Concrete Calculator

How to use Concrete Calculator

Enter finished slab length, width, and depth. Choose a waste allowance and premix bag size to compare ready-mix volume with bagged concrete.

Concrete slab formula

For a rectangular slab, cubic feet equal length in feet × width in feet × depth in feet. Because depth is entered in inches, the calculator divides it by 12 first. Cubic yards equal cubic feet ÷ 27. The selected waste percentage is added to volume before bag quantities are rounded up.

Worked example

A slab 10 feet long, 12 feet wide, and four inches deep has a depth of 4 ÷ 12 = 0.3333 feet. Its volume is 10 × 12 × 0.3333 = 40 cubic feet. Dividing by 27 gives about 1.48 cubic yards. With 10% waste, plan for about 44 cubic feet or 1.63 cubic yards. At a nominal yield of 0.80 cubic foot per 80-pound bag, the estimate rounds up to 55 bags.

Choosing a waste allowance

Waste covers uneven excavation, spillage, forms that are not perfectly square, and small measurement errors. A simple, well-formed slab may need less allowance than an irregular pour. Excessive allowance can leave costly surplus. Measure several points when the base or depth varies rather than assuming one ideal dimension.

Bag yield and ready-mix assumptions

The bag options use approximate nominal yields of 0.45, 0.60, and 0.80 cubic foot. Actual coverage is controlled by the product label and may vary with mix design, water, handling, and compaction. Ready-mix suppliers commonly sell by cubic yard and may impose minimum loads or delivery increments; this page does not estimate price or delivery requirements.

Accuracy and construction limitations

This is a material-volume estimate, not structural design. It does not determine slab thickness, reinforcement, footings, base preparation, drainage, frost protection, load capacity, curing, joints, or local code compliance. Openings, slopes, thickened edges, beams, and multiple shapes should be calculated separately and combined. Consult project plans, product instructions, and a qualified professional where structural performance matters.

Common mistakes

Keep length and width in feet and depth in inches as labeled. Do not enter four feet when the slab is four inches thick. Measure finished concrete dimensions, not only nominal lumber or excavation dimensions, and round bag counts upward because partial bags cannot normally satisfy the estimate.