FitAndFillConcrete Slab Calculator: Cubic Yards and Bags
Estimate the concrete for a rectangular slab in cubic yards and in 40, 50, 60, 80 or 90 lb bags, with a waste allowance.
How the calculation works
The calculator multiplies the three dimensions, converts to cubic yards and divides by how much finished concrete one bag makes.
volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12 cubic yards = volume (ft³) ÷ 27 bags = volume with waste ÷ yield per bag, rounded up Approximate yield per bag of standard concrete mix, from the manufacturer data sheets listed under Sources:
| Bag size | Yield (ft³) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.3 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.6 | 45 |
| 90 lb | 0.675 | 40 |
Worked example
A 10 ft × 10 ft slab that is 4 inches thick holds 10 × 10 × (4 ÷ 12) = 33.33 ft³, which is 1.23 yd³. That is 56 bags of 80 lb mix (55.6 rounded up).
Make the same slab 6 inches thick and it needs 50 ft³ (1.85 yd³), or 84 bags of 80 lb mix. Thickness has a bigger effect than most people expect: two more inches adds half again as much concrete.
With a 10 % allowance for uneven ground and spillage, the 4-inch slab becomes 36.67 ft³, or 62 bags.
Assumptions and limits
- The slab is a rectangle with the same thickness everywhere.
- Bag yields are for a standard concrete mix such as QUIKRETE No. 1101. Other products differ; sand/topping mix, for example, yields more per bag. Always use the yield printed on your bag.
- The gravel base, forms, rebar or wire mesh are not included.
- Bag counts are always rounded up to whole bags.
Frequently asked questions
How many 80-lb bags make one cubic yard?
About 45. One cubic yard is 27 ft³ and an 80-lb bag of standard mix makes about 0.60 ft³, so 27 ÷ 0.60 = 45. That is roughly 3,600 lb of dry mix.
How much extra should I add?
Subgrade is rarely perfectly level and some mix is always lost to spillage, so most people add a margin. This tool defaults to 10 %. Running short mid-pour is worse than having a little left over.
How thick should my slab be?
Thickness depends on the load, the soil, your climate and local building code. Light-duty patios and walkways are commonly poured about 4 inches thick, while driveways and slabs that carry heavy loads are usually thicker and often reinforced. Confirm the requirements with your local building department or a licensed contractor; this tool does not give structural advice.
Does this include the gravel base, rebar or mesh?
No. It only calculates the concrete itself. The base layer under a slab is a separate volume that you can estimate with the same length × width × depth method.
Should I use bags or order ready-mix?
One cubic yard is 45 bags of 80 lb, which is a lot of mixing by hand. For larger pours many people order ready-mix delivery. Ask suppliers about minimum order sizes and short-load fees, because they vary by location.
Why does the bag count round up?
Bags are sold whole, and buying one too few can ruin a pour. The tool always rounds up, so its count can be one bag higher than a rough hand estimate.
Sources
- QUIKRETE Concrete Mix No. 1101 product data sheet (yield per bag: 40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60, 90 lb ≈ 0.675)
- Unit definition: 1 cubic yard = 27 cubic feet
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