A calculator you can’t check is just an opinion with a button on it. This page explains, in plain terms, where our formulas come from, why our calculators use the constants they use, how we test them before they reach you, and what we do when we get something wrong. If you ever want to verify one of our results with a pencil and paper, this page plus the worked example on each calculator page should give you everything you need.
1. Where our formulas come from
Every calculator on ProjectCalc Pro is built from a published, checkable source — never from a figure we invented. Our sources fall into four groups:
- Basic geometry. Most material estimates start as area or volume: length × width for surfaces, and length × width × depth for materials sold by volume, such as concrete, mulch, and gravel. These are definitions, not opinions.
- Manufacturer specifications. Where a product’s yield is defined by its maker, we use the maker’s number. Examples you will see on this site: an 80 lb bag of concrete mix yields approximately 0.6 cubic feet, and a 60 lb bag yields approximately 0.45 cubic feet. Bag and box coverage figures are taken from the packaging conventions of major US manufacturers.
- Standard trade coverage rates. Some figures are conventions of the trade rather than properties of a single product. Paint is the clearest example: one gallon of paint is generally rated to cover about 350 square feet in one coat on a smooth, primed surface, a figure published by major paint manufacturers. Where we use a trade rate like this, the calculator page says so and states the number.
- Standard material dimensions and densities. Lumber, drywall sheets (4 × 8 ft and 4 × 12 ft), and bulk materials have widely published standard sizes and weights. For gravel, for example, we use a typical density of about 1.4 tons per cubic yard — and because real density varies by stone type, the calculator lets you change it.
Each calculator page shows the specific formula and the key inputs it uses, directly under the tool. If a figure on this site ever appears without a stated source or assumption, that is a defect — please report it on our Contact page.
2. Waste allowances and rounding
Real projects waste material. Tiles get cut, boards have defects, and paint stays in the roller. Ignoring that would produce calculators that are precisely wrong, so our tools add a waste allowance where the trade normally does. The defaults we use include:
- Tile: 10% extra, calculated on the installed layout including the grout gap between tiles.
- Flooring: 10% for laminate and luxury vinyl plank, 15% for hardwood, reflecting the extra cuts and board selection hardwood requires.
- Wallpaper: 15% extra to allow for pattern matching, using a standard US double-roll coverage of about 56 square feet.
- Paint and volume materials: results are rounded up to whole purchasable units — you cannot buy 0.8 of a gallon in most stores, and running short mid-job costs more than one extra bag.
Rounding is a place where calculators quietly go wrong, so we handle it deliberately. Quantities are rounded up only at the final step, using a method that treats a result that lands exactly on a whole number as that whole number — not one more. (This sounds trivial; it is not. Floating-point arithmetic can turn an exact 110 tiles into 110.00000000000001, and a naive calculator would tell you to buy 111. Our test suite specifically includes these exact-boundary cases.)
3. How we test our calculators
Before a calculator is published, and every time its code changes, it must pass a set of reference tests: inputs for which the correct answer has been worked out independently, by hand, from the published formula. Our current suite covers all ten of our launch calculators with 40 test cases, including:
- Ordinary cases (a typical room, a typical slab).
- Boundary cases, where the answer lands exactly on a whole purchasable unit.
- Multi-part shapes, such as L-shaped rooms calculated as a rectangle minus the missing corner, and projects with several rooms added together.
Every test must pass — not most of them. This process has already earned its keep: during development, our tests caught a real rounding bug in which exact results were being rounded up to one unit too many. The bug was fixed in the shared calculation code, so the fix applied to every affected calculator at once, and the failing case stays in the suite permanently so it can never come back unnoticed.
We do not publish a calculator we cannot test this way. If a type of estimate cannot be reduced to a checkable formula with a definite right answer, it does not go on this site.
4. How we handle prices
Material prices vary by region, store, and month, so we present costs as ranges, not promises. Our price data is collected from publicly available sources — published retail pricing and manufacturer information — and reviewed on a quarterly cycle.
Every cost table on this site carries the date it was last checked. If a price on a page has no date, treat the figure with suspicion and let us know. When prices move significantly between reviews — as lumber and concrete prices sometimes do — we update the affected tables rather than waiting for the next scheduled review.
Cost estimates cover materials only, in US dollars, unless a page says otherwise. They do not include tools, delivery, permits, or labor.
5. Imperial and metric
Our calculators are built for US projects and default to imperial units (feet, inches, pounds, gallons). Where metric equivalents are shown, they are converted using exact conversion factors (1 inch = 25.4 mm; 1 cubic yard = 27 cubic feet) and are for convenience — the underlying calculation always follows the formula shown on the page.
6. When we make a mistake
We will eventually publish a wrong number somewhere. What matters is what happens next, and our policy is fixed:
- Every report gets checked. If you report a suspicious result through our Contact page, we reproduce the calculation and compare it against the published formula and our reference tests.
- We fix the source, not the symptom. Our calculators share one calculation engine. A fix is made in the engine, so every calculator using that logic is corrected at the same time, and the corrected case is added to the permanent test suite.
- We correct openly. If an error affected results shown on a page, we update the page and, where the change is significant, note the correction and its date on that page.
If you spot something, we genuinely want to hear it. A reported error makes every future result on this site better, and we are grateful to the readers who take the time.
7. What our calculators do not do
Our tools estimate quantities and costs for common, non-structural home projects. They do not perform engineering. We do not publish calculators for structural loads, beam sizing, electrical circuits, gas lines, or plumbing systems, and no result on this site should be used to size or design anything safety-critical. For those jobs, hire a licensed professional — and see our Disclaimer for the full statement.