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Concrete Calculator
Find out exactly how many bags of concrete mix you need for a slab, footing, or post hole. Results include a 10% waste allowance, like the pros use.
Enter your measurements
Bag yields (US standard): 80 lb = 0.6 cu ft · 60 lb = 0.45 cu ft · 40 lb = 0.3 cu ft.
How to calculate concrete by hand
- Multiply length × width × (thickness ÷ 12) to get cubic feet.
- Divide by 27 to convert to cubic yards.
- Divide cubic feet by the bag yield (0.6 for an 80 lb bag) and round up.
Example: a 10×10 ft slab, 4 in thick → 10 × 10 × (4/12) = 33.3 cu ft → 1.24 cubic yards → about 56 eighty-pound bags with waste.
Quick reference
- 10×10 slab, 4″ thick: ~1.24 yd³ → 62 × 80 lb bags
- 12×12 patio, 4″ thick: ~1.78 yd³ → 88 × 80 lb bags
- Post hole (12″ dia, 3 ft deep): ~2 × 80 lb bags per hole
Concrete FAQs
How many 80 lb bags of concrete make a yard?
About 45. Each 80 lb bag yields 0.6 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.6 = 45).
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways and anything carrying vehicles should be 5–6 inches with reinforcement.
Should I order ready-mix instead of bags?
For anything over ~2 cubic yards (about 90 eighty-pound bags), ready-mix delivered by truck is usually cheaper and far less work.
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Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
One cubic yard of concrete needs about 45 bags of 80 lb concrete mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix. This calculator does the exact math for your slab size.
How thick should a concrete slab be?
A 4-inch slab works for patios, walkways and shed floors. Driveways and garage floors that carry vehicles should be 5 to 6 inches thick.
Should I order extra concrete for waste?
Yes. Add 5 to 10 percent extra to cover spillage, uneven ground and form variations. The calculator above includes a waste factor you can adjust.
How long does concrete take to cure?
Concrete is walkable after 24 to 48 hours, but it reaches full strength in about 28 days. Avoid heavy loads for at least a week.