How to use it
- Enter the slab length in feet.
- Enter the slab width in feet.
- Enter the thickness in inches.
- Click Calculate to see cubic yards, cubic feet, and bags.
This calculator turns your slab's length, width, and thickness into the exact concrete volume you need — in cubic yards, cubic feet, and 80-lb bags. That's the number that tells you whether to rent a mixer or call the ready-mix truck, and how many bags to buy if you're doing it yourself. For example, a 10 ft x 12 ft slab at 4 inches thick is 40 cubic feet, which is about 1.48 cubic yards and 67 bags. The result matters because concrete is heavy, expensive, and hard to undo once poured. Under-ordering means a cold joint; over-ordering means wasted money and a messy cleanup. Read it as: cubic yards for bulk orders, cubic feet for volume reference, and bags for the actual number of 80-lb sacks to grab. Use it before you dig, not after.
Pro tips
- Add 5–10% waste allowance for spillage, uneven subgrade, or slight over-excavation.
- Always round up to the nearest whole bag — you can't buy a fraction of a bag.
- When ordering ready-mix, order by the cubic yard and round up to the next quarter yard.
- Double-check that length, width, and thickness are in the right units before calculating.
Real-life examples
- A DIY homeowner calculating how many 80-lb bags to buy for a 4x4 concrete patio slab.
- A contractor estimating cubic yards for a 20x30 garage foundation before calling the ready-mix plant.
- A landscaper pricing a 2-foot-wide, 12-foot-long walkway with a 4-inch-thick pour.
Case study
Jake needed a 10x12 ft shed slab at 4 in thick. He guessed 80 bags, but checked the calculator: 1.48 cubic yards, 67 bags. He ordered 72 with waste, finished in one pour, and had zero leftover concrete.