Estimates for planning only. Always confirm measurements and order a little extra for waste and cuts.
How the concrete calculator works
Concrete is ordered by the cubic yard (27 cubic feet). This calculator converts your slab, footing, or pad dimensions into yards and the number of 60 lb or 80 lb bags, so you order the right amount the first time.
The formula
Volume is length × width × thickness, with thickness converted from inches to feet, then divided by 27 to get cubic yards:
Cubic feet = length × width × (thickness ÷ 12)
Cubic yards = cubic feet ÷ 27
An 80 lb bag of pre-mix yields about 0.6 cubic feet; a 60 lb bag yields about 0.45 cubic feet.
Bags vs. ready-mix
For small jobs (a few posts, a small pad), bagged concrete is convenient. But the bag count climbs fast: a single 10×10 four-inch slab needs roughly 56 eighty-pound bags. Once you're past about 1 cubic yard, ordering ready-mix from a truck is usually cheaper and far less work than mixing dozens of bags by hand.
Pro tips
- Add about 10% for spillage, uneven subgrade, and over-excavation.
- Order ready-mix once you pass ~1 cubic yard, bagging that much by hand is brutal.
- Standard slabs are 4 inches thick; driveways and heavy loads often call for 5 to 6 inches.
- A typical truck holds about 10 cubic yards, plan deliveries accordingly.
Frequently asked questions
How many 80 lb bags make a cubic yard?
About 45 eighty-pound bags equal one cubic yard, since each yields roughly 0.6 cubic feet and a yard is 27 cubic feet. Sixty-pound bags take about 60 per yard.
How much concrete do I need for a 10x10 slab?
A 10×10 slab at 4 inches thick is about 1.23 cubic yards (add waste for ~1.35). At 6 inches thick it's about 1.85 cubic yards.
How thick should a concrete slab be?
Four inches is standard for patios and walkways. Driveways, garage floors, and anything carrying vehicles or heavy loads are usually 5 to 6 inches with reinforcement.
Should I add extra concrete?
Yes, about 10%. Subgrade is never perfectly level, and running short mid-pour means a cold joint or an emergency bag run. A little extra is cheap insurance.
Is this an exact figure?
It's an accurate volume estimate. The real order can shift with subgrade depth, thickened edges, and waste, so confirm with your supplier.
