How the concrete slab calculator works
The yard math
Length × width × thickness (in feet) = cubic feet; ÷ 27 = cubic yards. A 20×12 slab at 4" is 80 cu ft — just under 3 yards. Every concrete question on earth reduces to this one multiplication, and every ordering mistake comes from doing it in inches or forgetting the overage.
Thickness is the engineering decision
4 inches carries foot traffic and sheds; 5 inches suits patios; driveways want 5–6 with rebar because cars concentrate load at wheel lines; RV pads want 6–8. Thickness is the cheapest strength you can buy (formwork unchanged, material linear) — under-building here is the classic first-slab regret.
Why the overage is non-negotiable
Subgrades are never perfectly flat: every low spot consumes concrete and the pour does not pause while you level. Pros order 10% over and expect the truck to leave with a little in the drum. Under-ordering produces a cold joint — the weak plane where yesterday's cured concrete meets today's — which no amount of wishful troweling repairs.