Simple Interest Loan Calculator

by

Calculate simple interest on a loan using principal, rate, and time. See total interest, total payback (including optional fees), and an equivalent monthly amount.

USD
%
Simple interest means it uses the original principal for interest math (no compounding [interest earning interest]).
yr
mo
USD
This adds to your total cost. It does not change the interest math unless you choose “roll into loan” below.
Either way, total interest is the same for simple interest. This only changes how the payment breakdown is shown.
How to use simple interest loan calculator
  1. Enter the loan amount: This is the principal you borrow.
  2. Enter the simple interest rate: Use a percent like 7.5 (not 0.075).
  3. Enter the loan term: Fill in years and months (months must be 0 to 11).
  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.
Simple interest does not compound (interest does not earn interest). Many real-world loans use amortisation (interest calculated on a shrinking balance), so your lender schedule can differ even if the total cost is similar.
Methodology and sources
This calculator uses the standard simple interest model. “Simple” means interest is calculated from the original principal only, not from a changing balance [no compounding].
Inputs
  • P = loan amount [principal].
  • APR = simple annual interest rate entered as a percent (example: enter 7.5 for 7.5%).
  • 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 = APR / 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.
  • Currency is formatted in USD to two decimals. Small rounding differences can happen because JavaScript uses floating point arithmetic.
Sources
Did we solve your problem today?