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 in the UK edition of WeekendCalc are produced and checked, and where they are only estimates.
Every formula is shown
Each of our 60 UK 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 many square metres a litre of paint covers, we say so, and you can change it. The label on the tin or bag always wins over our defaults.
Metric, in the sizes UK merchants sell
The UK calculators work in metres, square metres, litres and tonnes, and in the pack sizes sold in the UK: 25 kg bags, 2400 × 1200 mm plasterboard, 600 × 600 mm slabs, 10.05 m wallpaper rolls and MOT Type 1. They use the same method and the same tests as the US edition, with British spelling.
Project planners reuse the single-material calculators
Our whole-project planners (a patio, a fence, a room to paint) are built from the single-material calculators: they run exactly the same formulas on one set of measurements and add up the shopping list. A number in a planner and the same number in the stand-alone calculator therefore cannot disagree.
Constants, guidance and rules of thumb
We separate three kinds of number:
- Physical constants and unit conversions (1000 litres in a cubic metre, the density of common materials). These are exact or very close, and do not change.
- Published guidance, such as the Building Regulations Approved Documents (for example K for stairs and J for combustion appliances) and the figures printed on merchants' and manufacturers' product data. We name the source on the page where it matters. Building Regulations differ in Scotland, Wales and Northern Ireland, so check the rules for your own nation with your local authority building control.
- Typical values and rules of thumb (waste allowances, appliance wattages, coverage per litre). These vary by product and job, so they are defaults you can override.
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 tables on our pages are generated by the same formulas as the calculators, so a table 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-off job. While building the site we found and corrected several errors in our own text and tables, which is why we generate tables by computer rather than typing them by hand. If you 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 registered tradespeople. The results are estimates for planning and shopping, not design. Anything structural or safety-related (stairs, roofs, foundations, retaining walls, electrical or gas work) must follow the Building Regulations and, where required, a qualified professional. See the terms and disclaimer.