How these calculators are made
There are thousands of calculators online and most are the same. This explains where we try to be different, and where the limits are.
The idea
Most online calculators solve the easy case and give up on the hard one, which is exactly the one that sent you looking. They hand you a number without saying where it came from, do not mention what they left out, and expect you to take it on trust.
We try the opposite: fewer tools, each one solving a specific problem all the way through, showing its assumptions and its breakdown.
Five rules
1. The result explains itself
Every tool states what the calculation assumes before you have to ask, plus a breakdown of the intermediate figures. A loan calculator that gives you the payment but does not tell you that 59% of the first payment on a 30-year mortgage is interest is hiding the interesting part.
2. The maths is verified
Each calculation is a pure function with its own unit tests, including the awkward edge cases: zero, negatives, leap years, months of different lengths, rates of zero. Right now there are 10 tools published here, 4 of them with a chart and 10 with a proper question-and-answer section.
3. Nothing you type leaves your device
Every calculation runs entirely in your browser. Your salary, your mortgage, your electricity use: none of it is sent to a server, stored in a database, or seen by anyone. You can check it yourself by going offline after the page loads and watching it keep working.
4. Units and currency follow you, and never lie
You can switch between metric and imperial and pick your currency from the preferences menu. Two things worth knowing about how that works: the conversions handle the traps properly (a price per gallon is higher than a price per litre, and miles per gallon is the inverse of litres per 100 km, not a multiple of it), and the currency selector does not convert amounts. It changes the symbol and the formatting. Converting would need live exchange rates, and a static site has nowhere to fetch them from; serving a stale rate would be worse than not offering the feature at all.
5. Limits are generous on purpose
Validation limits are set absurdly wide: a 40-year mortgage, a lorry's fuel tank, an industrial-sized room. Hitting a wall on a legitimate value is far more annoying than being allowed to enter something silly. The sliders cover the comfortable range; the text field takes almost anything.
Where the limits are
These are indicative tools. They are not financial, legal, tax or medical advice, and they do not replace a professional who knows your particular situation.
Anything touching regulation is tied to Spain, because that is where the site is run from: public holidays, VAT rates, the way lenders quote interest. Tools that depend on those are marked in their assumptions. The general ones — percentages, dates, proportions, fuel economy, running paces — work anywhere.
How it is built
A static site with no server and no framework runtime, no web fonts, no charting library and no tracking beyond an analytics tag that only loads if you say yes. Each tool is a folder with three files, and the page, the form, the validation, the chart and the shareable URL are all generated from a single declared schema. That is why the fortieth tool costs about the same to build as the eleventh.
Start somewhere
- Compound interest calculator — Work out how much your savings will grow with compound interest and regular contributions, and how much of that total came from interest rather than from you.
- Days between two dates calculator — Enter two dates and we tell you how many days lie between them, broken down into weeks, spare days and hours.
- Exact age calculator — Enter a date of birth and we work out the exact age in years, months and days, plus how long is left until the next birthday.
- Real fuel economy calculator — Fuel put in and distance covered. That is all it takes to know what your car really does, rather than what the trip computer claims.
- Percentage calculator — Enter a percentage and a number. Instead of making you pick an operation first, we give you all four answers at once.
- Percentage change calculator — From one value to another, how much has it gone up or down in percent? And how many times one is of the other.
Missing something?
The best clue about what to build next is someone telling us what they came looking for and did not find. Tell us what is missing.
Updated on 27 August 2026.