The short version
- Earlier principal reduction usually saves more interest.
- A lender may reduce EMI, shorten tenure or apply another treatment.
- Prepayment fees and liquidity costs must be included.
- The final installment should be capped so the model never creates a negative balance.
Why prepayment saves interest
Reducing-balance interest is calculated on outstanding principal. An extra payment reduces that balance earlier, so future interest is calculated on a smaller amount. The earlier the reduction, the more periods are affected.
The saving is not simply the prepayment amount multiplied by the original rate. The amortization schedule must be recalculated month by month because each later principal and interest component changes.
Recurring extra versus one-time payment
A recurring monthly extra creates steady acceleration and may be easier to budget. A one-time prepayment produces a larger immediate balance reduction. Both should be applied after scheduled interest and principal in a transparent schedule.
If the extra payment would exceed the remaining balance, the last payment must be reduced to the amount actually needed. A trustworthy calculator must not report negative closing balances or extra interest after payoff.
Check the lender’s treatment
Confirm whether the lender shortens tenure, reduces EMI, charges a prepayment fee, imposes a minimum amount or restricts timing. A calculator’s default of keeping the scheduled EMI and shortening tenure may not match the contract.
For floating-rate loans, model the expected rate reset as well as prepayment. The apparent saving can change when the future rate changes.
Do not ignore liquidity
Paying debt early can reduce interest, but it also uses cash that may be needed for emergencies, taxes or business operations. Compare the interest saving with the prepayment fee and the value of retaining liquidity.
The useful output is therefore a set of figures: interest saved, months saved, payoff date, fees and total cash outflow.
Where to check the details
For an important payroll, filing or financial decision, open the source itself and check whether anything has changed since this guide was updated.
- Nepal Rastra Bank — regulatory directives
Check current directives and the lender’s approved terms.