How the Simple Interest Loan Calculator works
Most loans you'll encounter — mortgages, auto loans, most personal loans — use amortized interest, recalculated every month on your remaining balance. Simple interest is different: it's calculated just once, up front, on the full original principal for the entire loan term, with no compounding.
Because the total interest is fixed from day one, this calculator can find it directly, then divide the total repayment evenly across the term to get an equal monthly installment. There's no month-by-month amortization schedule to build, since the interest doesn't depend on how much balance remains.
The chart above shows the balance declining in a straight line from the full loan amount to zero, since each equal installment reduces it by the same amount every month — unlike an amortized loan, where the balance falls faster later in the term as more of each payment shifts to principal.
A worked example
On an $8,000 loan at 9% for 3 years, simple interest is P × r × t = 8,000 × 0.09 × 3 = $2,160. Total repaid is $10,160, or about $282.22 a month if split evenly.
| Simple interest | Amortized (same rate/term) | |
|---|---|---|
| Total interest | $2,160.00 | $1,158.32 |
| Monthly payment | $282.22 | $254.40 |
Add-on simple interest charges the full amount on the original balance for the whole term, so it costs more than an amortized loan where interest is recalculated on a shrinking balance. True simple-interest daily loans (common for auto loans) behave differently — interest accrues on the current balance each day, so paying early saves money.
How to read your results
Total interest is fixed up front under the add-on method: 8,000 × 9% × 3 years. Total to repay is principal plus that interest. There is no amortization schedule because interest is not recalculated month to month.
When to use this calculator
Use it for short-term or add-on loans quoted as “simple interest,” some buy-here-pay-here auto financing, and personal IOUs. To compare against a bank installment loan, run the same numbers through the Loan Payment Calculator and compare total interest.
Common mistakes to avoid
- Assuming “simple interest” means cheap. Add-on simple interest is often more expensive than an amortized loan at the same stated rate.
- Expecting an early-payoff discount. Under the add-on method the interest is already baked in; check whether the Rule of 78s or a rebate applies.
- Mixing up add-on and daily simple interest. They produce very different costs.