Reverse Compound Interest Calculator
This tool solves the compound interest formula backwards. Instead of asking what your money will grow to, tell it where you want to end up (or where you ended up) and it solves for the starting amount, the rate you actually earned, or how long it will take. Same formula as every other calculator on this site, just rearranged.
Time
Assumes the ending amount is greater than the starting amount, with no additional contributions along the way.
When you'd actually solve it this way
Most people meet compound interest going forward: start with this much, earn this rate, end up with that much. These three questions run the other direction, and each one is a real thing people ask.
"How much do I need to start with?"
You have a number in mind, say $50,000 in 10 years, and a rate you expect to earn, say 6% compounded monthly. Set Solve for to Starting amount, enter $50,000 as the ending amount, 6% as the rate, and 10 as the years. The answer is about $27,481.64: that's what you'd need today, assuming nothing else gets added along the way.
"What rate did I actually earn?"
An account went from $5,000 to $6,000 over 3 years. It's tempting to divide the $1,000 gain by 3 years and call it a 6.67% annual return, but that ignores compounding. Set Solve for to Annual rate with those four numbers (compounded monthly) and the real answer is about 6.09% a year. The naive method overstates it, because part of the third year's growth came from interest the account had already earned in years one and two.
"How long will this take?"
$10,000 growing to $20,000 at 7% compounded monthly, for instance, takes about 9.93 years. That's the default loaded into the calculator above, and it's a variation on the same doubling question the Rule of 72 estimates with a mental shortcut. This tool gives the exact answer instead of an approximation.
Built by Cedrick Reese. This is the same compound interest formula used across the site, algebraically rearranged to isolate P, r, or t instead of A, and checked against the U.S. Securities and Exchange Commission's methodology at investor.gov. Every result on this page is computed live and verified by plugging it back into the forward formula. Last reviewed: September 19, 2026.
Frequently asked questions
How do I find out what interest rate I actually earned?
Set Solve for to Annual rate, then enter your starting balance, your ending balance, how long it took, and your account's compounding frequency. The calculator solves r = n × ((A/P)1/(nt) − 1). An account that grew from $5,000 to $6,000 in 3 years compounded monthly worked out to about 6.09% a year, not the 6.67% you'd get from naively dividing the total gain by 3 years.
How much do I need to start with to reach a savings goal?
Set Solve for to Starting amount, enter your goal as the ending amount, your expected rate, and your time horizon. To reach $50,000 in 10 years at 6% compounded monthly, for example, you'd need to start with about $27,481.64 today, assuming no further contributions.
Is this the same math as the main compound interest calculator?
Yes, exactly the same formula, A = P(1 + r/n)nt, just rearranged to isolate a different variable. The forward calculator answers what will my money grow to; this one answers what do I need, what did I earn, or how long will it take.