Concrete Calculator
Figure cubic yards and bags for slabs, patios, sidewalks, and pads - then shop mix with a realistic waste buffer built in.
Concrete needed
1.235 cubic yards
33.33 cubic feet before bag rounding
- 80 lb bags
- 56
- With 10% waste
- 62
- Ready-mix order
- 1.36 ydÂł
- Estimated bag cost
- $403.00
Bag yields vary by brand. For pours over ~1 cubic yard, ready-mix delivery is usually cheaper and cleaner than bagged mix.
How the concrete formula works
Concrete volume is length Ă— width Ă— depth. Because most DIY plans list slab thickness in inches, this tool converts depth to feet first:
cubic feet = length (ft) Ă— width (ft) Ă— (depth inches Ă· 12)
Divide cubic feet by 27 to get cubic yards - the unit ready-mix suppliers use. For bagged concrete, divide cubic feet by the bag yield (about 0.45 cu ft for 60 lb bags and 0.60 cu ft for 80 lb bags), then round up.
Example: a 10Ă—10 slab at 4 inches deep is 33.33 cubic feet, or 1.235 cubic yards. That's about 56 bags of 80 lb mix before waste, or roughly 62 bags with a 10% buffer.
- Use 4" depth for most patios and shed pads
- Use 5–6" for driveways and heavy loads
- Order ready-mix once you cross ~1 cubic yard
Yields vary slightly by brand and water content, so treat bag counts as planning estimates and confirm with the bag label before you buy.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × depth (in feet) to get cubic feet, then divide by 27 for cubic yards. This calculator converts depth from inches for you and rounds bag counts up so you don’t run short.
How many 80 lb bags of concrete are in a yard?
An 80 lb bag typically yields about 0.6 cubic feet. One cubic yard is 27 cubic feet, so you need roughly 45 bags of 80 lb mix per cubic yard before any waste factor.
How thick should a concrete slab be?
Most patio and shed slabs are poured 4 inches thick. Driveways and areas with vehicle traffic often need 5–6 inches, and footings follow local code. Always check your project requirements.
Should I order ready-mix or buy bagged concrete?
Bagged mix works well for small pads, fence posts, and repairs. Once you need about a cubic yard or more, ready-mix delivery is usually more economical and consistent.
Why add a waste percentage?
Forms aren’t perfect, ground isn’t level, and spills happen. A 5–10% buffer is common for DIY slabs so you don’t end up short mid-pour.
Related calculators
Keep planning your project with these tools, or browse the Concrete & Masonry category.