Concrete Calculator — Yards & Bags
Enter your slab dimensions to get the cubic yards of concrete and the number of pre-mix bags you need.
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How it works
cubic yards = (length_ft × width_ft × thickness_in ÷ 12) ÷ 27
Concrete bags by slab size (4 inches thick)
| Slab size | Concrete | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft | 1.23 cu yd | 56 | 75 |
| 10 × 12 ft | 1.48 cu yd | 67 | 89 |
| 12 × 12 ft | 1.78 cu yd | 80 | 107 |
| 12 × 16 ft | 2.37 cu yd | 107 | 143 |
| 20 × 20 ft | 4.94 cu yd | 223 | 297 |
At 4 inches thick; a 6-inch slab needs 1.5× as much. Order about 10% extra for spillage and uneven subgrade.
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag of pre-mix yields about 0.6 cu ft, so it takes roughly 45 80-lb bags to make one cubic yard — or about 60 60-lb bags, or 90 40-lb bags.
How many bags of concrete do I need for a 10×10 slab?
A 10×10 ft slab at 4 inches thick is about 1.23 cubic yards (33 cu ft) — roughly 56 80-lb bags or 75 60-lb bags. At 6 inches thick it is about 1.85 cubic yards (~84 80-lb bags).
Is it cheaper to use bagged or ready-mix concrete?
For small pours under about 1 cubic yard, bags are convenient and cost-effective. Above ~1 cubic yard, ready-mix delivery is usually cheaper and far less labor than mixing dozens of bags. Order about 10% extra either way.