How to Calculate Concrete Quantity for Any Project
Calculating concrete quantity requires converting all measurements to the same unit, applying the right volume formula for your shape, and adding a waste allowance. This guide walks through slabs, footings, columns, and steps.
Step 1 – Gather Your Measurements
Before any calculation, measure the length, width, and depth (or height) of the concrete element you are filling. Use a tape measure for all three dimensions and record them in feet. If any measurement is given in inches, convert to feet by dividing by 12. A 6-inch thickness becomes 0.5 feet, and a 4-inch thickness becomes 0.333 feet.
For complex projects with multiple concrete elements — a slab plus footings plus post holes, for example — measure each element separately and calculate each one individually. Record all measurements before starting any calculations to minimize the chance of losing track.
Double-check every measurement before calculating. An error in one dimension affects the entire volume calculation. For critical structural pours, have a second person verify at least the depth measurement, as depth is the most difficult dimension to measure accurately and has a direct impact on structural strength.
Step 2 – Calculate Volume Using the Right Formula
For rectangular slabs, footings, and walls, use the formula: volume equals length times width times thickness (or height). For a slab that is 20 feet long, 15 feet wide, and 4 inches (0.333 feet) thick, the volume is 20 times 15 times 0.333, which equals 100 cubic feet.
For cylindrical shapes like round columns or post holes, use the formula: volume equals pi (3.14159) times the radius squared times the height. The radius is half the diameter. A 10-inch-diameter post hole that is 3 feet deep has a radius of 5 inches (0.417 feet). Volume equals 3.14159 times 0.417 squared times 3, which equals approximately 1.64 cubic feet.
For concrete steps, calculate the volume of the entire staircase as a solid rectangular block (length times width times total height), then calculate the volume of the empty triangular space at the back and subtract it. Alternatively, calculate each individual step as a rectangular prism and add them together.
Step 3 – Convert to Cubic Yards
Ready-mix concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet. To convert cubic feet to cubic yards, divide the cubic foot total by 27. For the slab example: 100 cubic feet divided by 27 equals approximately 3.70 cubic yards.
For projects with multiple elements, convert each element to cubic feet first, add all the cubic feet together, and then divide the total by 27. This prevents rounding errors that accumulate when converting each small element individually.
Fractional cubic yards can be ordered from most ready-mix suppliers in quarter-yard increments. Always round up to the next quarter yard. Ordering 3.70 cubic yards means ordering 3.75 cubic yards minimum. Never round down, as running out of concrete mid-pour creates cold joint problems that require remediation.
Step 4 – Add the Waste Factor
Add 5 to 10 percent to the calculated volume to account for uneven subgrade, form deflection, spillage during the pour, and normal variation in concrete placement. For most residential slabs and footings, a 10 percent waste factor is appropriate. Multiply your cubic yard total by 1.10.
For complex shapes with multiple curves, irregular forms, or rocky subgrade, use a 15 percent waste factor. For simple, well-prepared slabs with level subgrade and stiff forms, 5 percent may be sufficient. When in doubt, use 10 percent as a safe middle ground.
After applying the waste factor, round up to the next quarter cubic yard. For 3.70 cubic yards with a 10 percent waste factor: 3.70 times 1.10 equals 4.07 cubic yards. Order 4.25 cubic yards to give yourself a comfortable margin.
Step 5 – Decide Between Ready-Mix and Bagged Concrete
For projects under 0.5 cubic yards (13.5 cubic feet), bagged concrete is usually the more practical choice. Pre-mixed bags in 60-pound and 80-pound sizes yield approximately 0.45 and 0.60 cubic feet respectively. To find the number of 80-pound bags needed, divide your total cubic feet by 0.60 and round up to the next whole bag.
For projects between 0.5 and 1.0 cubic yards, consider whether ready-mix delivery is available in that quantity. Many suppliers have minimum orders of 1 cubic yard, though some offer smaller loads for a short-load surcharge. Compare the cost of the surcharge against the labor of mixing bags to determine which is more economical.
For projects over 1 cubic yard, ready-mix is almost always preferred. It provides a consistent mix throughout the entire pour, eliminates the labor of hand-mixing bags, and typically costs less per cubic yard than bagged product. Schedule your delivery for a time when you have adequate labor available to spread and finish the concrete before it begins to set.