Concrete Calculator
Enter the slab dimensions to estimate how much ready-mix concrete or how many bags you'll need, with a waste allowance built in.
Last updated: August 30, 2026
Frequently asked questions
How much concrete do I need for a 10x10 slab?
At 4 inches thick, a 10×10 ft slab needs about 1.23 cubic yards before waste allowance - roughly 74 bags of 80 lb mix.
Why add a waste allowance?
Uneven subgrade, spillage, and over-excavation typically use 5–10% more material than the exact math suggests.
Can I use this for a non-rectangular slab?
Break an irregular shape into rectangular sections, calculate each one separately with this tool, then add the results together - the length × width × thickness math only works cleanly on rectangles, so splitting an L-shaped or curved area into simpler pieces is the standard workaround.
How do I account for slab thickness in the calculation?
Multiply the slab's length by width to get the area, then multiply by the thickness (converted to feet) to get cubic feet, and divide by 27 to get cubic yards. A thicker slab needs proportionally more concrete - doubling the thickness doubles the volume for the same footprint.
Is a concrete measurement calculator different from a concrete calculator?
No - they're the same tool by different names. Both describe estimating how much concrete (cubic yards or bags) a project needs from the slab's length, width, and thickness measurements, which is exactly what this calculator does.
Is this the same as a concrete estimator, or 'figuring concrete' by hand?
Yes - a concrete estimator is another name for this same tool. "Figuring concrete" by hand means doing the length × width × thickness ÷ 27 math yourself to get cubic yards; this calculator does that math for you and adds a waste allowance automatically.