Interest Rate Calculator
Find the annual growth rate that turned a starting amount into an ending amount.
Try an example
7.177%
About this calculator
How This Interest Rate Calculator Works
Enter a starting amount, an ending amount, and a time period, and the calculator solves backward for the annual interest rate that would produce that growth — useful when you know how an investment performed but not the rate it earned.
Worked Example
An investment that grew from $10,000 to $20,000 over 10 years earned an annual growth rate of 7.18% — not the 10% a quick "doubled in 10 years" guess might suggest, because compounding means a slightly lower annual rate still doubles the total given enough time.
The Formula
r = (End ÷ Start)^(1/years) − 1. This is the compound annual growth rate (CAGR) formula — it finds the single steady annual rate that, compounded every year, would take the starting amount to the ending amount over the given period, smoothing out any actual year-to-year variation into one average figure.
Why This Differs From a Simple Average
A simple average of yearly returns can overstate real growth, because it ignores compounding and the order returns happen in. CAGR instead answers "what constant rate explains the total change," which is the more useful number for comparing investments, businesses, or accounts over multi-year periods.
Worked example
$10,000 that grew to $20,000 over 10 years implies an annual growth rate of (20,000 ÷ 10,000)^(1/10) − 1 ≈ 7.18% — the constant compound rate that would produce the same result.
Frequently asked questions
Does this assume compounding?
Yes — it solves for the single annual rate that, compounded once a year, turns the starting amount into the ending amount over the given time.
Can I use this for an investment's average annual return?
Yes, this is the standard way to compute a compound annual growth rate (CAGR) from a starting and ending value.
What if the ending amount is less than the starting amount?
The calculator still works — it returns a negative rate, representing a loss over the period.