Concrete Calculator
Calculate slab concrete in cubic yards and all common bag sizes, with buying guidance for bags vs ready-mix.
Need a narrower concrete answer?
Post holes and fence posts
Use the post hole concrete calculator or the fence-post concrete guide when the job is about bag counts per hole, not slab volume.
Rebar and control joints
Use the slab rebar and control-joint tool after the volume is clear and you need crack-control planning.
Gravel under the slab
Use the gravel-under-slab guide and the gravel calculator when base prep, not concrete volume, is the open question.
Concrete Buying Guide
The main decision after you get a volume is whether to mix bags or schedule ready-mix. In practice, small pours can use bags, but once the job gets into the 1+ cubic yard range, labor, mixing speed, and consistency start pushing you toward truck delivery.
Short-load fees can change the math, so borderline pours should be priced both ways before you buy. The calculator's recommendation is there to start that decision, not replace supplier quotes.
Recommended Slab Thickness
| Application | Thickness | Notes |
|---|---|---|
| Sidewalk / Walkway | 4″ | Standard for pedestrian traffic |
| Patio | 4″ | Adequate for furniture and foot traffic |
| Shed Pad / Utility Pad | 4″ | Standard for lightweight structures |
| Driveway | 5–6″ | Must support vehicle weight; check local codes |
| Garage Floor | 4–6″ | 4″ minimum; 6″ for heavy vehicles or equipment |
How Concrete Volume Is Calculated
Volume: Length × Width × (Thickness ÷ 12) gives the volume in cubic feet. Dividing thickness by 12 converts inches to feet so all measurements use the same unit.
Cubic yards: Divide cubic feet by 27. Ready-mix trucks sell by the cubic yard, so this is what you order.
Bag count: Each bag has a yield in cubic feet: 40-lb = 0.30, 50-lb = 0.375, 60-lb = 0.45, 80-lb = 0.60 cu ft. Divide total volume by yield and round up — you can't buy half a bag.
Buying recommendation: Based on the adjusted volume, the calculator suggests whether bags are practical (≤ 0.5 cu yd), worth comparing (0.5-1.0 cu yd), or whether ready-mix delivery is more practical (> 1.0 cu yd). These thresholds come from common industry practice and community feedback.
Gravel base: Most slabs need a compacted gravel base underneath (typically 4 inches). Use our gravel calculator to estimate the base material needed for your slab area.
Worked Example: Small Patio
A homeowner is pouring an 8 × 10 ft patio slab at standard 4-inch thickness.
- 1 Patio dimensions: 8 ft × 10 ft × 4″ thick
- 2 Volume: 8 × 10 × (4/12) = 26.7 cu ft
- 3 Add 10% overage: 26.7 × 1.10 = 29.3 cu ft = 1.1 cu yd
- 4 Bags needed: 98 bags of 40-lb, 78 bags of 50-lb, 66 bags of 60-lb, or 49 bags of 80-lb
- 5 Buying recommendation: Compare bags vs ready-mix at this volume
A homeowner is pouring a 12 × 20 ft driveway apron at 5-inch thickness.
- 1 Driveway dimensions: 12 ft × 20 ft × 5″ thick (driveway preset)
- 2 Volume: 12 × 20 × (5/12) = 100 cu ft
- 3 Add 10% overage: 100 × 1.10 = 110 cu ft = 4.1 cu yd
- 4 Bags needed: 367 bags of 40-lb or 184 bags of 80-lb
- 5 Buying recommendation: Ready-mix truck delivery recommended — mixing 184 bags by hand is impractical
A homeowner is pouring a 10 × 10 ft shed pad at standard 4-inch thickness.
- 1 Shed pad dimensions: 10 ft × 10 ft × 4″ thick (shed pad preset)
- 2 Volume: 10 × 10 × (4/12) = 33.3 cu ft
- 3 Add 10% overage: 33.3 × 1.10 = 36.7 cu ft = 1.4 cu yd
- 4 Bags needed: 123 bags of 40-lb, 98 bags of 50-lb, 82 bags of 60-lb, or 62 bags of 80-lb
- 5 Buying recommendation: Compare bags vs ready-mix — 62 bags of 80-lb is doable but labor-intensive
Frequently Asked Questions
How many bags of concrete per cubic yard? +
Should I buy bags or order a ready-mix truck? +
How thick should my concrete slab be? +
What is a short-load fee for ready-mix concrete? +
How much extra concrete should I order? +
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