Concrete Slab Calculator
Enter your slab dimensions to get cubic yards, the number of 80-lb bags, and an optional cost estimate — with a waste margin so you don’t run short.
Leave at 0 if you only need volume.
How it works
We convert your dimensions to cubic feet, divide by 27 to get cubic yards, then add your waste margin. Bag count assumes an 80-lb bag yields about 0.6 cubic feet.
Formula: (Length × Width × Thickness÷12) ÷ 27 × (1 + waste) = cubic yards
Good for
- Patio and shed slabs
- Footings and pier pads
- Walkways and step landings
- Post and fence footings
Tips
- For anything over ~1 cubic yard, ready-mix trucks are usually cheaper than bags.
- Round up to the nearest half yard or full truck.
- A 6″ slab uses 50% more concrete than a 4″ slab at the same footprint.
References
Estimates use standard industry volume math. Cross-check your numbers against the pros:
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Open calculator →Concrete Slab Calculator FAQ
How much concrete do I need for a 10×10 slab at 4 inches thick?
A 10×10 ft slab at 4″ thick is about 1.23 cubic yards before waste. With a 10% margin, order roughly 1.36 yd³ — round up to 1.5 yd³ or about 60 of the 80-lb bags.
Why add a waste margin?
Concrete is hard to return and slabs are rarely perfectly rectangular. A buffer covers spillage, slight over-excavation, and finishing loss so you don’t run short mid-pour.
How many 80-lb bags equal a cubic yard?
An 80-lb bag yields about 0.6 cubic feet. There are 27 cubic feet in a yard, so you need roughly 45 bags per cubic yard (we round up).
Should I order ready-mix or bags?
For anything over about 1 cubic yard, ready-mix trucks are usually cheaper and far less labor. Bags make sense for small pads, posts, and repairs.