Loan Interest Calculator
Calculate total interest paid, monthly interest charges, and effective cost of borrowing for any fixed-rate installment loan.
What this calculator does
The Loan Interest Calculator estimates your result instantly from the values you enter, using standard personal loans formulas. Calculate total interest paid, monthly interest charges, and effective cost of borrowing for any fixed-rate installment loan. 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 Loan Interest Calculator
The Loan Interest Calculator isolates the exact interest charges you pay across any personal, auto, or business loan, showing the cost of borrowing separate from principal repayment.
Formula & Mathematical Calculation
Monthly Payment = P × [ r(1 + r)^n ] / [ (1 + r)^n - 1 ]
Total Interest Paid = ( Monthly Payment × n ) - Principal (P)
Interest Percentage of Total Payout = ( Total Interest / Total Payments ) × 100
Calculation Example
$20,000 Loan @ 10% (36 Months, Payment = $645.34/mo): Total Repaid = $23,232.39 → Total Interest Paid = $3,232.39 (13.9% of total payments).
Key Benefits
- Calculates total lifetime interest charges on any loan amount
- Breaks down first month vs. last month interest amounts
- Shows interest as a percentage of total loan repayment
- Helps compare low-rate vs. high-rate financing options
- Simple and instant calculation
How to Use this Calculator
-
Enter Principal Amount
Input borrowed loan amount. -
Enter Interest Rate (APR)
Input annual interest rate percentage. -
Enter Term
Select loan duration in months or years. -
Calculate Interest
Click Calculate to view total interest paid.
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
- Loan Interest 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.