How To Calculate Yardage For Concrete

So, How Much Concrete Do You Actually Need?

Here's a scene I've watched play out a dozen times. Someone decides to pour a patio. The fix isn't experience or a good guess. It's math. They eyeball the space, guess at the depth, and then either order way too much concrete (expensive, and now you've got a pile curing in your driveway) or way too little (the truck leaves, you're mid-pour, and suddenly it's a panic). Boring, simple, save-you-a-thousand-dollars math.

Calculating concrete yardage isn't hard once you understand what a "yard" of concrete actually means, and what numbers you need before you ever pick up the phone. Let's walk through it the way I'd explain it to a friend on a Saturday morning, coffee in hand.

What a "Yard" of Concrete Really Means

Every time you order concrete, the supplier talks in cubic yards. And one cubic yard is a cube that's three feet on every side — so 3 ft × 3 ft × 3 ft, or 27 cubic feet. That's a lot of concrete. It's also the only unit your ready-mix company cares about, so getting comfortable with it upfront saves a lot of headaches later.

If you're pouring something small — a few post holes, a short walkway — you might buy bagged concrete instead. Bags are sold by cubic feet (typically 0.6 ft³ per 80-lb bag) or by yield after mixing. For anything bigger than, say, a small shed floor, ready-mix delivered by the yard is almost always cheaper and faster.

The Formula That Does All the Work

Here's the whole game in one line:

Volume in cubic yards = (Length × Width × Thickness) ÷ 27

Everything else is just measuring carefully. Here's the thing — the trick is that length and width usually come to you in feet, and thickness often comes to you in inches. In real terms, the "27" comes from those 27 cubic feet in a yard. Convert thickness to feet first (divide inches by 12) before multiplying.

Quick example. Say you're pouring a driveway apron that's 20 feet long, 10 feet wide, and 6 inches thick:

• 20 × 10 × 0.5 = 100 cubic feet
• 100 ÷ 27 = about 3.7 cubic yards

Most suppliers won't deliver less than one yard, and many charge a short-load fee under 3 or 4 yards, so that math matters even on small projects.

For Odd Shapes, Break It Into Rectangles

This is the part most people overcomplicate. Got an L-shaped slab? And approximate the curve with a series of straight sections. A patio with a curved edge? Just split it into two rectangles, calculate each one, and add them together. Don't try to find some fancy equation. Concrete doesn't care about elegance — it cares about volume.

For Cylinders (Sonotubes, Footings, Post Holes)

Round holes are the one place a calculator is genuinely useful. The formula is:

Volume = π × radius² × height

Measure the diameter, halve it for the radius, and use the full depth of the hole. Which means then convert to yards the same way — divide cubic feet by 27. Sonotube footings for a deck can add up fast once you multiply across six or eight holes, so don't skip this step.

Why People Get This Wrong (and Pay for It)

A few classic mistakes show up over and over.

Forgetting to convert inches to feet. This is the big one. Someone multiplies 20 × 10 × 6, gets 1,200, divides by 27, and orders 44 yards of concrete for a driveway. Real talk, this happens more than you'd think. Always write the thickness as a decimal of a foot: 4 inches = 0.33, 5 inches = 0.42, 6 inches = 0.5.

Underestimating thickness. "Four inches should be fine" is a phrase that has wrecked a lot of patios. Driveways usually want 5–6 inches. Garage slabs often 6. Footings depend on code. If you're not sure, check your local building requirements before you pour — it's a lot cheaper to add an inch in the math than to tear out a slab later.

Not accounting for waste. Spillage, over-excavation, uneven subgrade — you'll use more than the perfect-math number. The standard advice in the trade is to add 5–10% on top of your calculation. For a complex pour with rebar, forms, and weird access, lean toward 10%.

Forgetting the sub-base. If you're pouring 4 inches of slab over 4 inches of compacted gravel, the concrete volume doesn't change, but the excavation does. People sometimes confuse the two and end up with a hole that's too shallow or too deep.

Practical Tips That Actually Help on the Job

Measure Twice in the Same Units

Sounds obvious, but write everything down in feet before you start punching numbers. A 12-foot dimension in inches is the kind of thing that flips your decimal and triples your order.

Round Up, Not Down

Suppliers will typically deliver a little extra in the truck, and returning concrete is a hassle (often impossible). Always round up to the nearest quarter or half yard when placing your order.

Call the Supplier Before You Order

Most ready-mix companies have someone on staff who will run your numbers with you. Do it. They've caught miscalculations that would have cost thousands. Some will even send a rep to the site for larger jobs.

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Account for Slump and Air Content

This is more of a heads-up than a math issue. Which means concrete ordered at a higher slump (wetter) or with air-entrainment (for freeze-thaw climates) behaves a little differently in the forms, but the volume you order stays the same. Just don't add extra "to be safe" based on consistency — add it based on waste.

For Bagged Concrete, Count Bags Last

Work backwards from the cubic yards (or feet) you actually need, then divide by the yield per bag. The bag's label tells you the yield, usually around 0.6 ft³ for an 80-pound bag. A cubic yard works out to roughly 45 80-lb bags, which is also why nobody bags their own driveway slab if they can help it.

A Quick Cheat Sheet for Common Slabs

These are rough starting points only — always run the actual numbers for your project.

• Patio (10×10 ft, 4 in thick): about 1.25 cubic yards
• Two-car driveway (20×24 ft, 5 in thick): about 7.5 cubic yards
• Garage slab (24×24 ft, 6 in thick): about 10.5 cubic yards
• Standard footing (16 in × 8 in, 40 ft long): about 1.5 cubic yards

If your project is close to one of these, it's a useful gut check. If your numbers come out way off, you probably made a unit-conversion error somewhere.

FAQ

How thick should a concrete slab be?

It depends on what goes on top of it. That said, a backyard patio or shed floor is usually 4 inches. A driveway should be 5–6 inches, especially if heavy vehicles will park on it. Plus, garage floors and any slab housing heavy equipment lean toward 6 inches. Always check local code for footings and structural slabs.

How many 80-lb bags make a yard of concrete?

Around 45 bags, give or take depending on the mix and yield. That's roughly 3,600 pounds of bagged concrete, which is one of the reasons ready-mix wins on any sizeable pour — both in cost and in your back.

Can I order concrete in half-yards?

Most ready-mix suppliers deliver in quarter-yard increments. They'll often have a short-load fee for orders under 3–5 yards, so it's worth filling the truck if you can. For very small jobs, bagged concrete is usually the better call.

Do I really need to add 10% extra?

For a simple rectangular slab with clean forms, 5% is usually plenty. For anything with curves, odd shapes, rebar congestion, or a sloped subgrade, 10% is the safer number. The cost of one extra yard is almost always less than the cost of a "short load" panic mid-pour.

What if my numbers come out to a weird decimal?

Round up to the nearest half or quarter yard. Concrete is ordered in those increments

, and suppliers don’t want to argue over a tenth of a yard. If you're a stickler, remember that a cubic yard is 27 cubic feet, so anything less than about 0.37 ft³ per inch of slab thickness is basically negligible.

A Note on Slump and Strength

The "slump" you order refers to how wet the concrete is when it arrives — a higher slump means more workable concrete, but it also means slightly less strength if you overdo it. For most residential slabs, a 4- to 5-inch slump is standard. Strength is measured in psi (pounds per square inch), and 3,000 psi is the typical residential minimum. On the flip side, driveways and garage slabs in freeze-thaw zones often call for 3,500 to 4,000 psi with air-entrainment. Don't pay for 5,000 psi concrete unless your engineer or code requires it — it's not stronger in any meaningful way for a patio, and it costs noticeably more.

Subgrade Prep Affects Volume More Than You Think

A poorly compacted subgrade will settle under the weight of wet concrete, and you'll end up using more than your calculations suggest. Compact the soil in lifts, add a base of crushed stone or gravel where appropriate, and screed your forms to the correct height before the truck arrives. A 2-inch error across a 20×24 driveway is almost a full yard of concrete you didn't plan for.

Wrapping It Up

Estimating concrete accurately is mostly about three things: getting the dimensions right in consistent units, knowing your thickness, and adding a realistic waste factor. Still, the discipline is in the measuring, the form setup, and the willingness to order a little extra so you're not the person trying to hand-mix a half-yard into a finished slab at 6 p. Worth adding: m. The math itself is simple — volume equals length times width times thickness, with a unit conversion to cubic yards at the end. on a Saturday.

Measure twice, order once, and pour with confidence.