A calculator is only useful if you can trust it, and you should not have to take our word for it. This page explains how the numbers on WeekendCalc are produced and checked, and where they are only estimates.
Every formula is shown
Each of our 85 calculators has a "how this calculator works" section with the formula in plain language and a worked example you can repeat with a pencil. Where a figure is a rule of thumb, such as how far a gallon of paint goes, we say so, and you can change it.
Constants, standards and rules of thumb
We separate three kinds of number:
- Physical constants and unit conversions (27 cubic feet in a cubic yard, 7.48 gallons in a cubic foot, 91,500 BTU in a gallon of propane). These are exact or very close, and do not change.
- Published standards and guidance, such as the Energy Star room air conditioner sizing chart and attic insulation recommendations, the USGS classes of water hardness, the stair and ramp limits in the International Residential Code and the ADA, and the yields printed on bags of concrete mix. We note the source on the page where it matters.
- Typical values and rules of thumb (waste allowances, appliance wattages, coverage per gallon). These vary by product and project, so they are defaults you can override, and product labels always win.
How we test
For every calculator we work out at least one example by hand, independently of the code, and an automated test confirms that the calculator returns exactly that answer. The tests also try every dropdown option and unusual inputs, and check that the calculator never shows a blank, a "not a number" error or an impossible value. The reference charts and tables on our pages are generated by the same formulas as the calculators, so a chart and a calculator can never disagree.
We re-run all of these checks every time the site changes, and also test the pages in a real browser, including on a phone-sized screen.
We have found our own mistakes, and fixed them
Checking is not a one-time job. While building this site we found and corrected several errors in our own text and tables (a rounding slip in a chart, a worked example that used the wrong number), which is why we generate tables by computer rather than typing them by hand. If you ever spot a figure that looks wrong, tell us and we will check it and fix it, usually within a few days.
What we are not
We are not engineers, architects or licensed trades. The results are estimates for planning and shopping, not design. Anything structural or safety-related (stairs, decks, roofs, footings, retaining walls, electrical or gas work) must follow your local building code and, where required, a qualified professional. See the terms and disclaimer.