Concrete Calculator in Cubic Yards: How Many Yards Do You Need?

Here's the thing about ordering concrete: the truck comes once. If you're short, you're paying for a second delivery and you've got a cold joint in the middle of your slab — a visible line where the first batch started setting before the second arrived. Ask me how I know. (My first patio has a lovely stripe down the middle. It's "character" now.)

The formula (it's just one)

Cubic yards = Length × Width × Thickness ÷ 27

Everything in feet. That 27 is just 3×3×3 — converting cubic feet to cubic yards.

Real example. You're pouring a 10 × 12 patio, 4 inches thick:

1. 4 inches = 0.333 feet (divide by 12, always)
2. 10 × 12 × 0.333 = 40 cubic feet
3. 40 ÷ 27 = 1.48 cubic yards

That's the theoretical number. The real number is bigger — keep reading.

The waste nobody warns you about

Your subgrade is never as perfect as you think. There's always a low spot here, a form that bowed out a quarter inch there. The concrete fills all of it, and suddenly your "exact" calculation is short.

My rules after a few pours:

• Flat slab on good grade: add 5–10%. I use 10% because I'm not lucky.
• Footings or trenches: add 10%. Trench walls crumble, holes end up bigger than you dug.
• Bumpy ground you didn't grade well: 15%. Or just grade it better — cheaper.

So that patio: 1.48 × 1.1 = 1.63 yards. And since ready-mix sells by the full yard, you're ordering 2 yards.

Thickness: what actually goes where

People overthink this. For residential work:

• Patio, walkway: 4". That's it. Don't let anyone upsell you.
• Driveway (normal cars): 4" is fine, 5" if it helps you sleep at night.
• Driveway (heavy truck, RV, boat): 6", and throw rebar in there.
• Shed floor: 4". Garage: 4", or 6" if you're parking something ridiculous.

When in doubt, go thicker on driveways and don't bother on patios.

Bags vs. the truck (the real decision)

This is the question I get most. Here's my honest breakdown:

Under half a cubic yard — bags are okay. An 80 lb bag makes 0.6 cubic feet, so half a yard is about 23 bags. That's doable in a day with a rented mixer (~$50/day). Your back will hate you, but your wallet won't.

Over half a yard — call the plant. I know the short-load fee stings (they charge extra under ~4 yards), but have you ever mixed 40 bags by hand? I have. Once. Never again.

Cheat sheet

| Project | Yards (4" thick) | With 10% waste | |---|---|---| | 10 × 10 patio | 1.23 | 1.36 → order 2 | | 10 × 12 patio | 1.48 | 1.63 → order 2 | | 12 × 20 slab | 2.96 | 3.26 → order 3.5 | | 20 × 24 driveway | 5.93 | 6.52 → order 7 |

One more tip: order in the morning. Plants get backed up by afternoon, and concrete waits for no one — you've got about 90 minutes from batch to final finish before it starts setting up on you.

Want the exact number without the arithmetic? Our free concrete calculator does yards, bags, and even cost estimates.