Concrete Slab Calculator
By Concrete Knowhow Editorial Team · Updated April 2024 · 4 min read
Quick Answer
Enter your slab's length, width, and thickness to instantly calculate cubic feet, cubic yards, and how many 60 lb or 80 lb bags of concrete you need — with an optional waste buffer.
Length
Width
Thickness
Overage
Extra to cover spills, uneven forms, and short pours. Most pros add 5–10%.
Enter dimensions above to see results
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Common Slab Sizes — Quick Reference
| Slab Size | Thickness | Cubic Ft | Cubic Yd | 60 lb Bags | 80 lb Bags |
|---|---|---|---|---|---|
| 4×4 ft | 4" | 5.3 | 0.20 | 12 | 9 |
| 10×10 ft | 4" | 33.3 | 1.23 | 75 | 56 |
| 12×12 ft | 4" | 48.0 | 1.78 | 107 | 80 |
| 20×20 ft | 4" | 133.3 | 4.94 | 297 | 223 |
| 10×10 ft | 6" | 50.0 | 1.85 | 112 | 84 |
| 12×20 ft | 4" | 80.0 | 2.96 | 178 | 134 |
| 20×30 ft | 4" | 200.0 | 7.41 | 445 | 334 |
All bag counts assume no waste buffer. Add 5–10% for real projects.
How to Pour a Concrete Slab — Step by Step
- Plan and mark the area. Use stakes and string to mark the perimeter. Call 811 (USA) to have underground utilities located before digging.
- Excavate and prepare the sub-base. Dig down 4–8 inches depending on your slab thickness and frost depth. Compact the soil and add 4 inches of compacted gravel base for drainage.
- Build your forms. Use 2×4 or 2×6 lumber secured with stakes. Check for level (or deliberate slope for drainage). Oil the inside of forms for easy removal.
- Add reinforcement. For slabs subject to load, lay #3 or #4 rebar on 2-inch chairs, or use wire mesh. Keep reinforcement centered vertically in the slab.
- Calculate and order concrete. Use this calculator, add 5–10% waste, and order bags or schedule ready-mix delivery for your pour date.
- Pour and screed. Pour from one end, working toward the other. Use a screed board to level the surface flush with the top of the forms.
- Float and finish. Use a bull float immediately after screeding, then a hand float and trowel once bleed water evaporates. Add control joints every 8–12 feet.
- Cure. Keep the slab moist for at least 7 days. Curing blankets or a curing compound are good options. Full strength is reached at 28 days.
Frequently Asked Questions
- How many bags of concrete do I need for a 10×10 slab at 4 inches thick?
- A 10×10 ft slab at 4 inches thick is 33.33 cubic feet (1.23 cubic yards). You need 75 bags of 60 lb mix or 56 bags of 80 lb mix. At this volume, ready-mix delivery is worth pricing out.
- How thick should a concrete slab be?
- A standard residential driveway or patio slab is 4 inches thick. Garage floors are typically 4–6 inches. Footings and heavy-load slabs should be 6–8 inches. Always check local building codes.
- How many cubic yards is one 80 lb bag of concrete?
- One 80 lb bag of concrete mix yields approximately 0.60 cubic feet, which equals about 0.022 cubic yards. You need roughly 45 bags to make one cubic yard.
- How much does a yard of concrete cost?
- Ready-mix concrete typically costs $125–$175 per cubic yard delivered, depending on your region, mix design, and quantity. Short loads (under 5 yards) usually carry a surcharge.
- Should I add a waste buffer when ordering concrete?
- Yes. Add at least 5% (ideally 10%) to your calculated volume. Subgrades are rarely perfectly level, forms can have slight inaccuracies, and a little extra is far cheaper than a second delivery.
- When does ready-mix make more sense than bags?
- Ready-mix becomes cost-competitive above roughly 1 cubic yard (about 27 bags). For larger projects, the labor savings of not mixing bags typically outweigh the delivery cost.