How to Calculate Concrete Volume for Any Slab
The cubic-yard formula behind every concrete order, worked through with a real patio slab example.
Concrete is sold and delivered by the cubic yard, but every slab is designed in feet and inches — so the entire calculation is really just a unit-conversion problem wrapped around a basic volume formula.
The formula
Cubic Yards = (Length × Width × Thickness in feet) / 27
There are exactly 27 cubic feet in a cubic yard (3ft × 3ft × 3ft), which is where that divisor comes from.
Worked example: a patio slab
A 15ft × 15ft patio, poured 5 inches thick. First convert thickness to feet: 5/12 ≈ 0.4167ft. Volume: 15 × 15 × 0.4167 ≈ 93.75 cubic feet. Dividing by 27: 93.75/27 ≈ 3.47 cubic yards of concrete needed.
Why contractors always order extra
Most professional estimates add 5-10% on top of the calculated volume to account for spillage, uneven sub-grade (ground that isn't perfectly flat absorbs slightly more concrete at the low spots), and minor form leaks. For the 3.47 cubic yard patio above, a 10% buffer brings the practical order up to roughly 3.8 cubic yards — and most ready-mix suppliers sell in whole or half-yard increments anyway, so this would likely round up to an even 4 yards regardless.
Bag concrete vs. ready-mix
For smaller pours, pre-mixed bags are common instead of a truck delivery. An 80lb bag yields approximately 0.6 cubic feet of mixed concrete. That same 93.75 cubic foot patio would need 93.75/0.6 ≈ 156 bags — at that volume, a ready-mix truck delivery is almost always more practical (and often cheaper) than hauling and mixing that many individual bags by hand.
Why thickness matters more than it seems
Because thickness is one of three multiplied dimensions, a seemingly small change has an outsized effect on total volume. Increasing that same patio's thickness from 4 inches to 6 inches — a 50% increase in thickness — increases total concrete volume by exactly 50% as well (from roughly 2.78 to 4.17 cubic yards), which is why over-specifying slab thickness "just to be safe" can meaningfully increase both material cost and delivery weight.
Common mistakes to avoid
- Forgetting to convert thickness from inches to feet before multiplying — this is the single most common error, and it inflates the result by a factor of 12
- Ordering the exact calculated amount with zero waste buffer, then running short mid-pour (a genuinely difficult situation to recover from once concrete starts curing)
- Assuming all "cubic yards" quotes from suppliers already include their own waste margin — always clarify whether a quote is for calculated volume or delivered volume
Calculate your own slab volume with the concrete calculator, and estimate base material with the gravel calculator.