This estimator uses annual rates and contribution amounts for simplicity. You should use an interest rate assumption that is
conservative and plausable considering your likely investments. Historically, the stock market, or equities have returned about 10.5 % total earnings over long periods. Bonds have returned about 6% over long
periods. So, for example, if you are going to maintain a portfolio which is 50% stocks and 50% bonds until retirement, you might use 7.5% as a conservative estimate.