How the concrete volume works
Slab concrete is just a volume — length × width × thickness — with one unit trap: thickness is quoted in inches while length and width are in feet. Divide the thickness by 12 first, then:
ft³ = L × W × (T ÷ 12) and yd³ = ft³ ÷ 27
Ready-mix trucks sell in whole cubic yards, and every pro orders about 10% over the theoretical volume to cover uneven subgrade, spillage and form flex. An order short by half a yard costs a standby fee far larger than the concrete; the surplus spreads into a pad or pier blocks.
Worked example
A 20 × 10 ft patio slab at 4 in: volume = 20 × 10 × ⅓ = 66.7 ft³ = 2.47 yd³. Order 2.72 yd³ — the dispatcher will round to 2.75 or 3. Doing it by hand instead: roughly 112 eighty-pound bags at 0.6 ft³ yield each. That is the honest crossover point: past about a yard, ready-mix delivery beats bag mixing on both cost and backs.
Thickness sanity check
4 in suits patios and walkways, 5–6 in for a residential driveway (often with rebar or mesh), 3,500–4,000 psi mix for freeze-exposed work. If your thickness varies across the slab, calculate each zone separately — subgrade surprises usually mean the real pour needs that 10% overage and then some.