Calculator
Concrete Calculator
Estimate the volume of concrete for a slab and how many bags of premix you’ll need.
How much concrete do I need?
Concrete is bought by volume, not area — a slab’s length and width tell you the footprint, but you also need thickness to get a total volume. This calculator converts your slab dimensions into a volume (cubic yards or cubic meters, depending on the units you pick), then into a bag count based on the yield of the premix bag size you’re buying.
Frequently asked questions
How is the concrete volume calculated?
Volume = length × width × thickness, with all three dimensions converted to the same unit first. In US units, thickness (inches) converts to feet by dividing by 12, and the resulting cubic feet divides by 27 for cubic yards. In metric, thickness (centimeters) converts to meters by dividing by 100, and the result is already in cubic meters.
What thickness should I use?
A typical walkway or patio slab is 4 inches (10 cm); driveways for cars are usually poured at 5-6 inches (13-15 cm) to handle the extra weight; footings and structural slabs vary by local building code and should follow an engineer's or inspector's spec, not a general estimate.
Should I buy premix bags or order ready-mix?
Bags make sense for small pours — a cubic yard (0.76 m³) is about 60 bags of 80lb (36 kg) premix, which gets impractical fast. Most pros switch to a ready-mix truck once a pour exceeds roughly 1-2 cubic yards (0.75-1.5 m³), since it's cheaper per yard and cures more consistently than many hand-mixed bags.
Does this account for waste or spillage?
No — the result is the theoretical volume for the exact dimensions entered. It's common practice to buy 5-10% extra to cover an uneven subgrade, spillage, or a slightly deeper pour than planned, especially for a first-time DIY pour.