Simple Interest Loan Calculator

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Calculate simple interest when the principal stays unchanged for the full term. See total interest, total payback with optional fees, and a monthly equivalent, not a lender repayment schedule.

USD
%
Uses the same principal for the full term, including financed fees. Enter the annual interest rate for this model, not a fee-inclusive APR.
yr
mo

Years and months add together. For months only, leave “Loan term (years)” blank or enter 0.

USD
This adds to your total cost. It does not change the interest math unless you choose “roll into loan” below.
Both choices use the same total interest in this fixed-principal model. They show the interest as one total or an average per month, not a declining-balance repayment schedule.
How to use simple interest loan calculator
  1. Enter the loan amount: This is the principal you borrow.
  2. Enter “Annual interest rate”: Use the rate for this fixed-principal model as a percent like 7.5 (not 0.075), not a fee-inclusive APR.
  3. Enter the loan term: “Loan term (years)” and “Loan term (months)” add together. For a 60-month loan, enter 60 in “Loan term (months)” and leave “Loan term (years)” blank or enter 0. Use whole numbers; the combined term cannot exceed 600 months (50 years).
  4. Optional: open Advanced options: Add upfront fees if you want the “all-in” cost. If the fees are financed (rolled into the loan), switch “treat them as added principal” to Yes so the interest is calculated on principal + fees.
  5. Click Calculate: You will see total interest (P x r x t), total payback, and an equivalent monthly payment (principal plus interest divided by months; separate upfront fees are excluded).
  6. Use “More details” for the breakdown: It shows the principal used for the interest math, fees, and totals.
Many loans calculate simple interest on the remaining balance as payments reduce it. For periodic payments on a declining balance, use the Amortization Calculator; actual lender schedules can also depend on payment dates and contract rules.
Methodology and sources
This calculator holds P_used unchanged for the full term and applies I = P_used * r * t without interest on interest. It does not model the declining balance of a loan repaid in installments.
Inputs
  • P = loan amount [principal].
  • annual_rate = “Annual interest rate” entered as a percent (example: enter 7.5 for 7.5%). This is not a fee-inclusive APR.
  • n = term in months, where n = 12 * years + months.
  • fee = upfront fees (optional).
  • “Fees financed” toggle:
    • If “Yes”: fees are treated as added principal for the interest calculation.
    • If “No”: fees are added after the interest calculation as a separate cost.
Conversions
  • Rate as a decimal: r = annual_rate / 100.
  • Time in years: t = n / 12.
  • Principal used for interest math:
    • If fees are financed: P_used = P + fee.
    • Otherwise: P_used = P.
Core calculations
  • Simple interest: I = P_used * r * t.
  • Principal + interest: (P_used + I).
  • Total payback shown:
    • If fees are financed: Total = P_used + I (fees are already inside P_used).
    • If fees are separate: Total = (P_used + I) + fee.
  • Equivalent monthly payment shown: Monthly = (P_used + I) / n.
    • If fees are separate, this monthly figure does not include the fees [because fees are not part of the loan balance in the simple-interest math].
Display notes and limitations
  • The “When interest is charged” option does not change the simple interest total. It only changes the explanatory text in the details panel.
  • This is not an amortized loan model. It does not compute a month-by-month balance where interest is recalculated on a declining principal, a lender repayment schedule, or a fee-inclusive APR.
  • Currency is formatted in USD to two decimals. Small rounding differences can happen because JavaScript uses floating point arithmetic.
Sources
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