How this concrete calculator works
Concrete is ordered by volume, so the first job is to work out the volume of the space you are filling. The principle is the same for almost every pour: volume = area × thickness. For a rectangular slab or footing the area is length × width, and the thickness is usually given in inches, so it is converted to feet (divide by 12) before multiplying. For a round column or post footing the shape is a cylinder, so the volume is π × radius² × height, where the radius is half the diameter.
Volume is most often quoted in cubic yards for ready-mix orders and in cubic feet for smaller jobs. There are 27 cubic feet in one cubic yard. Pre-mixed bags are sold by weight, and each bag fills only a small, approximate fraction of a cubic yard — about 0.011 yd³ for a 40 lb bag, 0.017 yd³ for a 60 lb bag and 0.022 yd³ for an 80 lb bag. To estimate how many bags you need, divide your total volume by the bag's yield and round up.
Reference note: bag yields are nominal figures that differ between manufacturers, and real pours lose material to spillage, over-excavation and uneven sub-grade. Use the optional waste factor, treat bag counts as estimates, and confirm yields and slab thickness against the packaging and your local building code.
Frequently asked questions
- How much concrete do I need?
- The amount you need is the volume of the space you are filling. For a slab or footing, multiply length by width by thickness, converting the thickness from inches to feet first. Volume is usually ordered in cubic yards, where one cubic yard equals 27 cubic feet. Enter your dimensions above and the calculator returns both units.
- How many 80 lb bags of concrete are in a cubic yard?
- An 80 lb bag yields roughly 0.022 cubic yards, so about 45 bags fill one cubic yard. A 60 lb bag yields about 0.017 yd³ (roughly 60 per yard) and a 40 lb bag about 0.011 yd³ (roughly 90 per yard). These yields are approximate and vary by brand, so round up and buy extra.
- How thick should a concrete slab be?
- A typical residential slab such as a patio or shed floor is about 4 inches thick. Driveways and slabs carrying heavier loads are often 5 to 6 inches. Always follow your local building code and any engineer's specification, as thickness affects both strength and the volume required.
- What is the difference between slab and cylinder volume?
- A slab or footing is a rectangular box, so its volume is length × width × thickness. A round column or post footing is a cylinder, so its volume is π × radius² × height, where the radius is half the diameter. The calculator switches formulas when you change the shape selector.
- Should I add a waste factor?
- Yes. Spillage, uneven sub-grade, over-excavation and the bottom of the mixer all mean you use slightly more than the exact volume. A waste factor of about 5 to 10 percent is common so you do not run short mid-pour. The calculator applies whatever percentage you enter to the volume and bag counts.
- Is this concrete calculator accurate?
- The volume math is exact for the shapes and dimensions you enter. Bag counts are estimates because yields differ between manufacturers and real pours lose material. Treat the results as a close planning estimate, add a waste factor, and confirm bag yields on the packaging before buying.