Free interest rate calculator
Find the implied interest rate on a loan or investment — enter principal, final amount, and time to solve for the annual rate, updated live, as you type.
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Simple interest only — does not compound. I = P × r × t.
Results are estimates. Consult a professional.
How the interest rate calculator works
This calculator works backwards from the result you have to find the rate that produced it. Given a starting value, an ending value, and a time period, it solves for the annual interest rate — either as a compound annual growth rate (CAGR) or as a simple interest rate depending on the context.
The CAGR formula is the most common choice for investments and savings accounts because it accounts for compounding. For simple loans or bonds that do not compound, the simple-interest rate formula is more appropriate. Both are shown below so you can apply the correct method.
Worked example: $5,000 grows to $8,500 in 6 years
Jordan invested $5,000 six years ago. The account is now worth $8,500. Jordan wants to know the average annual rate of return — the CAGR — that explains this growth, to compare it against other investment options.
Implied annual rate for $10,000 growing to various targets
The table shows the CAGR required for a $10,000 initial investment to reach various target values over 3, 5, 7, and 10 years. Use it to quickly benchmark whether a rate target is realistic.
| Target value | In 3 years | In 5 years | In 7 years | In 10 years |
|---|---|---|---|---|
| $12,000 | 6.27% | 3.71% | 2.63% | 1.84% |
| $15,000 | 14.47% | 8.45% | 5.92% | 4.14% |
| $20,000 | 26.00% | 14.87% | 10.41% | 7.18% |
| $30,000 | 44.22% | 24.57% | 17.01% | 11.61% |
$10,000 starting investment. Rates are CAGR = (Target/10,000)^(1/t) − 1. Source: CFA Institute TVM methodology; Federal Reserve rate benchmarks.
Tips for calculating and using interest rates
Solving for a rate is just as useful as solving for a future value. These tips help you apply the result correctly and avoid common pitfalls.
- Use CAGR to compare investments over different holding periods — a 50% total return over 7 years (≈5.99% CAGR) is very different from 50% over 3 years (≈14.47% CAGR). Always annualise returns before comparing.
- CAGR is not average return — if a portfolio gained 50% in year 1 and fell 33% in year 2, the average return is 8.5% but the CAGR is 0%. CAGR measures the actual path from start to end; arithmetic average of annual returns does not.
- For monthly data, multiply the per-period rate by 12 or compound it — the simple annualisation (× 12) gives a nominal annual rate; compounding ((1 + monthly r)^12 − 1) gives the effective annual rate (APY). The compounded version is more accurate.
- Benchmark against risk-free rates — the Federal Reserve's published rates give you the risk-free baseline. Any investment rate higher than the comparable risk-free rate implies that extra return comes with extra risk.
- Use the reverse calculation to set a savings target — if you need $30,000 from $10,000 in 10 years, the table shows you need an 11.61% CAGR. That sets a clear benchmark for asset allocation decisions.
Accuracy and limitations
This calculator solves for the implied annual rate using the CAGR or simple interest formula as appropriate. The CAGR assumes a single compounding period per year and a smooth, constant growth path between start and end values. It does not capture the volatility of the path — two investments with the same CAGR may have had very different year-by-year experience. It does not account for taxes, fees, dividends reinvested, or inflation.
Results are for educational and planning purposes only and do not constitute financial advice. Consult a qualified financial adviser before making investment decisions.
Interest rate terms defined
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