Retirement & investment calculators

Retirement math is compound interest math wearing a tax wrapper. Every calculator here is some version of "starting balance plus monthly contribution at rate R for N years", what differs is the tax treatment and the question being asked.

The big-picture question, "will I have enough?", is what the retirement calculator answers. It projects your nest egg to a target retirement age and converts it to a sustainable monthly withdrawal using the 4% rule (or your own rate).

For account-specific projections, the 401(k) calculator handles employer matching and pre-tax growth, while the Roth IRA calculator handles after-tax contributions and tax-free withdrawals. Use both: most savers want a mix.

For taxable accounts or general "what does $X invested for Y years become?" questions, the investment calculator and compound interest calculator are interchangeable, same math, different framing.

When you're targeting a specific number rather than projecting forward, the savings goal calculator reverses the question: "what monthly contribution gets me to $X by year Y?"

For simpler comparisons, simple interest shows what a non-compounding return looks like, useful as a baseline to see how much compounding actually adds.

The hardest input is the expected return. The S&P 500 has averaged ~10% nominal / ~7% real since 1928, but any 20-year window can fall well short. Run the projection at 5%, 7%, and 9% to see the range.