How to Calculate Concrete (How Much You Need for a Slab)

You have a form built and a delivery to book, yet the one number the concrete supplier asks for, how many cubic yards you want, is the number nobody hands you. Order short and the pour stops halfway while a second truck gets dispatched. Order long and you pay for concrete that hardens in the chute. The math that closes that gap is short. This walks it through from a raw tape measurement to a yardage you can phone in.

Start with length, width and thickness

Every concrete estimate begins with three measurements of the space you plan to fill, namely how long it runs, how wide it sits and how deep the concrete goes. Multiply the three together and you have a volume. A slab 10 feet long by 6 feet wide by 4 inches deep is not 10 x 6 x 4, though, because those three numbers are not in the same unit.

Slab thickness almost always comes in inches while length and width come in feet, so the fix is to turn the inches into a decimal of a foot. Four inches divided by 12 is 0.333 feet, six inches lands at 0.5 feet, a full foot stays 1. Once the thickness reads in feet, multiplying length by width by thickness gives a volume in cubic feet. That single conversion is where most bad estimates start.

Why you divide cubic feet by 27

Concrete gets sold by the cubic yard, not the cubic foot, which is why a volume in cubic feet still has to be converted before it means anything to a supplier. A yard is 3 feet, so a cube one yard on each side measures 3 feet by 3 feet by 3 feet, which works out to 27 cubic feet.

A cubic yard, that is 27 cubic feet of concrete, is the block you are really ordering. Dividing the cubic-foot figure by 27 tells you how many of those blocks the pour needs. If the term itself is fuzzy, the cubic yard glossary entry lays out where the number comes from.

This is the exact step Pennsylvania's Local Technical Assistance Program teaches municipal crews in its concrete estimating sheet, where a sidewalk works out to 145 cubic feet and gets divided by 27 to reach 5.37 cubic yards. The Concrete Calculator runs that division for you, though it helps to know what it is doing.

Calculate concrete for a slab, step by step

Take a 24 by 24 slab: at 4 inches thick it is the standard two-car pad. It also makes a clean worked example. The surface measures 24 feet times 24 feet, which is 576 square feet. The 4-inch thickness becomes 0.333 feet once it is divided by 12. Multiplying 576 square feet by 0.333 feet gives 192 cubic feet of concrete. Dividing 192 by 27 brings the pour to 7.11 cubic yards before any waste margin gets added.

How much concrete a 12 by 12 slab needs

A smaller 12 by 12 pad, common for a shed floor or a backyard patio, runs the same route with smaller numbers. Twelve feet by 12 feet is 144 square feet. At 4 inches, that is 0.333 feet, the volume comes to 144 times 0.333, which is 48 cubic feet. Divide 48 by 27 and the slab needs 1.78 cubic yards, which most people round up toward 2 once waste is folded in.

Mixing that pad from bags rather than a truck changes the arithmetic into a bag count. An 80-pound bag yields about 0.60 cubic feet of wet concrete, so 48 cubic feet divided by 0.60 works out to 80 bags. That is a wheelbarrow marathon, which is exactly why a slab this size sits right at the line where ordering ready-mix starts to win.

Bags versus ready-mix and how many

Bagged concrete earns its place on footings, post holes and small pads, while anything approaching a full cubic yard comes out cheaper and faster from a ready-mix truck. Each bag size carries a rated yield, the volume of wet concrete it produces once water goes in. That yield is what turns a cubic-yard figure into a bag count.

Bag size	Yield per bag	Bags per cubic yard
80 lb	0.60 cu ft	45
60 lb	0.45 cu ft	60
40 lb	0.30 cu ft	90

Forty-five 80-pound bags to fill a single cubic yard is the reason crews stop bagging somewhere around half a yard and call for a truck instead. For the small stuff the bag route still wins. The Concrete Calculator will convert a slab straight into a bag count for whichever size you buy.

What one cubic yard of concrete covers

Coverage flips the question around, asking how far a fixed cubic yard spreads at a given depth rather than how much a given area swallows. One cubic yard poured at 4 inches thick covers 81 square feet, which is the single most useful coverage figure to keep in mind. Halve the depth to 2 inches and that same yard stretches to 162 square feet, while a heavy 6-inch pour drops it to just 54.

Thickness	Area one cubic yard covers
1 in	324 sq ft
2 in	162 sq ft
4 in	81 sq ft
6 in	54 sq ft

How thick the slab should be

Thickness drives the whole estimate, since a jump from 4 to 6 inches adds half again as much concrete to the same footprint. For a walkway or a patio that only carries foot traffic, 4 inches is the normal pour and the figure behind most of the examples above. A driveway or any slab that parks a vehicle usually goes 5 to 6 inches so it does not crack under the load.

Footings and the thickened edge of a slab run deeper still, often 8 to 12 inches. That extra depth around the perimeter is real volume the flat-area math quietly misses, which is one more reason the 10 percent cushion exists. Decide the thickness from what the slab has to carry, then measure, because guessing an inch too thin is how a driveway spiders within a season.

Working in meters and cubic yards

Outside the United States the same formula runs in meters while the volume lands in cubic meters instead of cubic feet. A slab 4 meters by 3 meters at 0.1 meter thick holds 4 times 3 times 0.1, which is 1.2 cubic meters. To read that back in the units a US ready-mix supplier quotes, one cubic meter equals about 1.31 cubic yards, so 1.2 cubic meters is roughly 1.57 cubic yards.

The conversion factor is the whole trick here. It works the same on any shape once the raw volume is in hand. For an odd footing or a column that is not a flat slab, the Volume Calculator handles the geometry, then the cubic-meter total drops into the same 1.31 factor.

Always add a waste margin

None of these figures should be the exact amount you order, because a real pour loses concrete to spillage, an uneven subgrade and forms that bow outward under the weight of wet mix. A trench dug an inch deeper in one corner quietly swallows extra yardage that no tidy calculation predicted.

The standard cushion is 10 percent, which turns the 7.11 cubic yards for the 24 by 24 slab into about 7.8, rounded up to a clean 8. Manufacturers such as Sakrete print the same 10 percent allowance on their coverage guidance, so it is a builder's habit rather than a fudge factor. Build that margin in first, then place the order. The truck leaves with the driveway finished.