Concrete Square Footage Calculator

Here's the thing that trips people up: concrete isn't sold by the square foot. It's sold by volume, in cubic yards from a truck or in bags from the store. So square footage alone can't tell you how much you need. You also need a thickness. Once you have both, the conversion is straightforward, and the calculator below turns your slab dimensions into cubic yards and a bag count.

Why square footage isn't enough

A patio and a driveway can cover the same square footage and need wildly different amounts of concrete, because one is 4 inches thick and the other is 6. Concrete fills a three-dimensional space, so the amount you order depends on area and depth together.

The formula for cubic yards is:

Cubic yards = (area in square feet x thickness in feet) / 27

There are 27 cubic feet in a cubic yard, which is where the 27 comes from. The one step people forget is converting thickness from inches to feet before plugging it in. Divide inches by 12:

4 inches = 0.333 feet
5 inches = 0.417 feet
6 inches = 0.5 feet

Measure your slab's length and width to get square footage, pick the thickness, run the formula, then add a little overage. Do that and you won't come up short mid-pour, which is the one outcome you truly want to avoid with concrete.

Common slab thicknesses

Thickness depends on what the slab has to carry.

4 inches covers most residential flatwork: patios, walkways, sidewalks, shed floors, and standard driveways for cars.
5 to 6 inches is the call for driveways that see heavier vehicles, RV or truck parking, and garage floors. Some codes and pros default to 5 inches minimum on driveways.
6 inches or more shows up under heavy loads, thickened edges, and footings.

When you're not sure, going up an inch is cheap insurance against cracking, and it's a lot cheaper to add depth before the pour than to tear out a failed slab later.

Slab size to cubic yards reference

Here's the volume for a range of common slab sizes at 4 and 6 inches thick, with no overage added yet. Add 5 to 10 percent to any of these before you order.

Slab size	Area	Cubic yards at 4 in	Cubic yards at 6 in
8 x 8	64 sq ft	0.79	1.19
10 x 10	100 sq ft	1.23	1.85
10 x 20	200 sq ft	2.47	3.70
12 x 12	144 sq ft	1.78	2.67
20 x 20	400 sq ft	4.94	7.41
24 x 24	576 sq ft	7.11	10.67

Notice how a 6 inch slab needs half again as much concrete as a 4 inch one over the same footprint. That's the whole reason square footage alone can't answer the question. The jump from a 10 by 10 patio to a 20 by 20 driveway isn't double, it's four times the area and therefore four times the concrete at the same thickness.

Bags versus ready-mix

Once you know the volume, you choose how to buy it.

Bags make sense for small jobs. Bag yield depends on the size, and the standard figures from Quikrete's concrete mix data sheet are the ones to plan around:

Bag size	Yield per bag	Bags per cubic yard	Bags per cubic foot
40 lb	0.30 cu ft	about 90	about 3.3
60 lb	0.45 cu ft	about 60	about 2.2
80 lb	0.60 cu ft	about 45	about 1.7

Since a cubic yard is 27 cubic feet, you get those per-yard counts by dividing 27 by the yield: 27 / 0.60 is about 45 bags of 80 pound mix, 27 / 0.45 is about 60 bags of 60 pound, and 27 / 0.30 is about 90 bags of 40 pound. The bigger bags mean fewer to open and dump, which matters once you're mixing more than a handful. Bags are fine for setting posts, small pads, and repairs.

Ready-mix (a truck) makes sense once the volume climbs. Mixing dozens of bags by hand is brutal work, and hand-mixed batches can vary in consistency across a pour, which shows up as cracking and cold joints. Most people switch to a truck somewhere around 1 to 2 cubic yards. Below that, bags are manageable. Above it, a truck is faster, more consistent, and often cheaper per yard once you count your time and the bags you'd otherwise haul.

There's a lot of hard-won advice on this in DIY forums. In a thread on TractorByNet about pouring your own garage floor, the repeated warning is that concrete waits for no one, so once a decent-sized slab is in play, ordering a truck beats trying to keep up with a mixer and a mountain of bags. A similar AnandTech thread titled "Do I dare a DIY concrete job?" lands in the same place: small pours are DIY-friendly, but a full driveway is a lot of yardage to move before it sets.

Footings and thickened edges

The flat slab isn't the whole order on a lot of jobs. Many slabs need a thickened edge or a separate footing, and that concrete has to go into your total.

A thickened edge is where the slab perimeter drops deeper than the field, often to 8 or 12 inches, to carry edge loads and resist frost heave. A footing is a deeper, wider trench of concrete under a wall or post. Both are calculated the same way, length times width times depth, then divided by 27 for cubic yards.

Say a 20 by 20 slab has a thickened edge 12 inches deep and 12 inches wide around all four sides. That perimeter is about 80 feet, and the extra depth below the 4 inch field is roughly 8 inches (0.67 feet), so the added concrete is about 80 x 1 x 0.67 = 53 cubic feet, or 2 more cubic yards. Forget it and you're well short. Whenever a slab has a deepened perimeter or sits on footings, figure those pieces separately and add them before you order.

Add overage

Always order 5 to 10 percent more than the raw calculation. Slabs are never perfectly flat underneath, forms bulge slightly, and the subgrade eats some concrete. Running out partway through a pour is a real problem, because the concrete already placed starts curing while you scramble. A cold joint between old and fresh concrete is a weak point. The extra half yard is cheap next to that risk.

Worked example: a 20 by 20 driveway at 4 inches

You're pouring a 20 foot by 20 foot parking pad, 4 inches thick.

Area: 20 x 20 = 400 square feet
Thickness in feet: 4 / 12 = 0.333 feet
Volume in cubic feet: 400 x 0.333 = 133.3 cubic feet
Cubic yards: 133.3 / 27 = 4.94 cubic yards
Add 10 percent: about 5.4, so order 5.5 cubic yards

That's a clear truck job. To see why, look at the bag count: 133.3 cubic feet divided by 0.6 is about 222 bags of 80 pound mix. Mixing 222 bags by hand is not a weekend, it's a small nightmare. Anything approaching this volume goes to ready-mix.

Flip it around for a small job. A 4 by 8 shed pad at 4 inches is 32 square feet: 32 x 0.333 = 10.7 cubic feet, or about 18 bags of 80 pound mix. That's a reasonable afternoon with a mixer or a wheelbarrow.

Worked example: a 12 by 12 patio at 4 inches

A patio is the classic bag-or-truck judgment call, so here's the full math on a 12 by 12 pad at 4 inches.

Area: 12 x 12 = 144 square feet
Thickness in feet: 4 / 12 = 0.333 feet
Volume in cubic feet: 144 x 0.333 = 48 cubic feet
Cubic yards: 48 / 27 = 1.78 cubic yards
Add 10 percent: about 1.96, so order about 2 cubic yards

Bag counts for that raw 48 cubic feet, before overage: 80 bags of 80 pound, about 107 of 60 pound, or 160 of 40 pound. Eighty bags of 80 pound mix is brutal by hand, so a patio this size sits right on the line where most people call a short-load ready-mix truck. A 10 by 10 patio (about 1.23 yards, roughly 56 bags of 80 pound) is on the same edge. Below about a cubic yard, bags win; above it, a truck almost always does.

Common mistakes

Leaving thickness in inches. The formula needs feet. Plugging in 4 instead of 0.333 overstates your order by a factor of 12.

Forgetting the 27. Cubic feet is not cubic yards. Divide by 27 to get the unit the truck company quotes.

Ordering the exact math number. Skip the overage and you risk a cold joint. Add 5 to 10 percent every time.

Mixing bag sizes in your head. An 80 pound bag is 0.6 cubic feet and a 60 pound bag is 0.45. Pick one and count consistently.

Under-thickening a driveway. Four inches is fine for a patio but marginal for a driveway that parks a heavy truck. Match thickness to the load.

To convert your square footage straight into volume, the square feet to cubic yards converter handles the depth math in one step. For measuring the slab area itself, especially odd shapes, see the how to calculate square footage guide. Planning the yard around a new slab? The lawn calculator helps size the rest, or start from the homepage.

Frequently asked questions

How much concrete do I need for a slab?

Multiply the slab's square footage by its thickness in feet, then divide by 27 to get cubic yards. A 20 by 20 slab at 4 inches is (400 x 0.333) / 27 = about 4.9 cubic yards. Add 5 to 10 percent for overage before you place the order.

How do I convert square footage to cubic yards for concrete?

Use cubic yards = (area in square feet x thickness in feet) / 27. Convert your thickness from inches to feet first by dividing by 12, so 4 inches becomes 0.333 feet. The 27 accounts for 27 cubic feet in a cubic yard.

How many bags of concrete are in a cubic yard?

About 45 bags of 80 pound mix, since each yields roughly 0.6 cubic feet and a yard is 27 cubic feet. A 60 pound bag yields about 0.45 cubic feet, so it takes around 60 of those per yard. Once you're mixing more than 40 or so bags, a ready-mix truck is usually the better call.

How thick should a concrete slab be?

Four inches works for patios, walkways, and standard car driveways. Bump up to 5 or 6 inches for driveways with heavier vehicles, RV parking, or garage floors. When in doubt, adding an inch of thickness is cheap insurance against cracking.

Should I order bags or a ready-mix truck?

Bags are fine for small jobs like posts, repairs, and pads under a couple of cubic yards. Beyond that, order a truck. Ready-mix is more consistent and far less labor than mixing dozens of bags, and it avoids the cold joints that come from a slow, piecemeal pour.

How many bags of concrete for a 10x10 slab?

A 10 by 10 slab at 4 inches is 100 square feet and about 33 cubic feet of concrete. That's roughly 56 bags of 80 pound mix, 74 bags of 60 pound, or 111 bags of 40 pound. At those numbers many people order a short-load ready-mix truck instead, since it's about 1.4 cubic yards once you add overage.

How many 80 lb bags make a cubic yard?

About 45, because each 80 pound bag yields roughly 0.60 cubic feet and a cubic yard holds 27 cubic feet. For 60 pound bags it's about 60 per yard, and for 40 pound bags about 90 per yard. Multiply your cubic yards by these to get a bag count fast.

Do I need to add concrete for footings or a thickened edge?

Yes. A footing or a thickened slab edge is extra volume that the flat-slab math misses entirely. Calculate each one as length times width times depth, convert to cubic yards by dividing cubic feet by 27, and add it to the slab total before ordering. On a 20 by 20 slab, a 12 inch thickened edge can add roughly 2 cubic yards on its own.

Concrete Square Footage Calculator