How the calculation works
Rectangle: length × width × thickness. Convert inches to feet, then divide cubic feet by 27. Add your selected allowance once.
A worked example
A 10 ft × 10 ft slab at 4 in thick measures 1.2346 yd³. With 10% extra, it needs 1.3580 yd³ or 62 bags at 0.60 ft³ per bag.
Bags of concrete per cubic yard
One cubic yard is 27 cubic feet. With QUIKRETE Concrete Mix #1101, that takes 45 bags at 80 lb, 60 bags at 60 lb or 90 bags at 40 lb, before any allowance. Other products can list different yields, so check your bag.
| Bag size | Yield per bag | Bags per yd³ | Bags per yd³ + 10% |
|---|---|---|---|
| 40 lb | 0.3 ft³ | 90 | 99 |
| 50 lb | 0.375 ft³ | 72 | 80 |
| 60 lb | 0.45 ft³ | 60 | 66 |
| 80 lb | 0.6 ft³ | 45 | 50 |
| 90 lb | 0.675 ft³ | 40 | 44 |
How thick should a concrete slab be?
Use the thickness in your plan or required by your local building department. For orientation, QUIKRETE's driveway guidance says 4 inches is sufficient for passenger cars only and recommends 5 or 6 inches where trucks or other heavy vehicles will drive. Published cost guides describe most residential slabs as 4 to 6 inches thick.
| Use | Thickness often cited | Concrete per 100 ft² |
|---|---|---|
| Walkway, patio or shed slab | 4 in | 1.235 yd³ |
| Driveway, passenger cars only | 4 in | 1.235 yd³ |
| Driveway with trucks or heavier vehicles | 5–6 in | 1.543–1.852 yd³ |
| Garage floor | 4–6 in | 1.235–1.852 yd³ |
Soil, base, reinforcement, climate and local rules can require something different. For quick sizes, see the concrete yardage chart.
Calculating a different shape?
Use the matching tool so the geometry is right: a strip footing, column, round pier or tube form, fence post holes, solid steps or a curb. For the layer underneath the slab, use the gravel base calculator.
How do I calculate cubic yards of concrete?
For a rectangular slab, multiply length in feet by width in feet by thickness in inches, then divide by 324. That is the measured volume. Add your selected allowance and apply supplier order rounding separately.
Do I use feet or inches for thickness?
In feet-and-inches mode, enter length and width in feet and thickness in inches. The tool converts thickness to feet before calculating. Metric mode converts meter and millimeter dimensions to the same internal geometry.
Is the result the exact amount to order?
It is a planning quantity based on your inputs. Uneven ground, actual dimensions, product yield, supplier increments and any minimum billed quantity can change a purchase. Confirm the order with your supplier.
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 ft slab is 33.3 cubic feet, or 1.235 cubic yards. With a 10% allowance, that is 62 bags of 80 lb concrete at 0.60 cubic feet per bag, or 82 bags of 60 lb concrete at 0.45 cubic feet per bag.
How many square feet does a yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches thick, 64.8 square feet at 5 inches and 54 square feet at 6 inches, before any allowance.
What to check before using the result
- Keep the geometric quantity separate from the allowance.
- Choose the thickness from your project plan; this tool does not select a structural design.
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References & product information
Method prepared 2026-10-21. See our calculation and editorial methods. Spotted a problem? Tell us about it.