Concrete Calculator

How much concrete do I need?

Concrete is measured by volume, so you'll need the length, width, and thickness of the pour. Multiply them together in feet to get cubic feet, then divide by 27, since one cubic yard is 27 cubic feet and yards are how ready-mix is ordered. The only catch is thickness, which is usually given in inches, so you'll divide it by 12 first. A 10 by 10 foot slab at 4 inches works out to 1.23 cubic yards. It's built to do that chain for any slab, footing, or column.

How many bags of concrete?

Each bag of pre-mixed concrete yields a set volume once water is added: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Divide your total cubic feet by the yield to get the bag count, so you'll see all three sizes above. Bags suit small jobs like a few post holes or a single footing. Once you're past about a cubic yard, ordering ready-mix from a truck is usually cheaper and far less work than mixing dozens of bags by hand.

Slab thickness and waste

Most slabs are 4 inches thick, which suits patios, walkways, and shed floors. A driveway holding cars wants 4 inches as well, going to 5 or 6 if heavy vehicles park on it. Whatever the thickness, you'll want a little extra, because subgrade dips, spillage, and edges all use more than the clean calculation suggests. Five to ten percent is the usual cushion, and the result above shows the figure with 10% added so you don't run short. For the area of an odd-shaped pour, the area calculator helps, and the volume calculator covers other shapes.

How much does concrete cost?

There are two ways to buy it, and the size of the job decides which is cheaper. Bags of pre-mix run about $4 to $6 each, so a cubic yard mixed from 80 lb bags (45 of them) lands near $225, plus your time and a mixer. Ready-mix from a truck runs roughly $120 to $160 a cubic yard, but most companies add a short-load fee, often $50 to $150, when you order less than about 3 or 4 yards. So bags win for tiny jobs, and ready-mix wins once you're past a yard or so, where it's both cheaper per yard and far less labor.

Here's how the bag count and a rough mix-it-yourself cost work out per cubic yard. The result above gives the bag count for your actual pour, and this chart shows the scale.

Prices are rough guides that shift with your area and the mix. For other materials, the gravel calculator sizes a sub-base, and the square footage calculator measures an irregular area first.

Do I need rebar or wire mesh?

Concrete is strong under squeezing but weak when pulled or bent, so most slabs get some reinforcement to stop cracks from spreading. For a typical 4-inch slab like a patio or shed floor, welded wire mesh or a grid of #3 or #4 rebar on about 12 to 18 inch spacing is common. Thicker slabs and driveways that carry vehicles usually step up to #4 rebar. The steel sits in the middle of the slab, lifted on chairs so it isn't resting on the ground.

Reinforcement doesn't change how much concrete you pour, so the volume above stands, but it does add to the budget and the prep. Control joints, cut or tooled into the surface every 8 to 12 feet, also help by giving cracks a planned place to form. For anything structural, like a foundation or a load-bearing footing, check your local building code or have an engineer size the steel, since that's where it really matters.