Concrete Slab Calculator
By Concrete Knowhow Editorial Team · Updated July 2026 · 7 min read
Quick Answer
Length
Width
Thickness
Overage
Extra to cover spills, uneven forms, and short pours. Most pros add 5–10%.
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 to 10 percent for real projects.
How the Concrete Math Works
Every concrete volume estimate comes down to one formula. Multiply length by width by thickness to get cubic feet, then divide by 27 to convert to cubic yards, because one cubic yard is a 3 ft by 3 ft by 3 ft cube (27 cubic feet). Thickness is usually measured in inches, so divide it by 12 first to convert to feet.
Worked example. Say you are pouring a 12 ft by 15 ft patio at 4 inches thick. Convert the thickness: 4 divided by 12 is 0.333 ft. Multiply: 12 times 15 times 0.333 equals 60 cubic feet. Divide by 27: 2.22 cubic yards. Add a 10 percent waste buffer and you should order about 2.45 cubic yards. Ready-mix trucks sell in quarter-yard increments, so you would order 2.5 yards.
Wet concrete weighs roughly 150 pounds per cubic foot, or about 4,050 pounds per cubic yard. That number matters for two reasons: it tells you why hand-mixing more than a yard is a serious workout, and it tells you a pickup truck cannot haul a yard of ready-mix in tow-behind form without checking its rated capacity.
Concrete Bag Sizes and Yields
Yields below are the standard manufacturer figures printed on bagged concrete mix (for example, Quikrete and Sakrete data sheets). Actual yield varies slightly with how much water you add.
| Bag Size | Yield (cubic feet) | Bags per Cubic Foot | Bags per Cubic Yard |
|---|---|---|---|
| 40 lb | 0.30 | 3.3 | 90 |
| 50 lb | 0.375 | 2.7 | 72 |
| 60 lb | 0.45 | 2.2 | 60 |
| 80 lb | 0.60 | 1.7 | 45 |
Bags or Ready-Mix: a Real Cost Comparison
For the 10 ft by 10 ft slab in the table above (1.23 cubic yards), you would need 56 bags of 80 lb mix. At a typical $6 to $8 per bag, that is $336 to $448 in material, plus mixing labor: a bag takes about 3 to 5 minutes to mix in a rented mixer, so budget 3 to 4 hours of continuous mixing. Ready-mix for the same slab at $125 to $175 per yard is $155 to $215 in concrete, plus a short-load fee that often runs $75 to $150 for orders under 5 yards.
The practical crossover: below about half a cubic yard, bags win on cost and convenience. Between half a yard and 1.5 yards, the numbers are close and the decision is really about labor and access. Above 1.5 yards, ready-mix nearly always wins, and it also removes the biggest quality risk of bagged pours, which is cold joints forming between batches mixed too far apart.
Common Mistakes When Estimating Concrete
- Measuring thickness at one spot. Subgrades are rarely level. If your excavation varies between 4 and 5 inches, calculate at the average depth, not the shallowest point.
- Forgetting thickened edges and footings. Many slabs are poured with a thickened perimeter (8 to 12 inches deep and wide). Calculate that volume separately and add it to the slab volume.
- Skipping the waste buffer. Running short mid-pour is the worst outcome in concrete work: the first batch starts setting while you wait, creating a weak cold joint. Order 5 to 10 percent extra, every time.
- Confusing cubic feet with square feet. Suppliers quote volume, not area. A 100 sq ft slab is meaningless to a dispatcher until you tell them the thickness.
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.