The coverage rates behind every calculator, where we got them, and what happens when one turns out to be wrong.
A square footage answer is just arithmetic, but a paint, tile, or gravel calculator is only as good as the coverage rate or density built into it. This page lists where those numbers come from, how often we recheck them, and how a reader-reported error gets fixed.
Area and volume math on this site is plain geometry: length times width for a floor, a perimeter times a ceiling height for walls, a length-width-depth product divided by 27 for a cubic-yard gravel order. Where a formula needs a real-world constant, a paint coverage rate, a tile waste allowance, a wallpaper pattern-repeat loss, a BTU-per-square-foot rule of thumb, that constant comes from a published manufacturer figure or an industry convention, not a number we picked because it felt right. Every calculator page shows the formula it used, so you can rerun the math by hand.
The constants that can vary get checked against a primary source and cited on the page where they matter most:
Where a figure is genuinely a range, installed flooring cost per square foot, for instance, we show the range rather than pretend there is one fixed answer.
The 10 percent default waste factor on most calculators here, 15 percent for diagonal or patterned layouts, reflects the range flooring and tile installers commonly quote for straight-lay versus cut-heavy jobs. It is a widely used planning convention, not a number a specific supplier promised you. Your own layout, a lot of doorways, an oddly shaped room, may call for more.
These tools size a shopping list. They are not a contractor's takeoff, an engineer's drawing, or a code inspector's sign-off. A subfloor that looks flat rarely is, a dye lot varies box to box, and a cut-up roofline wastes more shingle than a simple gable. That is why the waste factor exists and why every guide tells you to round up rather than order to the exact number. For anything structural, permitted, or over a full pallet of material, confirm the order with your supplier or a licensed contractor.
A calculator or guide is checked at publication and again whenever a cited coverage rate moves, a linked source updates its page, or a reader flags something that does not match what they saw at the store. The date on a page reflects the last time that happened. Chris Terry owns HomeImprovCalc through Encore Promotional Products and signs off on the sourcing sitewide; the authors page lists who actually drafts each guide.
Found a calculator returning a figure that does not match your can, your box, or your supplier's quote? Say so through the contact page. We would rather fix a coverage rate the same week than leave a defensible-looking error standing.
Display ads and a small set of affiliate links, Amazon Associates among them, are what keep every calculator free. A purchase through one of those links can earn a commission at no cost to you, and that revenue never decides a formula, a coverage rate, or which product gets named in a guide.