Concrete Calculator

What is a concrete calculator?

A concrete calculator estimates how much concrete a rectangular slab needs and translates that volume into the number of pre-mix bags you have to buy. You enter the slab’s length and width in feet and its thickness in inches, and the calculator returns the volume in both cubic feet and cubic yards along with a bag count for the two most common bag sizes, 60 lb and 80 lb. It is the quick check that keeps a patio, shed base, sidewalk, or footing project from running short halfway through the pour.

How does it work?

A slab is just a flat rectangular box, so its volume is length times width times thickness. The only wrinkle is units: length and width are entered in feet but thickness is entered in inches, so the thickness is divided by 12 to convert it to feet before multiplying. The result is a volume in cubic feet, which is then divided by 27 to give cubic yards (there are 27 cubic feet in a cubic yard).

To turn that volume into bags, the calculator divides the cubic-foot volume by the yield of one bag and rounds up, because you cannot buy a fraction of a bag. A standard 80 lb bag of pre-mix yields about 0.6 cubic feet of set concrete, and a 60 lb bag yields about 0.45 cubic feet.

Formula

For a rectangular slab with length $L$ and width $W$ in feet and thickness $T$ in inches, the volume in cubic feet is:

$V_{ft^3} = L \times W \times \frac{T}{12}$

Converting to cubic yards divides by 27:

$V_{yd^3} = \frac{V_{ft^3}}{27}$

The bag counts round the volume up to the next whole bag:

$N_{80} = \left\lceil \frac{V_{ft^3}}{0.6} \right\rceil \qquad N_{60} = \left\lceil \frac{V_{ft^3}}{0.45} \right\rceil$

Worked examples

Example 1: a 10 ft by 10 ft slab, 4 inches thick

$V_{ft^3} = 10 \times 10 \times \frac{4}{12} = 33.333 \ \text{ft}^3$

That is $33.333 / 27 = 1.2346$ cubic yards. Dividing the cubic feet by the bag yields and rounding up gives 56 bags of 80 lb concrete or 75 bags of 60 lb concrete.

Example 2: a 10 ft by 10 ft slab, 6 inches thick

$V_{ft^3} = 10 \times 10 \times \frac{6}{12} = 50 \ \text{ft}^3$

That works out to $50 / 27 = 1.8519$ cubic yards, which needs 84 bags of 80 lb concrete or 112 bags of 60 lb concrete.

Practical notes

• Bag yields are approximate and vary slightly by brand and how stiff you mix the concrete; treat the bag count as a planning estimate and add a small allowance for spillage and uneven subgrade.
• Once a job needs more than about a cubic yard, mixing dozens of bags by hand becomes impractical. Compare the bag count against ordering ready-mix concrete by the cubic yard or renting a mixer.
• For an L-shaped or stepped pour, split the footprint into rectangles, compute each separately, and add the volumes together.
• Measure the compacted thickness you intend to pour rather than the depth of a loosely dug trench, and keep the thickness consistent across the slab for an even result.