How we calculate

All Shopnauta calculators follow the same rules. You will find them here once, so you can see where every number on the receipt comes from.

Measurements and units

Internally, all calculations use SI units (metres, square metres, litres). The measurements you enter are converted only on the way in, and the result only on the way out. That is why switching from metres to feet does not change the amount to buy, apart from the rounding to the pack sizes on sale.

Rounding

We do not round the intermediate steps. We round up only at the end, on the packs to buy: we will never tell you to buy less than you need.

Choosing pack sizes

When a product is sold in several sizes, we look for the combination that wastes the least. For paint, for example, each extra can counts as half a litre of waste, and as a full litre if the can is smaller than one litre, because small sizes cost much more per litre. When options tie, we pick fewer packs, then the larger pack.

Constants and sources

Coverage, consumption and other typical values vary from product to product and from country to country. We use values taken from data sheets of products sold in your country, we cite them at the bottom of each calculator, and you can replace them in the advanced options with the value printed on the pack you are about to buy.

What we don't do

• We don't use prices or offers: they change too often.
• We don't give regulatory or safety guidance: for structural work, installations or doubts about fitting, ask a professional.
• The estimates do not replace the manufacturer's instructions.

Checks

Every formula has automated tests with hand-calculated examples, run on every update of the site. If a result doesn't add up for you, write to [email protected] and attach the link to your calculation (the "Copy link" button): it contains all the measurements you entered.