How to Calculate Concrete

Every concrete estimate comes down to one move: find the volume in cubic feet, then divide by 27 to get cubic yards. Here's the whole method.

Step 1 — Get the volume in cubic feet

For rectangular pours multiply three dimensions in feet: L × W × T. Thickness usually arrives in inches — divide by 12 first. A 20 × 20 ft slab at 5" is:

20 × 20 × (5 ÷ 12) = 166.7 ft³

Round shapes use πr²h: a 10-ft round pad at 4" is π × 5² × ⅓ ≈ 26.2 ft³.

Step 2 — Convert to cubic yards

Ready-mix sells by the cubic yard (27 ft³): 166.7 ÷ 27 = 6.17 yd³. Bagged mix skips this step — compare your ft³ directly to bag yield (an 80 lb bag mixes to about 0.6 ft³, so 45 bags per yard).

Step 3 — Add waste

Subgrade is never perfectly flat and some mix stays in the truck or wheelbarrow. Add 5–10%. On our slab that turns 6.17 into about 6.8 yd³ ordered. Ready-mix suppliers charge for the truck load you confirm — running out mid-pour costs far more than half a yard extra.

Bags or ready-mix?

Sanity checks

• A 10×10 pad at 4" is always ~1.23 yd³ — memorize it.
• Each inch of thickness on 100 ft² is ~0.31 yd³.
• If your number feels high, check that all dimensions are in feet.

Put it to work: open the concrete calculator and check your own pour — the steps panel shows every conversion.