How much concrete do I need?
Concrete is ordered by volume, so you need length × width × thickness, all in the same units. Because slab thickness is usually given in inches, divide it by 12 to convert to feet. A 10 by 10 foot patio that is 4 inches thick needs 10 × 10 × (4 ÷ 12) = 33.33 cubic feet. Ready-mix concrete in the US is sold by the cubic yard, which is 27 cubic feet (3 × 3 × 3 feet), so that patio needs 33.33 ÷ 27 = 1.23 cubic yards before waste. In metric, multiply meters by meters by the thickness in meters: a 3 by 3 meter slab 10 cm thick is 0.9 cubic meters.
Bags or ready-mix?
For small jobs, bagged premix is convenient. Typical yields are about 0.30 cubic feet for a 40 lb bag, 0.45 cubic feet for a 60 lb bag and 0.60 cubic feet for an 80 lb bag. The 10 by 10 foot patio above would need 56 bags of 80 lb mix without any waste allowance, which is about 4,500 lb of concrete to mix by hand. As a rule of thumb, once a job needs more than about one cubic yard, ordering ready-mix from a truck is usually cheaper and much less work, although many suppliers charge a short-load fee for small orders. Check the exact yield printed on the bag you buy, as it varies slightly by brand.
Why you need extra
It is better to have a little left over than to run short in the middle of a pour, which can create a weak cold joint. Ground is rarely perfectly level, forms can bow outward, and some concrete is lost to spills and the inside of the mixer. Most contractors order 5% to 10% extra; uneven sub-bases may need more. The calculator adds 10% by default, and the amount is shown in the note under your results.
Choosing the right thickness
Thickness depends on the load. Walkways and patios are commonly 4 inches (10 cm) thick. Driveways that carry cars are typically 4 to 5 inches, and 6 inches for heavier vehicles such as trucks or RVs. Garage floors are usually 4 to 6 inches, and shed bases 4 inches. Slabs should sit on a compacted gravel base, often 4 inches deep, and are frequently reinforced with wire mesh or rebar to control cracking. Control joints, cut to about a quarter of the slab depth at regular intervals, help cracks form in straight lines. Local building codes may specify requirements for driveways and structural footings, so check before you pour. For round columns or footings, use the area calculator to find the area of a circle and multiply by the depth.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick, a 10x10 foot slab is 33.3 cubic feet, which is about 56 bags of 80 lb mix or 75 bags of 60 lb mix, plus extra for waste.
How many cubic feet are in a cubic yard?
27 cubic feet.
How thick should a concrete patio be?
Usually 4 inches for a patio or walkway, and 5 to 6 inches for driveways.
How much extra concrete should I order?
Plan on 5% to 10% extra to cover uneven ground, spills and form movement.