How many cubic yards of concrete do I need for a slab?

Multiply length in feet by width in feet by thickness in feet (inches divided by 12), then divide by 27. For a 20 by 24 foot slab at 4 inches, that is 160 cubic feet divided by 27, or 5.93 cubic yards before waste. Add 5 to 10 percent and order about 6.5 yards.

How many bags of concrete are in a cubic yard?

It takes about 45 eighty-pound bags or about 60 sixty-pound bags to make one cubic yard, based on yields of roughly 0.60 and 0.45 cubic feet per bag. Bagged concrete usually only makes sense below about one cubic yard; above that, ready-mix is faster and more consistent.

How much waste should I add when ordering concrete?

Add 5 to 10 percent for a slab on grade. Use the low end for tightly formed slabs over a clean, level base and the high end for rough subgrade or hand-dug footings. Running short triggers a second truck, a cold joint, and short-load fees, so rounding up is cheap insurance.

How do I calculate concrete for footings and round piers?

Calculate footings and turn-downs as separate rectangular volumes and add them to the slab. For round piers, use pi times radius squared times height in feet. A 12-inch-diameter tube poured 4 feet deep is about 0.12 cubic yards, so multiply by your pier count and add it to the total.

How to Calculate Concrete for a Slab: Yards and Bags

Ordering concrete is one of the few estimating mistakes you cannot walk back once the truck is pouring. Here is the exact method for converting slab dimensions into cubic yards and bags, plus the rules of thumb and waste factors that keep you from coming up short.

The core formula: everything is cubic feet first

Concrete volume is just length times width times thickness, with every dimension in the same unit. The trick that trips people up is thickness, which is almost always in inches while the slab is measured in feet. Convert thickness to feet by dividing inches by 12 before you multiply. A 4-inch slab is 0.333 feet thick, a 5-inch slab is 0.417 feet, and a 6-inch slab is 0.5 feet.

So the working formula is: length (ft) times width (ft) times thickness (ft) equals cubic feet. Then convert cubic feet to cubic yards by dividing by 27, because a cubic yard is a 3-by-3-by-3-foot block, which is 27 cubic feet. That single number, 27, is the conversion you will use on every concrete order for the rest of your career.

Worked example: a 20 by 24 foot slab at 4 inches. That is 20 times 24 times 0.333, which equals 160 cubic feet. Divide by 27 and you get 5.93 cubic yards of neat concrete before any waste allowance.

Coverage rules of thumb for fast field checks

When you do not have a calculator handy, memorize the square-feet-per-cubic-yard figures. One cubic yard covers about 81 square feet at 4 inches thick, about 65 square feet at 5 inches, and about 54 square feet at 6 inches. To sanity-check the example above, divide 480 square feet by 81 and you land right back at roughly 5.9 yards.

These coverage numbers are the fastest way to catch a decimal-point error before you call the dispatcher. If your detailed takeoff says a 480 square foot patio needs 12 yards, the rule of thumb tells you instantly that something is wrong, because 480 divided by 81 is nowhere near 12.

For thickened edges, footings, and turn-downs, calculate those separately and add them in. A monolithic slab with a 12 by 12 inch thickened perimeter can easily carry one to two extra yards that a flat-area calculation misses entirely.

Add a waste factor before you order

Never order your exact calculated volume. Subgrade is never perfectly level, forms bow slightly, and you will spill and over-fill. For a standard slab on grade, add 5 to 10 percent. Use the low end on a tightly formed slab over a clean, screeded base, and the high end on rough subgrade, hand-dug footings, or anything with irregular depth.

Applying 10 percent to the 5.93-yard example gives 6.52 yards. In practice you would order 6.5 or even 7 yards, because most ready-mix suppliers sell in quarter-yard increments and running short means a costly second truck, a cold joint, and a short-load fee. Being a quarter yard over is cheap insurance; being a quarter yard under can ruin the pour.

One regional note on cost: ready-mix is typically sold by the yard and pricing varies widely by market, fuel surcharges, and minimum-load fees, so treat any per-yard figure you see as approximate and get a current local quote before bidding.

When to use bags instead of ready-mix

For small pours, fence-post footings, equipment pads, and repairs, bagged concrete is often more practical than meeting a ready-mix minimum. You size bags by volume yield, not by weight. A standard 80-pound bag yields about 0.60 cubic feet, a 60-pound bag yields about 0.45 cubic feet, and a 40-pound bag yields roughly 0.30 cubic feet.

To get bag count, divide your total cubic feet by the bag's yield. The 160-cubic-foot example slab would need about 267 eighty-pound bags or about 356 sixty-pound bags, which is exactly why nobody hand-mixes a 480 square foot slab. As a reference, it takes about 45 eighty-pound bags or about 60 sixty-pound bags to equal a single cubic yard.

The practical crossover is usually around one cubic yard. Below that, bags win on convenience and no truck minimum. Above it, the labor of mixing and the inconsistency between batches make ready-mix the clear choice.

Don't forget columns, footings, and odd shapes

Slabs are rectangles, but pours rarely are. For round footings and column piers, volume is pi times radius squared times height, all in feet. A 12-inch-diameter sonotube poured 4 feet deep holds about 3.14 cubic feet, or roughly 0.12 cubic yards each, so a deck with a dozen of those piers adds about 1.4 yards you must account for separately.

Break any complex pour into simple shapes, calculate each in cubic feet, sum them, then convert the total to yards and apply your waste factor once at the end. Mixing units mid-calculation is the most common source of ordering errors, so keep everything in cubic feet until the final divide-by-27.

This is also where measurement accuracy earns its keep. Field-measuring an irregular slab or a stepped footing with a tape invites small errors that compound across a takeoff. A LiDAR scan of the formed area with an app like ProBuildCalc captures real dimensions and feeds the volume math directly, which tightens the order on jobs where the geometry is anything but a clean rectangle.

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FAQ

How many cubic yards of concrete do I need for a slab?: Multiply length in feet by width in feet by thickness in feet (inches divided by 12), then divide by 27. For a 20 by 24 foot slab at 4 inches, that is 160 cubic feet divided by 27, or 5.93 cubic yards before waste. Add 5 to 10 percent and order about 6.5 yards.
How many bags of concrete are in a cubic yard?: It takes about 45 eighty-pound bags or about 60 sixty-pound bags to make one cubic yard, based on yields of roughly 0.60 and 0.45 cubic feet per bag. Bagged concrete usually only makes sense below about one cubic yard; above that, ready-mix is faster and more consistent.
How much waste should I add when ordering concrete?: Add 5 to 10 percent for a slab on grade. Use the low end for tightly formed slabs over a clean, level base and the high end for rough subgrade or hand-dug footings. Running short triggers a second truck, a cold joint, and short-load fees, so rounding up is cheap insurance.
How do I calculate concrete for footings and round piers?: Calculate footings and turn-downs as separate rectangular volumes and add them to the slab. For round piers, use pi times radius squared times height in feet. A 12-inch-diameter tube poured 4 feet deep is about 0.12 cubic yards, so multiply by your pier count and add it to the total.