How to calculate how much concrete you need for any project
Concrete is ordered by the cubic yard, and getting the volume wrong is expensive. Too little means a cold joint and a ruined pour. Too much means paying for material you cannot return. Here is how to calculate exactly what you need.
The basic formula
Concrete volume is length times width times depth. All three dimensions must be in the same unit. For slabs, measure in feet, then divide by 27 to convert cubic feet to cubic yards, since concrete is sold by the cubic yard.
A 10-by-10-foot slab that is 4 inches thick (0.33 feet) is 33 cubic feet, or 1.22 cubic yards. Multiply by 1.1 to account for waste and uneven subgrade, giving you 1.35 cubic yards. Order 1.5 yards to be safe.
Slabs and patios
For rectangular slabs, the formula is straightforward. Measure the length and width in feet, multiply by the thickness in feet, and divide by 27. Four-inch slabs are standard for patios and walkways. Six-inch slabs are used for driveways. Eight to 12 inches is typical for foundations.
Circular slabs use the formula π × r² × depth. A 12-foot-diameter circular patio (6-foot radius) at 4 inches thick is 3.14 × 36 × 0.33 equals 37.3 cubic feet, or 1.38 cubic yards.
Footings
Footings distribute the weight of a structure to the soil. Trench footings are calculated the same as slabs — length times width times depth. Stepped footings need to be calculated as individual rectangular sections and added together.
For column footings, calculate the square or rectangular base first, then add the volume of the column pedestal above. A 2-foot-square footing that is 12 inches deep is 4 square feet times 1 foot equals 4 cubic feet. An 8-inch-square by 6-inch-tall pedestal adds 0.22 cubic feet. Total is 4.22 cubic feet or 0.16 cubic yards per footing.
Columns and piers
Round columns use π × r² × height. A 12-inch-diameter column that is 10 feet tall is 3.14 × 0.5² × 10 equals 7.85 cubic feet. Square columns are length times width times height.
For multiple columns, calculate one and multiply by the total number. Group identical columns together for easier ordering.
Stairs
Stairs are more complex. Each step consists of a tread (horizontal) and a riser (vertical). Calculate the volume of one step: tread width times tread depth times step width, plus riser height times step width times riser depth. Multiply by the number of steps.
For a set of 8 steps each 4 feet wide with a 10-inch tread and 7-inch riser, each step has roughly 0.97 cubic feet. Eight steps total 7.76 cubic feet. Add the landing volume separately.
Waste and overage
Always add 5 to 10 percent for waste, spillage, uneven subgrade, and changes in planned thickness. For large commercial pours, 5 percent is usually sufficient. For small DIY projects, 10 to 15 percent is safer because the relative cost of a shortage is much higher on a small pour.
Ordering in quarter-yard increments is standard. Most ready-mix trucks carry a minimum load of 3 to 5 cubic yards, so small projects may require short-load fees. Use the Concrete Calculator to get precise numbers before calling the supplier.
Pre-mixed bags
For small projects, pre-mixed 80-pound bags yield about 0.6 cubic feet each. A 1-cubic-yard project needs about 45 bags. This is only practical for projects under 1 cubic yard due to the labor involved. For anything larger, order ready-mix.
Accurate concrete calculations save money, time, and frustration. Measure twice, calculate carefully, and always add the waste factor. The Concrete Calculator handles all the math so you can focus on the pour.