Birthday Problem
About Birthday
What this tool does
General-purpose calculators cover percentages, dates, time differences, BMI, tips, taxes, salaries, loans, compound interest, and dozens of everyday math problems. Results are computed instantly in the browser.
Why use this tool
Quick calculations come up constantly: splitting a bill, figuring out how many days until a deadline, converting an hourly rate to an annual salary. Having a purpose-built calculator is faster and less error-prone than a spreadsheet.
How it works
Each calculator applies the relevant mathematical formula to your inputs. Financial calculators use standard amortization and compound-interest equations. Date calculators leverage JavaScript's Date object with timezone-aware arithmetic.
Pro tip
For loan and mortgage calculations, small changes in interest rate matter more than you think. Try adjusting the rate by 0.25% to see how it affects total interest paid over the life of the loan.
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