Concrete Slab Calculator: Thickness, Cost & Yards

P

ProCalc.ai Editorial Team

Reviewed by Jerry Croteau, Founder & Editor

I Almost Ordered 3 Extra Yards of Concrete Once

True story. I was pricing out a garage slab — 24 by 24 feet, 4 inches thick — and I did the math in my head while standing at the ready-mix counter. I came up with something like 9.5 cubic yards, which felt right, and then I added a couple extra "just in case" because that's what the old guys always told me to do. Ended up ordering 12 yards. The actual pour took about 7.1 yards. I had a truck sitting there with almost 5 yards of concrete that I had to figure out what to do with, and let me tell you, wet concrete doesn't wait around while you brainstorm.

That's the kind of mistake that costs you real money — we're talking somewhere in the ballpark of 150 to 175 per cubic yard depending on your area and mix — and it's completely avoidable if you just run the numbers properly.

So yeah, that's why I built a calculator for this.

How the Math Actually Works (It's Not Hard)

The formula for figuring out how much concrete you need for a slab is honestly pretty straightforward. The tricky part is just keeping your units consistent, which is where most people mess up. You're working in feet for length and width but inches for thickness, and concrete gets sold in cubic yards. Three different units in one calculation. No wonder people get it wrong.

Let me walk through a real example so this actually sticks.

Say you're pouring a patio slab: 20 feet long, 12 feet wide, 4 inches thick. Here's what you do:

1. Convert thickness to feet: 4 inches ÷ 12 = 0.333 feet
2. Multiply everything together: 20 × 12 × 0.333 = 79.92 cubic feet
3. Divide by 27 to get cubic yards: 79.92 ÷ 27 = 2.96 cubic yards

So you'd need right around 3 cubic yards. Most contractors add about 5-10% for waste, spillage, and uneven subgrade — so you'd probably order 3.2 or 3.3 yards. That's a whole lot different from my "add a couple extra" approach from the garage story.

If you're working with a

, it handles all the unit conversions automatically, which honestly eliminates about 90% of the errors I see people make.

Thickness Matters More Than You Think

This is the part where homeowners especially get tripped up. Going from 4 inches to 6 inches doesn't sound like much — it's just 2 inches, right? But that extra 2 inches adds 50% more concrete to your order. Fifty percent! On a 600 square foot driveway, that's the difference between roughly 7.4 yards and 11.1 yards. At current prices, you're looking at an extra 500 to 650 just in material.

Slab Size (ft)	Thickness	Cubic Yards Needed	Approx. Cost (material only)
10 × 10	4 inches	1.23	185 – 215
20 × 20	4 inches	4.94	740 – 865
24 × 24	4 inches	7.11	1,065 – 1,245
24 × 24	6 inches	10.67	1,600 – 1,870
30 × 30	4 inches	11.11	1,665 – 1,945
30 × 30	6 inches	16.67	2,500 – 2,920

(Those cost estimates assume roughly 150-175 per cubic yard for standard ready-mix. Your local prices will vary — sometimes a lot.)

So when should you go thicker? A basic walkway or patio can usually get away with 4 inches. A driveway that's only seeing passenger cars, also 4 inches, though I personally prefer going to 5 on driveways because the subgrade is never as perfect as you planned. Anything that's going to see heavy trucks, equipment, or serious loads — you want 6 inches minimum, and you probably want to talk to an engineer about reinforcement too.

I've seen guys try to save money by going thin on a garage slab and then wonder why it cracked within the first winter. The

lets you toggle between thicknesses so you can see exactly what each option costs before you commit.

The Stuff Nobody Tells You About Ordering

Ready-mix trucks typically carry 8 to 10 cubic yards. Most plants have a minimum order — usually around 1 yard — and they'll charge you a "short load" fee if you order less than a full truck. I've seen short load fees anywhere from 40 to 100+ per yard under the minimum. So if you only need 3 yards, you might be paying a premium on every single yard.

One trick I've picked up: if your pour is under about 2 cubic yards, it might actually be cheaper to mix bags yourself. A standard 80-pound bag of concrete makes about 0.6 cubic feet, which means you need roughly 45 bags per cubic yard. That's a LOT of mixing (and a lot of bags to carry), but for a small pad or some

, it can save you the delivery fee and short load charge.

For bigger pours, always order a little extra. The 5-10% overage rule exists for good reason — your forms might bow slightly, the ground might not be perfectly level, and you absolutely do not want to come up short at the end of a pour. Running out of concrete mid-pour is one of those nightmares that stays with you. Trust me on this one.

Also worth knowing: if you're doing any

underneath (and you should be, for drainage), factor that into your project budget too. A good 4-inch compacted gravel base is pretty standard for most residential slabs.

Quick Cost Breakdown Beyond Just the Concrete

People always ask me "how much does a concrete slab cost" and I always say "more than just the concrete." The material itself is honestly the straightforward part. Here's what a typical residential slab project actually involves cost-wise:

• Ready-mix concrete: 150-175 per cubic yard (ballpark)
• Gravel base: varies, but use a

to estimate

Rebar or wire mesh: 75-150 per 100 square feet, roughly

Form lumber: depends on what you've got lying around (I always seem to have 2x4s everywhere)

Labor: if you're hiring out, 3 to 8 per square foot for finishing, sometimes more

Delivery/pump truck: pump trucks run about 150-250 per hour if the truck can't back up to your forms

A 20×20 slab at 4 inches thick, all-in with labor and materials, might run you somewhere between 2,800 and 5,500 depending on your market and site conditions. That's a wide range, I know, but concrete work varies wildly by region.

If you're trying to estimate the full scope, the

can help you get a tighter number. And if your project involves any

for irregular shapes, run those first so your volume numbers are accurate from the start.

How many bags of concrete make a cubic yard?

About 45 bags of 80-pound concrete mix, or roughly 60 bags of 60-pound mix. Each 80-pound bag yields about 0.6 cubic feet, and there are 27 cubic feet in a cubic yard. So: 27 ÷ 0.6 = 45 bags. It's a lot of mixing by hand, which is why most people go with ready-mix for anything over a yard or two.

What's the minimum thickness for a concrete slab?

4 inches for most residential applications — patios, walkways, shed pads. Go 5-6 inches for driveways and garages. Anything structural or load-bearing needs engineering specs, not internet advice.

Should I add extra when ordering concrete?

Yes. Always. Order 5-10% more than your calculated amount. Subgrade isn't perfectly flat, forms flex a little, and you'll lose some to spillage and what sticks to the chute. Coming up short mid-pour is exponentially worse than having a little left over — you can always pour a stepping stone or fill a post hole with the excess.

The whole point of running these numbers ahead of time is so you're not standing at the ready-mix plant doing mental math and hoping for the best. I've been that guy. Don't be that guy. Use the

and show up knowing exactly what you need.

Get smarter with numbers

Weekly calculator breakdowns, data stories, and financial insights. No spam.