Guides
Twelve short explainers on the math behind the calculators. Each one ends with a link to the tool that does it for you.
- Percentage Calculator
How to Calculate a Percentage of Any Number
The one formula behind every percentage question, with worked examples you can check in seconds.
3 min read - Percentage Calculator
How to Calculate Percentage Increase and Decrease
Compare an old value to a new value correctly — and avoid the classic mistake of dividing by the wrong number.
3 min read - Percentage Calculator
How to Calculate a Discount and Final Sale Price
Work out what you actually pay and what you actually save at any percent off.
2 min read - Paycheck Calculator
How to Calculate Hourly Pay With Overtime
Turn a rate, regular hours and overtime hours into weekly, monthly and annual gross pay.
4 min read - Paycheck Calculator
How to Convert a Salary to Hourly Pay
The standard divisors for monthly, biweekly, weekly, daily and hourly equivalents.
3 min read - Loan Payment Calculator
How to Calculate a Loan Payment
The amortization formula behind every fixed monthly payment, explained plainly.
5 min read - Loan Payment Calculator
How to Read an Amortization Schedule
Why early payments are mostly interest, and what changes when you pay extra.
4 min read - Age Calculator
How to Calculate Your Exact Age in Years, Months and Days
How day-borrowing works, and how leap-year birthdays are handled.
3 min read - Date Calculator
How to Count the Days Between Two Dates
Inclusive versus exclusive counting, and why the answer can differ by one.
3 min read - Date Calculator
How to Calculate Business Days Between Dates
Skip weekends and your own holiday list to get a realistic delivery or deadline date.
3 min read - Rent Affordability Calculator
How Much Rent Can I Afford?
Where the 30% guideline comes from and when it should be lower.
4 min read - Price Markup Calculator
Markup vs Margin: The Difference That Costs Sellers Money
Same profit, two different percentages — and why confusing them shrinks your prices.
4 min read