The compound interest formula
For a lump sum: A = P(1 + r/n)^(nt). Monthly contributions compound from the month they're added. The engine here simulates month by month, so contributions and any compounding frequency combine accurately.
FAQ
What rate should I use?
High-yield savings currently pay ~4–5%, the S&P 500 has averaged ~10% annually over decades (~7% after inflation). Match the rate to where the money will actually sit.
How powerful is starting early?
At 7%, money doubles roughly every 10 years — $10,000 at age 25 becomes ~$150,000 by 65, but only ~$76,000 if you start at 35. Slide the years and watch.