Interest rate calculator
Find the interest rate behind a loan payment or the growth rate between a starting and ending amount.
About the interest rate calculator
Find the interest rate behind a loan payment or the growth rate between a starting and ending amount.
Part of SmartCalcy’s investing & savings tools. It runs in your browser with no account, uses your own currency and units (change them at the top of the page), and gives estimates for information only, so check official guidance before you rely on a result.
How to use the interest rate calculator
What it calculates and when to use it
Finds the interest rate hidden inside a loan quote, or the yearly growth rate between a starting amount and an ending amount.
How to use it, step by step
- Choose a loan or an investment.
- For a loan, enter the amount, monthly payment and number of years.
- For an investment, enter the starting amount, ending amount and years.
What each input means
- Monthly payment – The fixed payment the lender asks.
- Starting and ending amount – What the investment was and what it grew to.
How to read the results
- Interest rate – Yearly rate compounded monthly (loan) or per year (investment).
- Effective yearly rate – The same rate with compounding included.
The formula
Loan: the rate r satisfies payment = amount × r ÷ (1 − (1 + r)^−n), solved numerically. Growth: rate = (end ÷ start)^(1 ÷ years) − 1.
A worked example
A 20,000 loan repaid with 400 a month for 5 years has a rate of 7.42% a year, an effective rate of 7.678% and total interest of 4,000.
Assumptions and limits
- Fixed payments with no fees.
- The growth rate is a compound average, not a guarantee.
Common mistakes
- Ignoring fees that raise the real rate (see the APR calculator).
- Using the total years when payments stop earlier.
Frequently asked questions
Why can no rate be found?
If your payments add up to less than the loan, the rate would have to be negative, so the calculator reports an error.
Related tools
Details
- Effective yearly rate
- 7.678%
- Total interest
- £4,000.00
- Total repaid
- £24,000.00