Frequently Asked Questions

Using the calculator

The calculator solves for whichever field you leave empty. Fill in any two of base, percentage and result, and the third appears as you type — there is no button to press.

The field the calculator worked out is drawn with a dashed border and a shaded background, so at a glance you can tell which numbers you supplied and which one was derived. Type into that field and the calculator switches to solving for one of the others, using the two most recent values you touched.

Under the answer, the panel lists the arithmetic in order using your own numbers. That section is there so you can check the result rather than trust it, and so the site is useful to someone learning the method rather than only to someone who wants the total.

When a field stops updating

The calculator always keeps the two fields you edited most recently and solves for the third. If it seems to be solving for the wrong field, clear everything and enter the two you know in the order you know them.

What it will not do

Some combinations have no answer, and the calculator leaves the field blank rather than inventing one. A percentage change from zero is undefined, since it would divide by zero, and so is asking what percentage a number is of nothing.

Accuracy and rounding

Results are shown to two decimal places, with a trailing ".00" removed. The intermediate figures in the step-by-step working are shown to more places, because rounding them to two would make the lines look inconsistent with the answer.

Underneath, the arithmetic uses ordinary double-precision floating point, the same as every spreadsheet and browser. That is exact for the sizes of number this site deals with, with the well-known caveat that a few decimal fractions cannot be represented precisely in binary.

For accounting or invoicing, round explicitly at the step your own rules require rather than relying on the displayed figure. The methodology page sets out the rounding behaviour in full.

About the site itself

Every page is a static HTML file served from a CDN. The calculator is JavaScript that takes over once the page has loaded, which is why the explanations are readable before any script runs, and why the tool keeps working if the connection drops afterwards.

Nothing you type is transmitted. There is no server to receive it, no account system and no stored history of your calculations.

Frequently Asked Questions

Which two fields should I fill in?

Any two. The calculator works out which value is missing and solves for it, so you can use the same form to find a percentage, a part or a total.

How do I switch between calculation types?

Use the tabs above the form: percentage of a number, what percent one number is of another, increase or decrease, and percentage change between two values.

Why is one of my fields greyed out with a dashed border?

That is the field the calculator solved for. Typing into it makes it an input again, and the calculator then solves for whichever field you edited least recently.

Can I enter a negative percentage?

Yes, in the increase/decrease mode, where a negative value means a reduction. The other modes only accept positive percentages, since a negative one has no meaning there.

Can I use commas for decimals?

Yes. Pick your separator in the header. The choice is saved on your device and applies to both input and results.

Does the site work without JavaScript?

The written explanations, tables and answers do, because every page is pre-rendered HTML. The interactive calculator needs JavaScript.

Is the site available in other languages?

The calculator interface is available in English, Portuguese, Spanish, French, German and Italian. The full set of articles exists in English and Portuguese; the other languages currently have the home page and the methodology page, and link through to English for the rest.

Does it work on a phone?

Yes. The layout adapts to small screens and the number fields open a numeric keypad on mobile devices.

Can I share a calculation with someone?

The share button passes the result sentence to your device's share menu, or copies it to the clipboard if sharing is unavailable. The text is in whatever language you are reading the site in.

How precise are the results?

Calculations use standard double-precision floating point and results are rounded to two decimal places for display. For accounting work, round explicitly at the step your rules require rather than relying on the displayed value.

How do I report an error?

Email franfactory.store@gmail.com with the mode you were using, the exact numbers you entered, what you saw and what you expected. Reproducible reports get fixed quickly.

Is the calculator free to use?

Yes, entirely free and with no account required. The site is funded by advertising placed outside the calculator itself.