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Flat Rate Interest Calculator

Enter the principal, annual flat rate, and term in years to find the total interest and payment.

How to use

  1. Enter your values in the fields above.
  2. Press Calculate to see your result instantly.
  3. Use the Share button to copy a link to your result.

About this calculator

Flat rate interest (also called add-on interest) charges interest on the full original principal for every period of the loan term, instead of on the declining balance as principal is repaid — the interest amount is fixed at the outset and simply added to the principal to set the total repayment, which is then divided evenly into monthly installments.

Because the balance actually owed keeps shrinking as you pay while the flat rate keeps charging interest as if it hadn’t, the true effective interest rate (APR) on a flat-rate loan runs roughly double the stated flat rate — a loan advertised at a flat 10% typically has an effective rate close to 18–20%. This calculator takes the principal, annual flat rate, and term to show the total interest charged and the resulting monthly payment.

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