🧮 Free Online Calculators
Percentages, ages, date arithmetic and random numbers — all worked out in your browser.
These are the calculations that come up often enough to be annoying and rarely enough that the method has to be looked up again each time. Each one shows its working rather than just an answer, because the usual problem is not the arithmetic but the question: whether a percentage change is measured against the old figure or the new one, whether a date range includes its last day, whether an age counts the birthday that has not happened yet.
All Calculator Tools
Percentage Calculator
A percentage of a value, a share, a change between two numbers, or an increase and decrease.
Age Calculator
Exact age in years, months and days, with totals and the countdown to the next birthday.
Date Difference
Days between two dates, split into weekdays and weekends, plus adding days to a date.
Random Number Generator
Draw numbers in any range from a cryptographic source, with or without repeats.
Why the working matters more than the answer
Every calculator on this page returns a number, and every one of them also shows how it got there. That is not decoration. The commonest error in everyday arithmetic is not a slip in the multiplication — it is answering a subtly different question: a share where a change was meant, a date range that silently includes or excludes its final day, an age that counts a birthday still weeks away.
A visible calculation makes that check possible in a second. If the percentage calculator prints "(150 − 100) ÷ 100 × 100" and you meant to divide by 150, the mistake is obvious before it reaches a spreadsheet. If the date calculator prints seven days where you expected eight, the checkbox that decides whether the last day counts is right there.
None of these calculations need a server, so none of them use one. The dates you type, the figures you enter and the numbers drawn stay in the browser tab, which is worth knowing before entering a date of birth into a web page.
What these calculators deliberately do not do
There is no loan, mortgage or investment calculator here, and that is a decision rather than an omission. Those results depend on compounding conventions, fee structures and tax treatment that differ by country and by product, and a number produced without them looks authoritative while being wrong in ways the reader cannot see.
The random number generator is a related case. It draws from the browser’s cryptographic source and corrects the distribution bias that most online generators carry, which makes it genuinely fair — but not auditable. A prize draw that has to be provably fair to other people needs a procedure that can be checked afterwards, and no web page can supply one.
What is here is arithmetic with no hidden assumptions: percentages, calendar dates and uniform random draws. Where an answer depends on a convention — whether the end date counts, how a 29 February birthday falls in an ordinary year — the choice is a visible control rather than a decision made silently on your behalf.