How much concrete do I need for a slab?

The basic calculation

Concrete volume is length times width times depth. The one catch: slab thickness is almost always given in inches, while length and width are in feet, so you convert the thickness first.

Divide the thickness in inches by 12 to get feet. A 4-inch slab is 4 / 12 = 0.333 ft; a 6-inch slab is 6 / 12 = 0.5 ft. With all three dimensions in feet, multiply them together for the volume in cubic feet.

Ready-mix is ordered and priced by the cubic yard, not the cubic foot. There are 27 cubic feet in a cubic yard, so divide your cubic-foot total by 27 to get yards.

The formula

Volume (cubic feet) = length (ft) x width (ft) x thickness (ft), where thickness (ft) = thickness in inches / 12.

Cubic yards = cubic feet / 27.

Knowing the two steps lets you sanity-check the calculator's output before you commit to an order.

Worked example: a 10x10 slab at 4 inches

Pouring a 10 ft by 10 ft slab, 4 inches thick:

Convert the thickness: 4 / 12 = 0.333 ft. Volume: 10 x 10 x 0.333 = 33.3 cu ft. Cubic yards: 33.3 / 27 = 1.23. Add 10 percent for waste and plan for about 1.36 cubic yards.

Mixing from bags instead of ordering ready-mix? Work from the cubic-foot figure. An 80-lb bag yields about 0.6 cu ft, so 33.3 / 0.6 is about 56 bags.

• Thickness: 4 / 12 = 0.333 ft
• Volume: 10 x 10 x 0.333 = 33.3 cu ft
• Cubic yards: 33.3 / 27 = 1.23
• With 10 percent waste: about 1.36 cubic yards
• In 80-lb bags: 33.3 / 0.6 = about 56 bags

Bagged concrete vs. ready-mix

For small jobs, bagged concrete you mix yourself is convenient. Yield is roughly 0.6 cu ft per 80-lb bag and about 0.45 cu ft per 60-lb bag. Divide your cubic-foot volume by the bag yield to get a count. The 33.3 cu ft example works out to about 56 of the 80-lb bags or about 74 of the 60-lb bags.

Past a small pad, mixing dozens of bags by hand becomes impractical, and ready-mix by the yard is usually easier and often cheaper. A shortfall hurts most with ready-mix: if a second batch arrives after the first has started to set, you get a cold joint. That is the main reason to round up rather than down.

These are planning and budgeting estimates, not engineering figures. Actual yield shifts with how wet you mix, how much you spill, and how level your subgrade is.

Why order about 10 percent extra

Forms are never perfect. The ground under the slab is rarely dead level, so low spots pull extra concrete. Spillage, material left in the mixer or wheelbarrow, and slightly over-dug edges add up. Ordering roughly 10 percent more than the calculated volume covers it.

On the 10x10 example, that turns 1.23 cubic yards into about 1.36. It feels like a lot until you are 10 minutes from finishing and come up short. Running out mid-pour costs far more than a little leftover, in both dollars and the quality of the finished slab.

Footings and trench pours

A footing is just a slab turned on its side, so the same three-dimension multiplication applies. Say a shed foundation calls for a continuous footing 40 feet long, 20 inches wide, and 10 inches deep. Convert both inch measurements to feet first: 20 / 12 is 1.667 ft and 10 / 12 is 0.833 ft.

The volume is 40 x 1.667 x 0.833 = 55.56 cubic feet, which is 2.06 cubic yards. Entering 40 for length, 1.667 for width, and 10 for thickness in the Concrete Calculator returns the same figure, along with about 93 bags of the 80-lb size. That is far past the point where hand-mixing is sensible.

Trenches dug in soil are less forgiving than lumber forms. Sidewalls crumble and the bottom wanders. A trench that ends up just 2 inches deeper than planned along its whole length adds 40 x 1.667 x 0.167 = about 11 cubic feet, or 0.41 cubic yard. That single overdig is a 20 percent increase on this footing, which is why concrete placed directly against earth often needs a bigger cushion than a formed slab does.

Round holes: piers, tubes, and fence posts

Anything poured into a round hole or cardboard form tube is a cylinder. Its volume is pi x radius squared x depth, with the radius and depth in feet. A 12-inch tube has a 0.5 ft radius, so a pier 4 feet deep takes 3.1416 x 0.25 x 4 = 3.14 cubic feet. Ten of them need 31.4 cubic feet, or 1.16 cubic yards.

Fence and deck posts add a wrinkle, because the post itself takes up room in the hole. Take a 10-inch hole, 30 inches deep, with a 4-inch round post set in it. The hole holds 1.364 cubic feet and the post displaces 0.218, leaving 1.145 cubic feet of concrete per hole. For 12 holes that is 13.74 cubic feet, or 31 bags of 60-lb mix, which matches the Post Hole Concrete Calculator.

Skipping the subtraction would have you buying about 19 percent more concrete than the holes can hold. On a single post that is trivial; across a long fence line it adds up to several extra bags.

Why a 4-inch slab is often 3.5 inches

Lumber is sold by nominal size, not actual size. A 2x4 really measures about 1.5 inches by 3.5 inches, and a 2x6 about 1.5 by 5.5. Stand a 2x4 on its edge to form a slab and you have built a 3.5-inch pour, not a 4-inch one. A 2x6 on edge gives you 5.5 inches.

That half inch matters to the order. A 10 ft by 10 ft slab at 3.5 inches is 29.17 cubic feet, or 1.08 cubic yards, about 49 bags of 80-lb mix. At a true 4 inches it is 1.23 cubic yards, and at 5.5 inches it jumps to 1.70. Plug your actual form height into the formula, not the name printed on the board.

Thickness is also measured from the compacted base surface to the top of the form, so a base that sags in the middle quietly adds volume. If you rake and tamp the base to a consistent depth before the pour, your calculated number lands much closer to what you really use.

A metric example in cubic meters

Outside the United States, slabs are specified in meters and millimeters and concrete is ordered in cubic meters. Take a terrace 4.5 m long and 3 m wide at 100 mm thick. Convert the thickness first: 100 mm is 0.1 m. Volume = 4.5 x 3 x 0.1 = 1.35 cubic meters.

Adding 10 percent for waste gives 1.485, so you would request about 1.5 cubic meters. If you are comparing against a yard-based quote, 1 cubic meter is about 1.308 cubic yards, which puts the unrounded 1.35 at roughly 1.77 cubic yards. Going the other direction, multiply cubic yards by 0.7646.

The classic metric slip is mixing centimeters and millimeters. A thickness of 10 cm and one of 100 mm are the same, but typing 100 into a field expecting centimeters produces a volume ten times too large. Say the unit out loud when you write the number down.

Big pours: trucks, minimums, and the clock

A 24 ft by 24 ft garage slab at 6 inches thick is 288 cubic feet, or 10.67 cubic yards. With a 10 percent allowance you would order about 11.7. Full-size mixer trucks commonly carry somewhere around 9 to 10 cubic yards, though capacity varies by truck and local weight limits, so a job this size can mean two trucks. Ask the supplier how they stagger deliveries so the second load arrives before the first has begun to set.

Small orders have the opposite problem. Many suppliers charge a short-load fee when an order falls below a minimum quantity, often a few cubic yards, so a 1.5-yard pad can cost more per yard than a large pour. Bagged mix often wins up to about a yard, then loses quickly beyond it.

Ready-mix is on a timer. A widely used specification, ASTM C94, calls for discharge within 90 minutes of batching or 300 drum revolutions, whichever comes first, unless the purchaser approves otherwise. Line up helpers, wheelbarrows, and finishing tools before the truck arrives, and do not add water at the site to buy time. Extra water weakens the finished slab.

Quick checks, and what the volume leaves out

A fast sanity test is to remember how much area one cubic yard covers. Dividing 27 by the thickness in feet gives 108 square feet at 3 inches, 81 at 4 inches, and 54 at 6 inches. The 10x10 example has 100 square feet, and 100 / 81 is 1.23 yards, matching the full calculation. If your answer is far from this shortcut, a unit slipped somewhere.

The volume number covers only the concrete. The gravel base is a separate purchase, and reinforcement such as wire mesh or rebar takes up too little space to change the order. Control joints are a separate cost in labor and saw cuts. A common rule of thumb places them at a spacing in feet of two to three times the slab thickness in inches, so a 4-inch slab gets joints roughly every 8 to 12 feet.

After the pour, keep the surface damp for the first several days if you can. Concrete gains strength by chemical reaction rather than by drying, and the slab is generally treated as reaching its design strength at about 28 days.