What happens to $500,000 at 4% over 15 years?
If you invest $500,000 today at a 4% annual interest rate and leave it untouched for 15 years — with interest compounding annually — you end up with $900,471.75. That means your original principal earns $400,471.75 in compound interest, bringing your total return to 80.1% over the investment period.
The key driver is compounding: each year you earn interest not only on your original $500,000, but also on all the interest that has accumulated in prior years. In year one you earn $20,000.00, but by year 15 that annual interest payment grows to $34,633.53 — the same percentage applied to a much larger base.
At 4%, money doubles approximately every 17.7 years (Rule of 72: 72 ÷ 4 = 18.0). Over a 15-year horizon that translates to a 1.80x growth multiple.
These figures assume a constant 4% rate, annual compounding, and no withdrawals or additional deposits. Use the interactive calculator below to model monthly contributions, different compounding frequencies, or any custom scenario.