Compound Interest Calculator

Compounding means the growth earned in one period is added to the balance and itself earns growth in the next period. A balance growing at a constant rate therefore rises on a curve rather than a straight line, and the gap between the amount deposited and the ending balance widens the longer the money is left alone. This calculator compounds monthly and adds deposits at the end of each month.

For a lump sum: A = P × (1 + r/n)^(n × t), where P is the starting balance, r the annual rate, n the compounding periods per year, and t the years. For recurring deposits the future value of an ordinary annuity applies: PMT × ((1 + i)^m − 1) ÷ i, where i is the monthly rate and m the number of deposits. This calculator applies both together.

The rate is an assumption you supply, not a projection produced by Equity Rank. Savings accounts and certificates quote a rate directly. For a diversified investment account, people often model with a long-run historical average and then re-run the calculation at a lower rate to see how sensitive the result is. Historical averages are not a prediction of any future period, and real results vary year to year and can be negative.

Inflation reduces what a future balance can purchase. Entering an inflation assumption deflates the ending balance into present-day purchasing power: a $1,000,000 balance in 30 years buys roughly what $412,000 buys today at 3% inflation. The nominal and real figures are both shown because they answer different questions.

Time is the input compounding is most sensitive to, because it appears in the exponent. Depositing $500 a month for 30 years at 7% produces materially more than depositing $1,000 a month for 15 years at the same rate, despite the second scenario contributing the same total. Change the years field above to see the effect on your own figures.

It raises the monthly deposit by a fixed percentage at the start of each year, which models a deposit that tracks pay rises. Entering 3% means a $500 monthly deposit becomes $515 in year two and $530.45 in year three. Leave it blank for a flat deposit.

Yes, and the calculator models it as a decline rather than as growth. The figure that would otherwise read as growth is labelled a loss and shown as a share of what was paid in, because a loss expressed as a share of the ending balance is not a readable number — at −6% a year it works out to −350%. One limit applies: a rate at or below −1200% a year is a monthly rate at or below −100%, which erases the balance every month or flips its sign, so no projection is produced for those and the calculator says so.

Yes. Both optional fields accept either sign and the calculator models what was entered rather than replacing it with zero. A negative deposit increase tapers the monthly deposit once a year, which models a contribution being wound down; at exactly −100% the deposits stop after the first year. A negative inflation figure is deflation, and the balance in today's dollars is then higher than the nominal balance rather than lower. Each field has one limit. A deposit increase below −100% would flip the deposit into a withdrawal every other year and, past −200%, grow without bound, and an inflation figure at or below −100% divides by zero or by a negative number, which produces an infinite or negative purchasing power. Neither is projected, and the calculator explains why instead of showing a figure.

No, and that is deliberate. The deposit is a nominal schedule: the dollars paid in each month are the dollars the account receives, so the amount needed to reach a stated balance is the same whatever inflation does. What inflation changes is what that balance will buy, so entering a figure in goal mode restates the goal itself in today's dollars — a $250,000 goal 25 years out buys roughly what $119,000 buys today at 3% a year. If the goal is meant to hold a certain amount of purchasing power, raise the goal until the figure in today's dollars reads what you intended, and the deposit will follow.

A blank starting balance or monthly deposit is treated as $0, and an entered 0 means exactly the same thing — either box can be zero, and the calculator projects from what is left. A negative amount is different and is not treated as zero. Because the ending balance, the amount deposited and the growth figure would all then follow from a scenario that was not entered, the calculator names the field and withholds the projection instead. The same rule applies to the savings goal in goal mode, to a rate box that holds no number, and to a horizon outside the 1-to-100-year range the calculator covers.

Both are answered rather than treated as missing information. A goal of $0 is already met before the first deposit — every starting balance the calculator accepts, $0 included, is at or above it — so the required deposit is $0 at any rate and over any horizon, and the calculator says so instead of asking for the goal again. A starting balance above a positive goal also solves to $0 a month, and the fourth figure then reports the amount you are above the goal by at the start. That figure is a surplus, not a loss: it is only labelled as lost to the rate when deposits overshoot the goal because the rate is working against the balance, which happens on a negative rate.

No. This calculator runs entirely in your browser. Nothing you type is transmitted to Equity Rank or saved anywhere, and no account is required to use it.