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.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 slab at 4 inches thick requires about 1.23 cubic yards of concrete. With 80-pound bags yielding 0.6 cubic feet each, you would need approximately 56 bags. For a 6-inch-thick slab, the need increases to about 84 bags, making ready-mix a more practical option.
How do I calculate concrete volume in cubic yards?
Multiply the length by width by depth (all in feet), then divide by 27. For a 10-foot by 10-foot slab that is 4 inches thick (0.33 feet): 10 × 10 × 0.33 = 33 cubic feet, divided by 27 equals 1.22 cubic yards. Always add 5-10% for waste.
How much does an 80lb bag of concrete cover?
An 80-pound bag of premix concrete yields approximately 0.6 cubic feet. At 4 inches thick, one bag covers about 1.8 square feet. For a 4-inch-thick slab, you would need roughly 56 bags for a 10x10 area (100 square feet).
How do I calculate concrete for a round post?
Use the formula π × r² × height, where r is the radius of the post hole in feet. For a 12-inch-diameter hole (0.5-foot radius) that is 2 feet deep: 3.14 × 0.5² × 2 = 1.57 cubic feet, or about 0.058 cubic yards. An 80-pound bag of concrete will fill roughly two-thirds of this hole.
Should I use bagged or ready-mix concrete?
Use bagged concrete for projects under 1 cubic yard, such as small patios, fence posts, or repairs. For anything larger, ready-mix is more cost-effective and practical — a 1-cubic-yard project requires about 45 bags of 80-pound concrete versus a single truck delivery. Ready-mix trucks typically have a 3- to 5-cubic-yard minimum.