Concrete Calculator
Calculate how much concrete a rectangular slab needs in cubic feet, cubic yards, and cubic meters, plus the number of 60lb or 80lb bags required.
How It's Calculated
Formula
V_{ft^3} = L_{ft} \times W_{ft} \times \frac{T_{in}}{12}Concrete needed for a rectangular slab is simply its volume: length times width times thickness. Since thickness is usually measured in inches while length and width are measured in feet, thickness is first divided by 12 to convert it to feet, so all three dimensions share the same unit before multiplying. The result in cubic feet is then converted to cubic yards (dividing by 27, since a cubic yard is 3ft x 3ft x 3ft) and cubic meters (multiplying by 0.0283168), since ready-mix concrete is typically ordered by the cubic yard. For smaller pours, this calculator also estimates how many standard pre-mixed bags are needed, using the manufacturer-published yields of 0.45 cubic feet per 60lb bag and 0.6 cubic feet per 80lb bag, rounded up to the next whole bag since partial bags cannot be purchased. All three slab dimensions must be greater than zero.
Worked Examples
10ft x 10ft patio slab, 4 inches thick
- Thickness in feet: 4 / 12 = 0.3333 ft
- Volume: 10 x 10 x 0.3333 = 33.33 cubic feet
- Cubic yards: 33.33 / 27 = 1.23 cubic yards
- 60lb bags: ceil(33.33 / 0.45) = 75 bags
2ft x 2ft fence post footing, 6 inches thick
- Thickness in feet: 6 / 12 = 0.5 ft
- Volume: 2 x 2 x 0.5 = 2 cubic feet
- 60lb bags: ceil(2 / 0.45) = 5 bags
- 80lb bags: ceil(2 / 0.6) = 4 bags
Frequently Asked Questions
Why is thickness divided by 12 in the formula?
Length and width are entered in feet, but thickness is typically measured in inches since slabs are usually only a few inches deep. Dividing by 12 converts the thickness to feet so all three dimensions are in the same unit before multiplying them together.
Should I order slightly more concrete than the calculated amount?
This calculator reports the exact geometric volume of the slab. In practice, many contractors add 5-10% extra to account for uneven subgrade, spillage, and form imperfections — that buffer is a project-planning decision left to you, not built into this calculation.
Where do the 0.45 and 0.6 cubic foot bag yields come from?
These are the standard yields published by major pre-mixed concrete bag manufacturers for their 60lb and 80lb bags respectively. Bag counts are rounded up to the next whole bag, since you can't purchase a fraction of a bag.
Can I use this for footings, sidewalks, or driveways?
Yes — the formula works for any rectangular concrete pour. For a driveway or sidewalk with a consistent thickness, just enter its overall length, width, and thickness the same way as a slab.