Concrete is sold in two very different ways — by the cubic yard for truck-delivered ready-mix, or by the bag for smaller DIY pours — and getting the quantity wrong in either direction is expensive: too little means a mid-pour scramble, too much means wasted material and money. Our Concrete Calculator handles both, from the same three inputs.
The volume math
Concrete volume is just length × width × thickness — but thickness is almost always specified in inches while length and width are in feet, so the calculator converts thickness to feet first, then divides the resulting cubic-foot volume by 27 (the number of cubic feet in a cubic yard) to get the figure ready-mix trucks actually price by.
Converting volume to bag count
For smaller pours using pre-mixed bags, the standard conversion is that one 80 lb bag yields about 0.6 cubic feet of mixed concrete once water is added — a figure published by major concrete mix manufacturers and widely used across DIY construction calculators. Dividing your total cubic-foot volume by 0.6 gives the number of bags, rounded up to the next whole bag since you can’t buy a fraction of one.
A worked example
A 10 ft × 10 ft slab, 4 inches thick: that’s 10 × 10 × (4/12) = 33.33 cubic feet, or 1.23 cubic yards. In bags, that’s 33.33 ÷ 0.6 = 55.6, rounded up to 56 bags of 80 lb pre-mix.
Buy a little extra
This calculator’s bag count is the exact theoretical minimum — it doesn’t add a buffer for spillage, an uneven sub-grade, or slightly over-excavated forms. Most contractors and DIY guides recommend adding 5-10% extra material on top of the calculated amount for exactly that reason, so treat the raw output as your floor, not your final shopping list.
Beyond slabs
The same length × width × thickness math works for any rectangular pour — footings, patios, sidewalks. For round post holes, you’d instead use cylindrical volume (π × radius² × depth) and convert to cubic yards the same way, though that’s not what this particular calculator handles.
Try it yourself: Concrete Calculator.