Every PourMath calculator is built on the same explicit formulas and assumptions, laid out here so you can check the math and judge whether our defaults fit your job. The numbers are planning estimates, not quotes: confirm structural dimensions against engineered plans and final quantities with your ready-mix supplier before you order or pour.
Volume: the core calculation
Concrete volume is geometry. For flatwork we multiply length × width × depth, with depth converted from inches to feet, then convert cubic feet to cubic yards: 1 cubic yard = 27 cubic feet. Ready-mix is sold by the cubic yard, so that is the unit every calculator reports first.
Worked example: a 10 × 10 ft slab at 4″ thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 33.3 ÷ 27 = 1.23 cubic yards before waste.
Waste allowance
Real pours need more than the theoretical volume: subgrade is never perfectly flat, forms bow, and trenches over-dig. Every calculator adds an editable waste allowance defaulting to 10%. Lower it for a tightly formed slab on a level base; raise it for footings, uneven ground, or hand-dug trenches. The 10 × 10 slab above becomes about 1.36 cubic yards ordered.
Bags vs. ready-mix
Bag counts come from published bag yields (an 80 lb bag of standard concrete mix yields roughly 0.60 cubic feet). We divide the waste-inclusive volume by the bag yield to get the count. As a rule of thumb, past about 1 cubic yard (~45 × 80 lb bags) ready-mix delivery is cheaper and far less work than mixing bags by hand, so the calculators flag that threshold.
Where cost figures come from
Material cost multiplies your cubic yards by an editable ready-mix price, defaulting to a U.S. national ballpark per cubic yard. Installed-cost ranges come from published national cost guides and are intentionally wide because region, site prep, thickness, finish, and access move real quotes substantially. The full national reference set, with a citable CSV, lives in the Concrete Cost Index. Cost data last reviewed: June 2026.
How we keep numbers consistent
The size pages, cost guides, and Cost Index all derive their figures from the same underlying constants, so a 12 × 12 slab shows the same yards and bag count wherever it appears, and the pre-filled answer in the served HTML matches what the calculator recomputes in your browser.
Corrections
Spotted a number that looks off? Tell us the page and we will check the math: contact@pourmath.com.