Calculators that show their working
Loans, tax, percentages, dates and body metrics — each with the formula written down and the awkward cases handled on purpose.
7 tools · free · no signup
Finance
Compound Interest & SIP Calculator
Grow a lump sum, a monthly contribution, or both — with the compounding stated.
On your deviceEMI & Loan Calculator
Work out a loan instalment, the total interest, and the full month-by-month schedule.
On your deviceGST & VAT Calculator
Add tax to a price or pull it back out of a tax-inclusive one, with the CGST/SGST split.
On your devicePercentage Calculator
Every percentage question, including the two that are always confused with each other.
On your deviceWhy this category is here at all
Calculators are the part of the old purffle.tools that failed worst, and it is worth being specific about how. Its wind-chill tool ran a Fahrenheit-and-miles-per-hour regression against Celsius and kilometres-per-hour inputs and reported −22 °C where the real answer was −5.9 °C. Its horsepower calculation accepted an engine displacement, ignored it entirely, and returned a number derived from the cylinder count. Its EMI calculator returned an HTTP 500 because the form sent a tenure in years and the code read months, then divided by zero. Several others silently returned zeros.
None of that announced itself. Every one of those tools produced a confident, plausible-looking number, which is worse than producing nothing.
What is different here
Three things, and they are all visible on the pages themselves. The formula is written out next to the answer, so you can check it rather than trust it. The boundary cases are handled deliberately rather than falling out of the arithmetic — a loan at 0% interest, a percentage change from zero, 31 January to 1 March, a leap-day birthday. And where a question genuinely has no answer, the tool says so instead of printing Infinity or NaN.
Tested against a second implementation
Every number these calculators produce is checked against an independent implementation written in a different language by a different route: where the JavaScript uses a closed-form annuity factor, the checker runs the loop; where it counts months forward, the checker counts upward from zero. Two implementations that share an algorithm share its bugs, so agreement would prove nothing — two that arrive by different roads is evidence.
It has already earned its keep. It caught an amortisation schedule that added a spurious extra instalment of twenty paise because rounding drift was never absorbed, and a date difference that returned minus one day between 31 January and 1 March. Both were fixed before anything shipped.
They run as you type
Nothing here touches a server. On the old site, counting three words took a 3.3-second round trip to another continent; these answer immediately, work offline once the page has loaded, and never send your salary, your loan balance or your date of birth anywhere.