How the calculator works
Concrete is sold by volume, so the math is length × width × depth, with one catch: your thickness is in inches while everything else is in feet. The formula is:
Cubic yards = (length ft × width ft × (thickness in ÷ 12)) ÷ 27
The calculator adds a 10% margin to the order figure because subgrades are never perfectly level. A low spot of just half an inch across a 12×10 slab swallows an extra 5 cubic feet of mix. Running short mid-pour is the most expensive mistake in concrete work, since a cold joint between loads weakens the slab.
Worked example: a 12 × 10 ft patio at 4″ thick is 12 × 10 × 0.333 = 40 cu ft = 1.48 cubic yards. With 10% waste, order 1.6 yards, or 67 × 80 lb bags if you're mixing by hand (each yields about 0.60 cu ft).
Ready-mix or bags?
- Under ~1 cubic yard: bags are cheaper and simpler. A rented mixer pays for itself past ~25 bags.
- 1–4 yards: compare a short-load ready-mix fee against roughly 45 bags per yard of mixing labor. Most people's backs vote for the truck.
- Over 4 yards: always ready-mix. Hand-mixing this much before the first batch sets is a race you'll lose.
Thickness guide
- Walkways, garden pads: 4″
- Patios, shed bases: 4″ (5″ if heavy equipment will sit on it)
- Driveways: 5″ minimum, 6″ for trucks/RVs
- Garage floors: 6″