Use a three-step process: measure the shape, calculate its net volume, then decide what ordering allowance and product fit the job. Keep square feet separate from cubic feet and cubic yards. A surface area alone is not enough to find concrete volume unless you also know thickness.
1. Measure the shape and use consistent units
For a rectangular slab, record length and width in feet and thickness in inches. The shortcut formula is cubic yards = length × width × thickness ÷ 324. It works because 12 inches make a foot and 27 cubic feet make a cubic yard. If you prefer, convert thickness to feet, multiply length × width × thickness for cubic feet, then divide by 27.
If dimensions are mixed, convert them before multiplying. A 10 ft length and a 120 in length are physically equal, but multiplying those numbers as though they used the same unit gives a meaningless result. For a circular pad or post hole, measure diameter and height; calculate radius as half the diameter. Do not enter diameter as radius.
2. Split multi-part projects without overlaps
For an L-shaped pad, driveway widening, or sidewalk with turns, divide the footprint into simple shapes and add their volumes. Draw the boundaries on paper so adjoining sections meet but do not overlap. If one rectangle already includes a corner, subtract that shared area from the other rectangle. For a wall opening, subtract the opening volume only when it passes through the measured thickness.
A round project uses the circle formula: π × radius² × thickness. For a project with multiple thicknesses, calculate each section separately. A curved edge does not need to be guessed as a rectangle if you can measure its plan area accurately. The goal is to represent the actual concrete shape, not to force every project into one rectangle.
A complete example: 10 ft by 10 ft by 4 in
First find the net volume: 10 × 10 × 4 ÷ 324 = 1.2345679 yd³. In cubic feet, 10 × 10 × 4 ÷ 12 = 33.3333 ft³. These are two ways to describe the same quantity. The area is 100 ft², but the volume is 33.3333 ft³; do not order 100 cubic feet because the surface area number is 100.
If you select a 10% ordering allowance, multiply the net volume by 1.10: 1.2345679 × 1.10 = 1.3580247 yd³, or 36.6667 ft³. Using the QUIKRETE Concrete Mix No. 1101 reference yield of about 0.60 ft³ for an 80 lb bag, divide 36.6667 by 0.60 and round up to 62 bags. The manufacturer data sheet is product-specific; another product may have a different yield.
| Quantity | Result for this example |
|---|---|
| Net concrete volume | 33.3333 ft³ = 1.2346 yd³ |
| With 10% ordering allowance | 36.6667 ft³ = 1.3580 yd³ |
| 80 lb QUIKRETE Concrete Mix No. 1101 bags | 62 bags at about 0.60 ft³ per bag |
3. Choose an allowance once
An allowance accounts for measuring uncertainty, uneven forms, small spills, and placement conditions. The calculator’s 10% default is a planning setting, not a mandatory code value. Adjust it when you have a reason, and do not apply it twice. If the number you enter is already a supplier order quantity that includes extra, treat it as an order quantity rather than adding another percentage.
For multiple sections, add their net volumes first and apply the allowance to the combined total. Applying 10% to each segment and then applying 10% again to the sum would overstate the order. Keep the unrounded volume through the calculation; round only the displayed result and the final bag or order quantity.
Choose ready-mix or bagged concrete
Ready-mix is ordered by volume, usually in cubic yards. Ask the supplier how it rounds loads and whether it has a minimum order, short-load charge, delivery charge, or waiting-time rule. Do not assume a truck size or order increment is universal. A calculator can show volume, but the supplier determines what can be scheduled and delivered.
Bagged premixed concrete is purchased by whole bags. Divide the order volume in cubic feet by the selected product’s published yield and round up. Match the bag weight to the exact product data; do not infer yield from bag weight alone. Portland cement is not the same product as premixed concrete: a cement bag still needs aggregate and mix planning.
Check what the volume estimate leaves out
A measured volume does not select thickness, reinforcement, base preparation, joints, footing dimensions, or a suitable mix for a structural use. It estimates the concrete shape you specified. Confirm design inputs from project documents and the appropriate local professional. For post holes, subtract any post section that displaces concrete; for a slab, include separate thickened edges only when they do not overlap the slab volume.
Before placing an order, recheck units, dimensions, holes, and section boundaries. The slab calculator handles a simple flat slab, while the yard calculator is useful when you already know a volume. Use the bag calculator when you need to compare a named bag product with the required cubic feet.
Frequently asked questions
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. For example, 33.3333 ft³ is 1.2346 yd³.
Should I add 10% to my concrete order?
Ten percent is a planning default, not a rule. Choose an allowance that fits measurement and placement uncertainty, then apply it once to the net total.
Do I use a circle’s radius or diameter?
The circle formula uses radius. Measure diameter, then divide it by two before calculating the area.
How many 80 lb bags make one cubic yard?
For QUIKRETE Concrete Mix No. 1101 at about 0.60 ft³ per bag, 27 ÷ 0.60 is 45 bags before any allowance. Other products can yield different volumes.
Can I order the exact calculator result from a ready-mix supplier?
Confirm the order quantity and delivery terms with the supplier. Minimums, rounding, truck availability, and site access are supplier-specific.