Material takeoff
How much concrete do I need?
Enter your slab size and thickness. The result rounds up to a real ordering increment, because trucks do not deliver 8.83 yards.
How the math works
cubic yards = square feet × thickness in inches ÷ 324
Then round up. Ready-mix is typically sold in quarter-yard increments with a short-load fee below about a yard. This calculator rounds to whatever increment your supplier uses.
For reference, an 80 pound bag of concrete mix yields about 0.6 cubic feet, so it takes 45 bags to make one cubic yard. Bag mixing is realistic up to about half a yard. Past that, order ready-mix.
Standard slab thicknesses
Four inches covers most residential flatwork: patios, walkways, shed floors, and driveways carrying passenger vehicles.
Five to six inches is for driveways that will see heavy pickups, RVs, or delivery trucks, and for garage floors.
Six inches or more, with engineered reinforcement, is where you should be talking to a professional rather than a calculator.
Everything under the slab matters as much as the slab. A compacted gravel base, proper drainage, and control joints cut at the right spacing determine whether it cracks. Concrete does not crack because it was thin; it cracks because the ground moved or it was not jointed.
Where people get it wrong
- Ordering the exact calculated volume
- Uneven subgrade eats concrete. Ten percent extra is normal and cheap insurance against a cold joint.
- Forgetting thickened edges and footings
- This calculator sizes the slab only. Edge thickenings, footings, and steps are additional volume you must add.
- Skipping the base
- A slab on soft or uncompacted ground will crack regardless of thickness.
- No control joints
- Concrete shrinks as it cures and will crack. Control joints decide where. Cut them at roughly 24 to 30 times the slab thickness in feet.
Background
Concrete is the one material on this site where running short is genuinely serious. A slab poured in two sessions has a cold joint through it, which is a permanent structural and cosmetic defect. Order more than you think you need — the cost of an extra quarter yard is trivial next to the cost of being short.
What you need for the job
Common questions
How many bags of concrete in a cubic yard?
About 45 bags of 80 pound mix, or 60 bags of 60 pound mix. Above roughly half a yard, ready-mix is usually easier and cheaper.
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab at 4 inches thick is 1.23 cubic yards. With 10% waste, order 1.5 cubic yards.
How thick should a concrete patio be?
Four inches is standard for a residential patio on a properly prepared and compacted base.
How much does a yard of concrete cover?
At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches, 54 square feet.
What happens if I order too little concrete?
The pour stops while you wait for more, and the joint between the two pours becomes a permanent weak line called a cold joint. This is why ordering slightly over is standard practice.
Measuring by hand is where estimates go wrong
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