To estimate concrete for a rectangular patio, multiply length by width by the designed slab thickness, using the same units throughout. If the dimensions are in feet, the result is cubic feet; divide by 27 for cubic yards. Cement is only one ingredient, so the concrete volume is not the amount of cement powder to buy.
First establish the required slab design. The examples below explain quantity, not the correct thickness for every patio. Loads, ground conditions, climate and local requirements can change the specification.
The basic formula
For measurements in feet and thickness in inches: cubic feet = length × width × thickness ÷ 12. Then cubic yards = cubic feet ÷ 27. Keep full precision until the final order rather than rounding the thickness conversion early.
For metric measurements: cubic metres = length in metres × width in metres × thickness in millimetres ÷ 1,000. Do not mix inches, centimetres and metres in the same multiplication without converting them first.
Worked example: 10 by 12 feet
A patio measuring 10 feet by 12 feet with a specified 4-inch slab has a theoretical volume of 10 × 12 × 4 ÷ 12 = 40 cubic feet. Dividing by 27 gives about 1.48 cubic yards. This is the slab’s geometric volume before any allowance for unevenness or waste.
As an illustration only, adding a 10% allowance gives 44 cubic feet, or about 1.63 cubic yards. Ask the supplier or installer what allowance and ordering increment suit the actual job. Ten percent is not a rule that compensates for poorly measured excavation.
Convert volume to bags using the product’s yield
Bag weight alone is not enough: read the mixed-concrete yield on the bag or current data sheet. For example, the QUIKRETE Concrete Mix data sheet lists approximately 0.60 cubic foot from an 80-pound bag.
At that yield, the 40-cubic-foot example needs 40 ÷ 0.60 = 66.67 bags, rounded up to 67 whole bags before allowance. With the illustrative 44-cubic-foot allowance, it needs 44 ÷ 0.60 = 73.33, rounded up to 74 bags. Different products and bag sizes require their own yield.
Check the manufacturer’s calculator against your measurements and remember that approximate yields do not include every site loss. A large number of bags also means substantial handling and mixing time. Compare delivery and placement options before committing to hand mixing a whole slab.
Irregular shapes and thicker sections
Divide an L-shaped patio into rectangles that do not overlap, calculate each volume and add them. For a triangle, calculate the area as base × perpendicular height ÷ 2, then multiply by thickness in matching units. A circle uses π × radius² for its area; the radius is half the diameter.
Calculate designed thickened edges, footings or steps separately. If you already included the ordinary slab thickness across an edge strip, add only the extra depth there to avoid double-counting. Have the installer check complex shapes or changing levels rather than estimating them by eye.
What should be in the order?
Give the supplier the dimensions, total calculated volume, intended use, access conditions and project specification. Discuss delivery charges, minimum loads, placing arrangements and the agreed allowance. Concrete strength and exposure requirements are separate from volume; ordering the correct number of yards does not select the correct mix.
For bagged material, confirm storage space, mixing equipment and enough people to maintain the required placing sequence. Cement, mortar and concrete mix are not interchangeable products. Adding a guessed percentage of cement to a volume calculation is not a dependable batch design.
Check these measurements before paying
Measure inside the forms, confirm the specified thickness and check whether the prepared base creates deeper pockets. Keep the base and slab volumes separate. A gravel base needs its own estimate and should not be counted as concrete.
Review the quantity once the site is prepared, then record the agreed order and allowance. For the construction sequence, see planning, pouring and curing a concrete patio.