How Much Concrete Do I Need for a Slab?

The honest answer is a two-minute calculation you can do in your head — once you know the one formula and the single number that trips everyone up.

Whether you are pouring a shed floor, a patio or an equipment pad, the amount of concrete comes down to volume. Get the volume right and everything else — bags, ready-mix, cost — falls into place. Here is the method the pros use, plus a worked example you can copy.

The one formula: volume = area × thickness

A slab is just a very wide, very thin box. Its volume is length × width × thickness. The catch is units: length and width are in feet, but thickness is almost always in inches. You have to convert the thickness to feet before you multiply, or your answer will be twelve times too big.

To turn inches into feet, divide by 12. A 4 inch slab is 4 ÷ 12 = 0.333 feet thick. Then:

That divide-by-27 is the second thing people forget. Concrete is ordered in cubic yards, and there are 27 cubic feet in a cubic yard (a yard is 3 ft, and 3 × 3 × 3 = 27). Quote a supplier in cubic feet and you will get a blank look.

Worked example: a 10 × 10 slab, 4 inches thick

Take the most-searched slab of all, a 10 by 10 foot pad at 4 inches:

• Thickness in feet: 4 ÷ 12 = 0.333 ft
• Cubic feet: 10 × 10 × 0.333 = 33.3 cu ft
• Cubic yards: 33.3 ÷ 27 = 1.24 cu yd

So a 10 × 10 slab needs about 1.24 cubic yards of concrete. In bags that is roughly 56 of the 80 lb size, 75 of the 60 lb, or 112 of the 40 lb — before waste. Our concrete calculator runs these numbers instantly and lets you switch bag sizes and shapes.

Footings and columns are round, not square

Deck footings, fence posts and porch columns are cylinders, so the area is a circle, not a rectangle. The volume of one cylinder is π × radius² × height. For a 12 inch diameter footing 4 feet deep: the radius is 6 inches (0.5 ft), so π × 0.5² × 4 = 3.14 cubic feet each. Four of them is 12.6 cubic feet, just under half a yard. Enter the diameter, depth and count in the calculator and it totals them for you.

Always add a waste factor

Your calculated volume assumes a perfect box or cylinder. Reality is messier: the subgrade dips, hand-dug footing holes bell out at the bottom, and concrete sticks to the mixer, the wheelbarrow and your boots. Running short in the middle of a pour is the worst outcome — you get a cold joint where the new batch meets the half-set old one, or you make a panicked second trip.

Add 10 percent as a default. On rough ground or for deep footings, go to 15. It is far cheaper to return a leftover bag than to rescue a failed pour.

Bags or a ready-mix truck?

The break-even is around one cubic yard, which is roughly 45 to 60 bags depending on size. Below that, bagged mix is convenient and you control the pace. Above it, mixing dozens of bags by hand is exhausting and slow — slow enough that the first batches start curing before the last go in, which weakens the slab. A ready-mix truck delivers a uniform load in one continuous pour and usually costs less per yard. Watch for the supplier’s minimum load and short-load fee.

Common mistakes to avoid

• Leaving thickness in inches. Convert to feet first, or divide the final cubic-inch figure by 1,728.
• Forgetting the divide-by-27. Cubic feet is not cubic yards.
• Skipping waste. Ten percent is not optional on a real site.
• Ignoring the base. Most slabs sit on 4 inches of compacted gravel — size that with our gravel calculator.

Ready to run your own numbers? Open the concrete calculator, pick slab or footing, and read cubic yards and bags side by side.

This guide is for general planning only and is not engineering advice. Concrete yield and slab requirements vary with mix, reinforcement and local code. Confirm with your supplier and building department before pouring.