Concrete Slab Calculator

How to calculate concrete for a slab

Slab volume is length × width × thickness, with thickness converted from inches to feet (divide by 12). The result in cubic feet is divided by 27 to get cubic yards. A 20 × 20 ft slab at 4 inches is 400 sq ft × 0.333 ft = 133.3 cu ft, or 4.94 cubic yards before waste. Adding 10 percent brings it to about 5.43 yards.

Thickness drives the order more than anything else. 4 inches is the typical thickness for patios, walkways and shed pads. Garage floors and slabs that carry cars are usually 4 to 6 inches, with 5 or 6 inches used where heavier trucks, RVs or equipment will park. Going from 4 to 6 inches adds 50 percent to the concrete.

Ready-mix plants typically sell in quarter- or half-yard increments, so the calculator rounds your total up to the next quarter yard. Many suppliers add a short-load fee for orders under a set minimum (often around 4 to 6 yards, depending on the plant), so ask when you get your quote. Below about a yard, bagged mix is usually simpler; the bag counts assume 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag.

The cost line multiplies the calculated yards by your price per yard. The default of $180 is close to the 2025 national average, but local prices vary, and delivery, fuel surcharges, fiber, color or accelerator add-ons are billed separately.

Where these numbers come from

The formulas and default values in this calculator are based on:

• Concrete Network, Concrete Prices (2025 average ready-mix price about $180 per cu yd, range $160–$195)
• QUIKRETE Concrete Mix No. 1101 technical data sheet for bag yields
• Portland Cement Association residential slab guidance

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.