About CalcBoxes

CalcBoxes is a small, independent collection of free online calculators and unit converters, built to give you a clear answer fast — and then get out of your way.

Why CalcBoxes exists

Most calculator websites are slow, cluttered with pop-ups, and bury the actual tool under a wall of ads. We wanted the opposite: a set of focused tools that load in under two seconds, work perfectly on a phone, and show their working so you can trust the result. Every CalcBoxes tool does one job and does it well.

Who’s behind CalcBoxes

CalcBoxes is built and maintained by Fendy, an independent developer who got tired of ad-choked calculator sites and decided to build the clean, fast alternative he wanted to use. It isn’t a faceless content farm: it’s a personal project, written and checked by hand, and every tool is one that solves a problem the author has actually run into. If you email us, you’re emailing the person who made the site.

How our tools work

All of our calculators run entirely in your browser using plain JavaScript. Nothing you type is sent to a server, which keeps the tools fast and your data private. Each one also explains the formula behind the answer, with worked examples, so you can understand and verify what you’re seeing rather than just trusting a black box.

Our commitment to accuracy

Getting the number right is the whole point of a calculator, so we take it seriously. Every formula is based on standard, verifiable definitions — for example, conversions use the internationally agreed exact factors (an inch is exactly 2.54 cm, a pound is exactly 0.45359237 kg), and financial tools use well-established formulas such as the standard amortised-loan payment equation and the 4% rule from the Trinity Study. We show the formula and worked examples on each page precisely so you can check our working, and if you ever spot a result that looks off we want you to tell us so we can review it.

That said, our finance and health tools are for education and estimation, not professional advice. A BMI figure is a screening number, not a diagnosis, and a FIRE or loan projection uses simplifying assumptions. For decisions that matter, please consult a qualified doctor or licensed financial adviser.

What we cover

We currently offer everyday tools such as the age calculator, percentage calculator and BMI calculator; a full unit converter with dedicated pages for common conversions like cm to inches and kg to lbs; and finance tools including the salary calculator, loan and mortgage calculator and FIRE calculator. We add and refine tools regularly based on what people find useful.

How we keep it free

CalcBoxes is funded by unobtrusive display advertising. That lets us keep every tool completely free, with no accounts, paywalls or usage limits. We deliberately keep ads out of the way so the calculator always stays the focus of the page.

Get in touch

Spotted a bug, or want a calculator we don’t have yet? We’d genuinely like to hear from you — head to our contact page. You can also read how we handle data in our privacy policy.