Concrete Calculator
Estimate concrete for slabs and patios by cubic feet, cubic yards, bags, waste allowance, and ready-mix or bagged concrete cost.
Concrete slab calculator
Phase 1 calculates rectangular slabs. Footer, post hole, and circular pad calculators are planned, but not shown here until fully built.
Slab dimensions
Results
| Cubic feet | 0 |
| Cubic yards | 0 |
| Cubic yards with waste | 0 |
| Concrete bags | 0 |
| Ready-mix estimate | $0 |
| Bagged estimate | $0 |
Common concrete thickness assumptions
| Project | Common concrete thickness |
|---|---|
| Sidewalk | 4 inches |
| Patio | 4 inches |
| Shed slab | 4 inches |
| Driveway | 5 to 6 inches |
| Garage slab | 4 to 6 inches |
| Heavy equipment pad | 6+ inches |
Waste allowance
Default 10%. Concrete is hard to stretch once the pour starts. For small jobs, odd shapes, uneven ground, or first-time DIY projects, 10% extra is usually safer than ordering the exact calculated amount.
Concrete calculator FAQ
How many 80 lb bags are in a cubic yard?
One cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 cubic feet, so one cubic yard takes about 45 bags before waste.
Is bagged concrete cheaper than ready-mix?
Bagged concrete is convenient for small jobs. Ready-mix is often more practical for larger pours because bag counts get high quickly.
How thick should a concrete patio be?
Many patios are 4 inches thick, but soil conditions, reinforcement, loads, and local code can change the right answer.
How much concrete do I need for fence posts?
Post holes depend on hole diameter, depth, and post size. A dedicated fence/post calculator is planned; for now, use slab mode only for rectangular slabs.
Should I add waste to concrete?
Yes, a waste allowance helps cover uneven ground, measurement error, spillage, and small variations during the pour.
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