Concrete Calculator
Find the cubic yards of concrete for a slab and the number of premix bags, using standard published bag yields.
Learn how it works: How Much Concrete Do I Need?Four inches is the standard thickness for a patio or walkway slab. Concrete is ordered by the cubic yard; order about 10% extra to allow for spillage and uneven subgrade.
Enter the slab size to find the concrete volume.
How to use the concrete calculator
Enter the slab length and width in feet and the thickness in inches. The calculator works out the volume in cubic yards and cubic feet, then translates that into the number of 40, 60, and 80 pound bags of premix for smaller pours.
Thickness defaults to four inches, the standard for patios, walkways, and shed bases. For a driveway that takes vehicle weight, step up to five or six inches and the volume rises with it.
Cubic yards or bags of premix
Concrete is ordered from a ready-mix truck by the cubic yard, which is the practical unit for anything larger than a small pad. The cubic yard figure is the one to quote when you call a supplier for a delivery.
For a small job, bags of premixed concrete are easier than a truck. An 80 pound bag yields about 0.6 cubic feet of mixed concrete, a 60 pound bag about 0.45, and a 40 pound bag about 0.30, which are the yields printed on the bags. The calculator rounds bag counts up to whole bags.
Choosing the right slab thickness
Four inches is enough for foot traffic and light loads like a patio, a garden path, or an air conditioner pad. The extra inch or two for a driveway spreads vehicle weight and resists cracking under repeated load.
A solid, compacted base under the slab matters as much as the thickness. Soft or uneven ground lets a slab flex and crack no matter how much concrete sits on top, so prepare the subgrade before you calculate the pour.
Always order a little extra
Forms are never perfectly even, the subgrade dips here and there, and some concrete is always lost to spillage. Ordering about 10 percent more than the calculated volume keeps a pour from coming up short, which is far costlier than a small surplus.
Running out mid-pour leaves a cold joint where fresh concrete meets a set edge, a weak line that can crack later. A small overage avoids that risk, so treat the calculated figure as the minimum, not the order.
Frequently asked questions
- How much concrete do I need for a 10x10 slab?
- A 10 by 10 foot slab at four inches thick is about 33.3 cubic feet, or 1.23 cubic yards. In 80 pound bags at 0.6 cubic feet each, that is roughly 56 bags before any extra allowance.
- How many bags of concrete are in a cubic yard?
- A cubic yard is 27 cubic feet. At about 0.6 cubic feet per 80 pound bag, that is roughly 45 bags; at 0.45 per 60 pound bag, about 60 bags; at 0.30 per 40 pound bag, about 90 bags.
- How thick should a concrete slab be?
- Four inches is standard for patios, walkways, and light-use pads. Driveways and slabs that carry vehicles are usually five to six inches over a well-compacted base.
- Should I order extra concrete?
- Yes. Order about 10 percent more than the calculated volume to cover uneven subgrade, form flex, and spillage. Coming up short during a pour creates a weak cold joint, so a small surplus is worth it.
- Is it cheaper to use bags or a ready-mix truck?
- Bags suit small jobs where mixing by hand is manageable. Once a project needs more than about a cubic yard, a ready-mix delivery is usually cheaper and far less labor than mixing dozens of bags.