Calculators

Which calculator do you need?

• Dates and ages — Age Calculator for an exact age in years, months and days; Date Calculator for the gap between two dates or a date plus or minus a number of days; Hours Calculator for time worked between two clock times.
• Money — Loan and Mortgage Calculators for monthly payments and total interest, Compound Interest for savings growth, Discount, Sales Tax and Tip Calculators for the price you will actually pay.
• Health — BMI, Calorie (TDEE) and Protein Calculators, each using a published formula rather than an invented one.
• School and everyday maths — Percentage, Average (mean, median, mode), GPA on the 4.0 scale, a full Scientific Calculator, Aspect Ratio for images and screens, and Number to Words for cheques and legal documents.
• Fuel Cost for a trip, and a Swedish taxi driver salary calculator for a very specific audience.

How the results are produced

Every calculator runs as JavaScript inside your browser. The page loads once, and after that your inputs are never sent anywhere — there is no server call, no account and no stored history. That also means the calculators work offline once the page is open, and the result updates as you type instead of after a submit button.

Where a calculation has a standard definition, the tool uses it: BMI is weight in kilograms divided by height in metres squared; loan payments use the standard amortisation formula; percentage change is the difference divided by the original value. The formula and a worked example are written out on each tool's page so the arithmetic can be checked by hand.

Rounding and precision

Money results are rounded to two decimal places only when displayed; the underlying arithmetic keeps full floating-point precision, so totals built from several steps do not drift by a cent. Health and unit results show two or three decimals, which is more than any measurement they are based on justifies — treat the last digit as noise.