. Tell the plant the job (patio, driveway, footing) and they will suggest a mix strength.

How long before you can use it

Stage	Typical time after the pour
Light foot traffic	24 to 48 hours
Remove forms	1 to 2 days
Cars on a driveway	About 7 days
Close to full design strength	28 days

Keep the slab damp, or covered with plastic or a curing compound, for the first week. Concrete that dries out too fast is weaker at the surface and dusts and scales. Times stretch in cold weather, so follow the supplier's advice for your season.