How Much Concrete Do I Need for a Slab?

The measurement everyone forgets is the thickness

A slab feels like a flat thing, so people describe it in square feet. Concrete is sold by the cubic yard. The number that turns one into the other is thickness, and it's in inches while everything else is in feet.

That mismatch is where nearly every bad concrete order comes from.

The arithmetic

A 10 by 10 foot patio, 4 inches thick.

Convert the thickness to feet:

4 ÷ 12 = 0.333 feet

Multiply all three dimensions:

10 × 10 × 0.333 = 33.3 cubic feet

Convert to cubic yards, which is how ready-mix is sold:

33.3 ÷ 27 = 1.23 cubic yards

A cubic yard is 3 × 3 × 3 feet, so 27. That's a definition, not a rule of thumb.

Then add waste. At 10%, you'd order 1.36 cubic yards — and in practice you'd ring the supplier, because they sell in quarter-yard steps and usually have a minimum load.

Why you order more than the slab holds

This isn't padding for the sake of it. Concrete goes missing in specific, predictable ways:

• The sub-base isn't flat. A dip of half an inch across a 10 by 10 slab is about 4.2 cubic feet of extra concrete — roughly an eighth of a 4-inch pour, from a dip you can barely see.
• Forms bow. Timber flexes under the weight of wet concrete, and a slab that bulges outward holds more than the one you measured.
• Not all of it arrives. Concrete sticks in the truck's drum, in the chute, and in every barrow.

Ten percent is a sensible starting point on a small flat pour. The asymmetry matters: too much concrete is an annoyance, too little is an emergency. Concrete sets on a schedule you don't control, and a second load an hour later leaves a visible cold joint through your patio.

How thick should it be?

• 4 inches — patios, shed bases, footpaths, garden slabs
• 6 inches — driveways, or anything a vehicle sits on

Thickness is driven by load, not area. A big patio that only ever holds furniture doesn't need to be thicker than a small one. A short driveway does.

If it's structural — a foundation, a retaining element, anything holding a building up — the thickness isn't yours to pick from an article. Check what's required locally.

What this calculation doesn't include

It's a rectangular slab of even thickness. Real pours often aren't:

• Thickened edges are common on slabs carrying a load at the perimeter, and they add real volume
• Footings are separate — calculate them as their own rectangles and add them
• Steps and kerbs likewise
• A slope for drainage means one end is thicker than the other; use the average thickness

For anything shaped awkwardly, break it into rectangles, work out each one, and add them together.

Where people get it wrong

Not converting inches to feet. 10 × 10 × 4 gives 400 "cubic feet", which is twelve times the real answer. It's the classic mistake and it turns a one-yard order into a fifteen-yard one.

Dividing by the wrong number. 27 is cubic feet to cubic yards. 12 is inches to feet. Mixing them up quietly gives you an answer that looks plausible and is wildly wrong.

Measuring only the corners. The corners are where the forms are, so they're the flattest part. The middle is where the volume hides.

Ordering the exact figure. See above — this is the error with the expensive failure mode.

Forgetting the minimum load. Many suppliers won't deliver under a set volume, or charge as though you'd hit it. On a small pour that minimum often decides the price entirely, which sometimes makes bagged mix the cheaper route.

Have everything ready before it arrives

Once concrete is on site you're on its clock, not yours. Forms braced, sub-base compacted, reinforcement supported off the ground, tools out, and enough people for the whole pour rather than the first ten minutes.

And check the weather. Pouring into frost or heavy rain damages the surface in ways you can't fix afterwards.

The short version

Length × width × thickness-in-feet gives cubic feet. Divide by 27 for cubic yards, then add around 10% for the ground being imperfect. Convert inches to feet before you multiply, measure the depth in the middle as well as the corners, and always round up — running short mid-pour is the one mistake you can't undo.