How many 80 lb concrete bags do I need?

Divide your volume in cubic feet by the bag yield (80 lb ≈ 0.60 ft³ per bag), add your waste factor, then round up. Our calculator does this live, e.g. a 12×10×4 in slab with 10% waste needs 74 bags.

Should I buy bags or ready-mix?

Small jobs under ~1 yd³ are usually cheaper and easier with bags. Larger slabs and driveways are ordered by the cubic yard from a ready-mix supplier. We show both bag count and order-ready yards.

Why include a waste factor?

Spillage, uneven subgrade, and partial bags mean you rarely use every ounce. A 10% waste default helps you avoid a second trip to the store, you can adjust from 0–15%.

How do I convert inches of thickness to cubic yards?

Multiply length × width × thickness (in feet), divide by 27. Thickness in inches is divided by 12 first. See our Methodology page for the full formula and sources.

Concrete Calculator & Cost Estimator

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Frequently asked questions

How many 80 lb concrete bags do I need?: Divide your volume in cubic feet by the bag yield (80 lb ≈ 0.60 ft³ per bag), add your waste factor, then round up. Our calculator does this live, e.g. a 12×10×4 in slab with 10% waste needs 74 bags.
Should I buy bags or ready-mix?: Small jobs under ~1 yd³ are usually cheaper and easier with bags. Larger slabs and driveways are ordered by the cubic yard from a ready-mix supplier. We show both bag count and order-ready yards.
Why include a waste factor?: Spillage, uneven subgrade, and partial bags mean you rarely use every ounce. A 10% waste default helps you avoid a second trip to the store, you can adjust from 0–15%.
How do I convert inches of thickness to cubic yards?: Multiply length × width × thickness (in feet), divide by 27. Thickness in inches is divided by 12 first. See our Methodology page for the full formula and sources.

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Frequently asked questions

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Bill this job

Frequently asked questions

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