Compound Interest

How to use this compound interest calculator

1. Enter your initial amount, monthly contribution, and interest rate.
2. Set the number of years to project.
3. See your final balance and a year-by-year breakdown instantly.

What is compound interest?

Compound interest is interest earned on both your original balance and the interest already accumulated. Over long periods, and with regular contributions, it can grow savings significantly faster than simple interest.

The math behind compound growth

Compound interest calculations rest on a straightforward idea, but the formula looks intimidating written out: each period's interest is calculated not just on the original principal, but on the principal plus every dollar of interest and every contribution added in previous periods. This calculator compounds monthly, applying (1 + monthly rate) as a multiplier to the running balance every month, then adding that month's contribution on top. The compounding frequency matters mathematically — dividing an annual rate by 12 and applying it monthly produces a slightly higher effective annual return than applying the full rate once a year, because interest starts earning its own interest sooner.

Why time matters more than most people expect

The defining feature of compound growth is that its curve isn't a straight line — it accelerates. In the early years, most of a portfolio's growth comes from contributions; in later years, most of it comes from the balance itself compounding, since a larger base generates more interest even at the same rate. This is why the number of years you let money compound tends to matter more than the exact monthly amount contributed: two savers depositing the same total amount, but starting a decade apart, can end up with meaningfully different final balances, because the earlier saver's contributions had more time to compound on top of themselves.

Nominal returns vs. real purchasing power

Every number this calculator shows is a nominal figure — the actual dollar amount your balance would reach, assuming the interest rate you entered holds constant for the entire period. It doesn't subtract inflation, so the purchasing power of that final balance will be lower than the number itself suggests if prices rise over the same years. A common approach is to estimate a "real" (inflation-adjusted) rate of return by subtracting an assumed average inflation rate from the nominal rate before running the projection, which gives a rough sense of growth in today's purchasing power rather than in future dollars.

Common mistakes when projecting compound growth

The single biggest source of unrealistic projections is assuming a constant rate of return for the entire period. Real investment returns vary year to year, sometimes significantly, and a rate that looks reasonable as a long-term average can still produce a very different outcome than a smooth projection suggests, especially over shorter timeframes where a few bad years can't be averaged out by enough good ones. It's also easy to forget taxes and fees when comparing a projected balance to a real account — an account subject to annual taxes on gains, or a fund charging an ongoing management fee, will compound more slowly than the raw interest rate implies.

Limitations of this calculator

This tool models a simplified, idealized scenario: a fixed interest rate, monthly compounding, and a constant monthly contribution for the entire period, with no taxes, fees, or withdrawals factored in. Real savings and investment accounts rarely behave this predictably — rates change, contributions vary, and unplanned withdrawals happen. Use this as a way to build intuition about how principal, contributions, rate, and time interact with each other, not as a guaranteed forecast of what any specific account will actually be worth.