Calculators

Small calculations come up constantly in everyday life: working out a discount at checkout, checking a BMI reading, finding out exactly how old someone is down to the day, or counting the days left until a deadline. Getting them right by hand is easy to fumble — a misplaced decimal, a discount applied to the wrong price, a leap year forgotten. This page collects five calculators that handle each of these cases directly, with the formula shown on every result so you can see exactly how the number was produced.

Money and percentages

The percentage calculator is the general-purpose one: it finds what percent one number is of another, adds or subtracts a percentage from a value, or works out the percentage change between two numbers. It covers anything from splitting a bill to comparing year-over-year figures, including the case most people get wrong — a percentage change measured against a negative starting value.

The discount calculator is the shopping-specific version. Give it a price and a percent off to see the final price, give it two prices to find out what percentage you actually saved, or use its stacked-discount tab when a store applies two reductions one after another — a case where the two percentages never simply add up, as the calculator's own page explains with worked numbers.

Time and dates

The age calculator turns a birth date into an exact age in years, months, and days, along with the total number of days lived and the date of the next birthday. The date difference calculator is the more general tool for the same kind of question: it finds the days, weeks, months, and years between any two dates, in either order, and includes a business-day count that skips weekends — useful for a rental period, a contract term, or a project deadline.

Health

The BMI calculator computes body mass index from height and weight in either metric or imperial units and shows the WHO adult weight category the result falls into. Like any health-adjacent figure, a BMI number is general information rather than a medical assessment — it can't tell a muscular build from a higher body-fat percentage, and it doesn't apply to children.

Which calculator do I need?

Comparing prices while shopping: use the discount calculator. Any other percentage question — a tip, a raise, a rate of change: the percentage calculator. Your own age or an upcoming birthday: the age calculator. The distance between two arbitrary dates: the date difference calculator. A quick check of your weight relative to your height: the BMI calculator. All five work on mobile, need no account, and keep every number you enter on your own device.