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Concrete Calculator
Enter your slab dimensions to get the concrete volume in cubic yards and cubic feet, and how many 40, 60 or 80 lb pre-mix bags you need if you are mixing it yourself. A waste allowance is included because real pours always use a little more than the pure geometry.
Results update as you type. Estimates only — confirm final quantities before ordering.
The formula
Cubic yards = ft³ ÷ 27 • Bags = ft³ ÷ yield per bag
Bag counts use the yields published in the QUIKRETE Concrete Mix data sheet: an 80 lb bag yields about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³ of mixed concrete. One cubic yard therefore takes 45 bags of 80 lb mix, 60 bags of 60 lb, or 90 bags of 40 lb. The waste allowance (10% by default) is added before the bag counts are rounded up.
Worked example
Example: a 10 ft × 10 ft patio slab, 4 inches thick, with 10% waste.
- Raw volume: 10 × 10 × (4 ÷ 12) = 33.33 ft³
- With 10% waste: 36.67 ft³ = 1.36 yd³
- Bags: 36.67 ÷ 0.60 = 62 × 80 lb | ÷ 0.45 = 82 × 60 lb | ÷ 0.30 = 123 × 40 lb
Reference chart
| Slab size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 5 × 5 ft | 0.34 | 16 | 21 |
| 8 × 8 ft | 0.87 | 40 | 53 |
| 10 × 10 ft | 1.36 | 62 | 82 |
| 10 × 20 ft | 2.72 | 123 | 163 |
| 12 × 12 ft | 1.96 | 88 | 118 |
| 20 × 20 ft | 5.43 | 245 | 326 |
Frequently asked questions
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed bases. Driveways and garage floors are typically 5–6 inches, and heavy vehicle or RV pads may need more plus reinforcement. Local code and soil conditions govern — this calculator only sizes the volume.
Should I buy bags or order ready-mix?
Above roughly one cubic yard, a ready-mix truck is usually cheaper and far less labor than mixing: one cubic yard is 45 bags of 80 lb mix. For small jobs — a few post holes, a small pad — bags win on convenience and there is no short-load fee.
Why add a waste allowance?
Forms bow, subgrade is never perfectly level, and some mix is lost in the wheelbarrow and on tools. Five to ten percent covers normal jobs; use the higher end for hand-dug or uneven bases.
How many bags of concrete make one cubic yard?
At the published QUIKRETE yields: 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb mix per cubic yard, before any waste allowance.
Does rebar or mesh change how much concrete I need?
No — reinforcement displaces a negligible volume. It does not change the order quantity, but it does need the correct cover depth inside the slab.
Related calculators
- Gravel Calculator — Gravel in cubic yards and tons, by depth and density.
- Fence Calculator — Posts, sections, rails and pickets for a wood fence.
- Board Foot Calculator — Board feet and lumber cost from thickness, width, length and quantity.
Sources & method: QUIKRETE Concrete Mix No. 1101 technical data sheet (yields 0.30 / 0.45 / 0.60 ft³ per 40 / 60 / 80 lb bag); 27 ft³ = 1 yd³; cross-checked against Inch Calculator — Concrete Calculator.