How we calculate
Every one of our 104 calculators answers a planning question with arithmetic you could do yourself on paper, given enough time. This page explains which formulas we use, where the outside numbers come from, how we check our work and when we update it, so you can judge how far to trust a result before you act on it.
The core formulas
Growth calculators compound monthly. A yearly return is converted to an equivalent monthly rate, each month's deposit is added at the end of the month, and the balance grows until it reaches your goal. That is the same method behind the year-by-year tables and the growth charts, which simply print the balance at each step instead of only the final answer. When a tool offers a "real" or inflation-adjusted view, we subtract inflation from the return in the same way, so the result is shown in today's dollars.
Loan and debt tools use standard amortization: each month interest is charged on the remaining balance, your payment covers that interest first, and the rest reduces the principal. Credit card tools follow the common rule of a minimum payment set as a percentage of the balance with a dollar floor, and they flag when a payment is too small to ever clear the debt. Retirement tools work in the other direction, drawing a monthly amount from a balance that keeps growing, and report how long the money lasts.
Where outside numbers come from
Some tools need real-world data, and each of those pages lists its sources at the bottom. Inflation figures use the US consumer price index (CPI-U) published by the Bureau of Labor Statistics, through September 2026. Historical market returns, used for the S&P 500 and 4% rule backtests, are yearly total returns including dividends for the S&P 500 and the 10-year US Treasury bond from the dataset maintained by Aswath Damodaran at NYU Stern, through 2025. Net worth percentiles come from the Federal Reserve's Survey of Consumer Finances, weighted the same way the Fed weights its own tables.
Country comparisons use World Bank price level data from the International Comparison Program, which compares the cost of the same basket of goods across countries, carried forward to 2025 with each country's inflation. Rents and local currency figures are converted with exchange rates checked on October 28, 2026. Because averages hide big differences between cities and lifestyles, these tools are best for comparing countries with each other rather than for setting an exact budget.
How we check our work
Before a calculator goes live, we run every mode through an automated test that tries typical inputs and edge cases, such as a zero interest rate, a goal you have already reached or a payment that can never clear a loan, and we compare the results against hand calculations. Every number quoted in a guide is recalculated with the same formulas the calculators use, so the article and the tool always agree. When a guide cites a statistic, it links to the original source rather than to a summary of it.
What the results can and cannot tell you
A projection is only as good as its assumptions. Markets do not deliver the same return every year, prices and exchange rates move, and tax rules change, so we show our assumptions on each page and let you change them. Treat the answers as a way to compare options and to see which levers matter most, not as a promise. For decisions that depend on your full situation, a qualified professional can help.
Updates and corrections
We review data-driven tools whenever their source publishes new figures, and at least every three months. The date at the top of each calculator and guide shows when it was last checked. If you find a mistake, please write to hello@moneyyears.com and we will fix it and note the change.