Compound Interest Calculator
LiveProjection
Compound interest is the single most important math concept in personal finance, and it is also the most underestimated. The intuition most people start with is linear: if you save a dollar a year for forty years, you have forty dollars. The reality is that those forty dollars, if they earn even a modest return along the way, become a multiple of what you put in. The earnings start earning their own earnings, and the curve bends upward sharply somewhere around year fifteen.
This calculator shows the curve. Drop in a starting amount, a target annual return, a time horizon, and how much you can add each month. The output is your projected balance at the end of the period along with the split between what you contributed and what the market did for you. The chart makes the relationship obvious. In the early years the two lines are close together because contributions dominate. Later they diverge because compounding takes over.
A few rules of thumb help calibrate expectations. The historic long term return of a broad US stock market index has been around 10 percent before inflation, or roughly 7 percent after. Bond portfolios run lower, perhaps 4 to 5 percent. A diversified mix often lands between those numbers. Twenty percent is not a real long term return, no matter what one quarter or one year shows. Pick a rate you can defend with decades of evidence and adjust expectations from there.
Why compounding feels slow at first and then fast
In year one, a $10,000 balance earning 7 percent generates $700. Not life changing. In year ten, with steady monthly contributions, that same setup may be earning closer to $3,000 a year purely from interest. By year twenty the annual interest can exceed your annual contribution, which is the moment your money is genuinely working harder than you are. By year thirty the interest alone can dwarf the principal multiple times over.
The famous shorthand is the rule of 72. Divide 72 by your annual rate of return and you get the rough number of years for money to double. At 6 percent it takes 12 years. At 8 percent, 9 years. At 10 percent, just over 7 years. The rule is approximate but close enough to do back of the envelope math in your head.
The single most important variable
Time. Not return, not contribution. Time. A 25 year old who saves $200 a month for 10 years and then stops, with no further contributions, will usually end up with more money at 65 than someone who waits until 35 and then saves $200 a month for 30 years straight. The reason is the extra decade of compounding on the early principal.
This is also why the cost of waiting is so steep. Each year you postpone saving is not just one year of contributions missed. It is one fewer year of compounding on every dollar you ever save. The calculator makes this visible. Run it with a 30 year horizon and then with a 25 year horizon, keeping everything else the same. The five year delay can cost a third or more of your final balance.
Inflation and real returns
The number this calculator shows is nominal. It does not account for inflation, which steadily erodes what each dollar will buy. To estimate what your future balance is worth in today's purchasing power, run the result through the inflation calculator with your projected average inflation rate. For decisions about whether you are on track for a retirement target, the real return is the number that matters.
A common discipline is to use a target real return of 4 to 5 percent for planning. That builds in a margin against both lower than expected market returns and higher than expected inflation. It also means you contribute more, which is the variable you actually control.
Choosing inputs that match reality
For return: the long run real return of a global equity portfolio has been roughly 5 to 7 percent. For US large cap stocks specifically, around 7 percent real after inflation. Bond portfolios run 1 to 3 percent real. A balanced 60/40 portfolio lands near 5 percent real over long horizons.
For contributions: be honest about what you can sustain through a downturn. A contribution rate you cannot keep up during a recession is not really your rate. Aim low enough that you keep going during years 2008 or 2020, not just years 2017 or 2024.
For years: use your real horizon. If you are 30 and saving for retirement at 65, that is 35 years. If you are saving for a house down payment in 5 years, that is 5 years, and you should be choosing a much more conservative return assumption than 7 percent, because 5 years is too short for the stock market to be a reliable engine.
What this calculator does not capture
Taxes. Real accounts pay taxes on dividends, on rebalances, and on withdrawals, depending on the account type. Use this calculator to see your gross trajectory, then apply your account's tax treatment to get a more accurate net figure.
Fees. A 1 percent annual expense ratio sounds small but compounds against you the same way returns compound for you. Over 30 years it can shave 20 to 25 percent off your final balance. If you have any control over fees, push them down as far as possible.
Variable returns. Real markets do not return 7 percent every year. They return 30 percent one year and minus 25 percent the next. The calculator uses an average, which is the right tool for planning but not for predicting any particular year.
Frequently asked questions
It is interest earned on both your original principal and on the interest you have already earned. Each compounding period, the new interest is added to the balance, and the next period's interest is calculated on the larger balance. Over long time periods this snowball effect dominates the result.
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