Interest Rate Calculator
Work backwards to the interest rate hiding in any growth: enter a starting amount, an ending balance, and the time elapsed to solve for the annual rate. Shows both the simple-interest rate and the compound annual growth rate (CAGR) so you can compare returns fairly.
How to Use
- Enter the present value — the starting amount or principal.
- Enter the future value — the ending balance you reached or expect.
- Enter the number of years between the two amounts.
- Read the simple annual rate and the compound rate (CAGR) the tool solves for.
- Use a preset such as "Double in 10y" to see the rate needed to hit a growth target.
Show Work
Formulas
About the Interest Rate Calculator
Need a hand with personal-finance and money decisions? The Interest Rate Calculator does the work for you — free, and right here in your browser. Work backwards to the interest rate hiding in any growth: enter a starting amount, an ending balance, and the time elapsed to solve for the annual rate. Shows both the simple-interest rate and the compound annual growth rate (CAGR) so you can compare returns fairly.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What is the difference between the simple and compound rate?
The simple rate spreads total growth evenly across the years with no compounding: (FV − PV) / (PV × years). The compound rate, or CAGR, assumes each year's growth builds on the last: (FV/PV)^(1/years) − 1. For multi-year periods the CAGR is the more honest measure of an annualized return.
What is CAGR and why does it matter?
CAGR (compound annual growth rate) is the single steady rate that would turn your starting amount into your ending amount over the period, as if it grew smoothly each year. It is the standard way to compare investments with different time horizons because it normalizes everything to an annual figure.
Can I use this to check an investment's return?
Yes. Put your original investment as the present value and its current or sold value as the future value, with the holding period in years, to get the annualized return. Note that it assumes a single lump sum with no added contributions or withdrawals along the way.
Why is the compound rate lower than the simple rate?
Because compounding does more work each year, a lower compound rate reaches the same ending balance that a higher simple rate would. Over longer periods the gap between the two widens.
Does this account for fees, taxes, or contributions?
No — it solves purely from the start amount, end amount, and time. Fees and taxes reduce real returns, and any deposits or withdrawals during the period would change the picture. For cash-flow streams, use an IRR calculator instead.
How do I use the Interest Rate Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Measuring an investment's annualized return
Enter what you paid, what it is worth now, and how long you held it to get the true CAGR.
Checking a savings or CD offer
Verify the effective rate implied by a "grow $X into $Y" promotion before committing.
Setting a growth target
Find the rate you would need to double or triple a sum in a given number of years.
Comparing two opportunities
Convert different gains over different time spans into comparable annual rates.
Sanity-checking a quoted rate
Confirm whether a lender or salesperson's headline figure matches the actual compound growth.
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