Every guide here works through a real money decision with actual numbers, charts, sources and a link to the free calculator that runs the same math. A new guide is added every day. Browse by topic, or go straight to the full list of calculators.
Student Loan Repayment in 2026: What Changed and How to Run Your Numbers
Federal student loan repayment changed in 2026: SAVE is gone, a new Repayment Assistance Plan began July 1, and many borrowers face a deadline this fall. Here is how to compare plans using real payment math.
What Was $100 Worth in 2000? Inflation, Calculated
Inflation turns prices into a time machine. Here is how to convert any past dollar into today's money, what the latest CPI says, and how fast your savings lose purchasing power.
Margin vs Markup: The Difference, the Formulas and a Conversion Table
Mixing up margin and markup is the most common pricing error in small business. A 50% markup does not give a 50% margin. Here are the formulas and a table that converts one to the other.
Sales Tax by State: Base Rates, No-Tax States and How to Calculate Total Tax
There is no national sales tax, so the rate depends on where you buy. Here are the base state rates, the states without one, how local taxes change the total and the formulas to add or remove tax.