About PercentBox

PercentBox is a free educational website for everyday percentage and finance arithmetic. The calculators give an instant number. The guides explain the base, the formula, and the mistakes that make a correct formula answer the wrong question.

Who runs it

PercentBox is published by Ricardo Rodrigues, working from Portugal. It is an independent educational project, not a bank, tax office, or licensed advisory firm. There is no office address published here because the site does not offer in-person services.

The same publisher also runs Portugal Net Pay (SalarioBox), a sister site for Portugal-specific salary and tax estimates in English and Portuguese. PercentBox stays on general percentage and finance math — VAT arithmetic, markup, compound interest, tips, discounts, and fixed-rate loan illustrations — that is not tied to one country’s payroll tables.

What the site is for

• Students checking percent-of, reverse percent, and percent-change problems
• Shoppers comparing a real sale price with a stacked discount
• Diners splitting a tip when the bill does not divide cleanly
• Freelancers separating markup (on cost) from margin (on price)
• Anyone who wants a fixed-rate loan payment or a compound-interest scenario labeled as a scenario

How the calculators work

Calculations run in your browser. There is no account, and the numbers you type are not stored on a PercentBox server. Optional history (the last few calculations) and the light/dark theme live in localStorage on your device. You can clear them in the calculator or in your browser settings.

Editorial standards

Guides are original explanations with worked numbers, not copied textbook chapters and not filler wrapped around a keyword. When a result depends on a local rule — a VAT rate, a tipping custom, a lender’s fee — the article says so and points you to an official source or the contract rather than inventing a universal answer.

How a calculator page is checked

Each tool page states who the formula is for, what every field means, the operation the page actually runs, at least one worked example with the inputs written out, the mistakes that produce a confident wrong number, and a short FAQ. The formula line in the calculator is the same operation as the prose. If they disagree, the page is wrong and should be corrected.

Worked examples use amounts you can redo by hand. Where a figure is rounded to the cent, the page says it is rounded. VAT presets are memory aids for commonly cited rates, not a decision about which rate a supply takes. Percent change is kept separate from percentage points. A loan schedule is not a compound-interest projection, and a markup is not a margin. An updated date means the prose was reviewed. It does not mean a tax authority changed a rate that day.

Tools and articles are educational estimates. They are not tax, legal, investment, or credit advice.

Pages show an author and an updated date when the content model has one. If you find a wrong factor, a swapped base, or a stale rate description, write to hello@percentbox.com or use the contact page. Corrections are part of the job.

Advertising and measurement

The site may show ads through Google AdSense so the tools can stay free, and it may use Google Analytics 4 to see which pages are actually used. The AdSense library tag is on the page whenever ads are enabled. A first-party banner still decides whether ad requests are allowed to run. Visitors whose browser looks like the EEA, the UK, or Switzerland do not get analytics or ad requests until they accept. Details, including what this banner is not, are in the privacy policy.

Start with the percentage calculator or open a guide if you want the method before the number.