Concrete
calculator
Slab math that saves a second trip to the store: length × width × depth gives the volume, and the volume gives the truck order or the bag count — with 10% safety included.
Slab math in one line
Volume is footprint times thickness — with thickness converted from inches to feet first. Ready-mix trucks quote in cubic yards (27 cubic feet each); bagged mix yields about 0.60 cu ft per 80 lb bag and 0.45 per 60 lb.
1.63 yd³ ≈ 74 × 80 lb bags — clearly truck territory.
Bags or truck?
The crossover is roughly one cubic yard: below it, bags are practical; above it, mixing 45+ bags by hand is brutal and ready-mix wins on both effort and consistency. Order with the 10% margin — running short mid-pour creates a visible cold joint you can't fix.
Thickness guide
Common practice: 4 inches for patios and walkways, 5–6 for driveways and garage floors carrying vehicles. Footings and slabs with loads have code requirements — check locally before pouring. The square footage calculator preps the footprint for odd shapes.
Concrete FAQ
Length × width × thickness (in feet) gives cubic feet; divide by 27 for cubic yards. A 12×10 ft slab at 4 inches is about 1.5 yd³ — 1.63 with a 10% margin.
About 45 × 80 lb bags or 60 × 60 lb bags per cubic yard — which is why anything near a full yard usually means ordering ready-mix instead.
Typically 4 inches for patios and walkways, 5–6 inches for driveways and anything supporting vehicles. Local building codes govern loaded or structural slabs.
Uneven subgrade, spillage and forms that sit slightly deep all eat concrete. Running out mid-pour leaves a cold joint — a permanent weak line — so the margin is cheap insurance.