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How to Calculate Concrete for a Slab
The Formula
Length (ft) x Width (ft) x Thickness (ft) / 27 = Cubic Yards
That’s it. Every concrete calculation starts here. The 27 comes from the fact that one cubic yard contains 27 cubic feet (3 x 3 x 3).
Most people trip up on thickness because slabs are measured in inches, not feet. A 4-inch slab is 0.333 feet. A 6-inch slab is 0.5 feet. Divide your inches by 12 to convert.
A Real Example
Say you’re pouring a 20 x 24 foot garage pad, 4 inches thick.
20 x 24 = 480 square feet
480 x 0.333 = 159.84 cubic feet
159.84 / 27 = 5.92 cubic yards
You’d order 6 yards. But you shouldn’t.
The Waste Factor
Concrete doesn’t pour into a perfect rectangle. The ground isn’t perfectly level. Your forms bow a little. The guy running the screed bar doesn’t get it laser-flat on the first pass. Spillage happens.
Add 5-10% for waste. On a small residential slab, go with 10%. On a large commercial pour with experienced finishers, 5% is usually fine.
Back to our garage: 5.92 x 1.10 = 6.51 yards. Order 6.5 or round up to 7.
Running short mid-pour is a disaster. The ready-mix plant might not have another truck available for two hours, and by then your first batch is setting up. A cold joint in the middle of a garage floor is ugly and structurally weak. Always order a little extra. You can build something with the leftover.
Bags vs Ready-Mix
An 80-pound bag of Quikrete makes 0.6 cubic feet of concrete. For our 6.5-yard garage, that’s:
6.5 x 27 = 175.5 cubic feet
175.5 / 0.6 = 292.5 bags
At $6 per bag, that’s $1,755 in material alone. Plus you’d need a mixer, a team willing to hand-mix 293 bags, and about 14 hours of brutal labor.
A ready-mix truck delivering 6.5 yards costs roughly $900-$1,200 depending on your area.
The rule of thumb: If you need more than 1 cubic yard, call a ready-mix truck. Below that, bags make sense. A yard is about 45 bags, which is already a hard day for two people.
Small projects where bags work well:
- Fence post footings (2-4 bags each)
- Setting a mailbox or basketball pole
- A small 3x3 landing pad
- Patching and repair work
What About Thickness?
IRC Section R506.1 requires a minimum 3.5-inch slab for residential construction. Most builders pour 4 inches as standard. Go 6 inches for garage slabs that’ll support heavy vehicles, or anywhere you’re putting serious point loads.
Every extra inch of thickness adds roughly 25% more concrete. Jumping from 4” to 6” on a 20x24 slab takes you from 5.92 yards to 8.89 yards. That’s $300-$400 more in material, but the slab is 50% thicker and substantially stronger.
Reinforcement Changes the Quantity
Concrete volume stays the same whether you add rebar or not. Rebar displaces such a small amount of concrete it’s not worth accounting for (see how to calculate rebar for a slab for the full reinforcement math). What changes your order is if you’re adding thickened edges (for a monolithic slab) or footings around the perimeter.
A thickened edge that drops from 4 inches to 12 inches around the perimeter adds concrete. For a 20x24 slab with 12-inch thickened edges, that’s an extra 0.7-0.9 yards on top of the flat slab calculation.
Irregular Shapes
Not every slab is a rectangle. For L-shapes, split it into two rectangles and add them together. For circles (like a fire pit pad), the formula is:
Pi x Radius(ft)^2 x Thickness(ft) / 27
A 10-foot diameter circle, 4 inches thick: 3.14159 x 25 x 0.333 / 27 = 0.97 yards. Close enough to a yard that you’d order one yard from the truck or grab 45 bags.
Common Mistakes
Forgetting to convert inches to feet is the most common error. A 4-inch slab entered as 4 feet gives you a result 12x too high. If your concrete calculator spits out 70 yards for a patio, you probably left the thickness in inches.
Using inside form dimensions when the forms aren’t straight is another one. Measure in multiple spots. If one end of your 20-foot form is 20’2” and the other is 19’10”, use the longer measurement. You’ll need the extra.
Not accounting for grade changes. If one side of your slab sits 2 inches lower than the other because the ground drops, you’re pouring an average of 5 inches instead of 4. That’s 25% more concrete.
Running the Numbers Faster
SiteCalc has a concrete calculator that handles all of this, including waste factor, thickened edges, bags-vs-truck comparison, and irregular shapes. You punch in the dimensions and it gives you yards, bags, and cost. The show formula feature lets you verify the math, and the reverse budget calculator tells you how much slab your dollar amount buys. Saves the math for when you’ve got six estimates to write before lunch.