Interest Rate Calculator
Determine the exact annual interest rate (APR) charged on a loan or earned on an investment based on principal, payment, and term.
What this calculator does
The Interest Rate Calculator estimates your result instantly from the values you enter, using standard investing & wealth formulas. Determine the exact annual interest rate (APR) charged on a loan or earned on an investment based on principal, payment, and term. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate for planning rather than financial advice.
About Interest Rate Calculator
The Interest Rate Calculator computes the implied annual interest rate (APR / Rate of Return) of a loan or investment given the initial principal, monthly payment, and repayment tenure.
Formula & Mathematical Calculation
Solves for r in: Monthly Payment = P × [ r(1 + r)^n ] / [ (1 + r)^n - 1 ]
Calculation Example
$20,000 Loan with $600/mo Payment for 36 Months: Total Repaid = $21,600 → Implied Annual Interest Rate = 5.14% APR.
Key Benefits
- Calculates actual interest rate from loan payments and principal
- Reveals true financing rates on dealer quotes and equipment leases
- Solves for nominal rate across any compounding frequency
- Helps identify hidden high-rate lending terms
- Instant mathematical solver
How to Use this Calculator
-
Enter Loan Principal (P)
Input initial borrowed amount. -
Enter Monthly Payment (PMT)
Input monthly payment amount. -
Enter Term (n Months)
Input total number of monthly payments. -
Calculate Interest Rate
Click Calculate to determine exact annual interest rate.
Assumptions & Limitations
What the calculation assumes
- Every value you enter is treated as an exact, known amount.
- Rates and contributions are assumed to stay constant for the whole period unless the form asks for them separately.
- Results use standard financial formulas and ignore fees, penalties and promotional terms that are not entered above.
- Inflation, tax changes and market volatility are not modelled unless a field for them is provided.
- Amounts are unit-agnostic: results are returned in the same currency you enter.
What it does not cover
- Interest Rate Calculator produces estimates for education and planning, not a quote, offer or professional advice.
- Real-world outcomes differ when fees, taxes, rounding rules or provider-specific terms apply.
- Local regulations and tax rules vary by country and change over time; verify current rules for your jurisdiction.
- Figures are only as accurate as the inputs you supply — check them before acting on the result.
- For decisions with lasting financial impact, confirm the numbers with a qualified professional.
Frequently Asked Questions
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