How Much Concrete Do I Need?
Concrete is sold by the cubic yard, and running short mid-pour is a headache no one wants. Here is how to estimate the volume for a slab, driveway, or patio.
Concrete is measured by cubic yard
One cubic yard equals 27 cubic feet. Ready-mix trucks price and deliver by the cubic yard, so translating your slab dimensions into yards is the first step.
The formula
Cubic yards equals length times width times thickness, divided by 27. Length and width are in feet, and thickness has to be converted from inches to feet by dividing by 12.
Worked example
A 12 by 16 patio poured at 4 inches: 12 times 16 times (4 divided by 12) equals 64 cubic feet. Divide by 27 and you get roughly 2.4 cubic yards.
Order about 10 percent extra
Subgrade is rarely perfectly flat, and forms bulge slightly under the weight of wet concrete. Ordering 5 to 10 percent extra prevents a short pour.
Bags vs ready-mix
An 80 lb bag of premixed concrete yields about 0.6 cubic feet, so a cubic yard is roughly 45 bags. Bags work well for small projects like fence posts or a single step, but a slab or driveway is almost always cheaper and easier with a ready-mix truck.
Frequently asked questions
How many bags of concrete are in a cubic yard?
About 45 eighty-pound bags make one cubic yard, since each 80 lb bag yields roughly 0.6 cubic feet and a yard is 27 cubic feet.
How thick should a slab be?
Four inches is standard for patios and walkways. Driveways and pads for heavy loads are usually 5 to 6 inches, sometimes with rebar or wire mesh.
Should I round up my concrete order?
Yes. Most crews order 5 to 10 percent extra to allow for spillage, uneven subgrade, and slight over-excavation. Running short of concrete mid-pour is a much bigger problem than a small overage.
Try the calculator
Get cubic yards and a bag-count breakdown with our Concrete Volume Calculator.