What the model assumes
The model keeps the original scheduled principal-and-interest amount after the lump sum, then adds the extra principal field to each monthly payment. The final payment is capped at the amount due. Interest is calculated monthly on the remaining modeled balance.
What is not included
- Escrow for property taxes or homeowners insurance.
- Servicing fees, late charges, deferred interest, or other unpaid charges.
- Prepayment penalties or loan-specific restrictions.
- Recasting, recast fees, or a servicer changing the required scheduled payment.
- Timing differences when a servicer receives and posts extra payments.
Confirm with your servicer
Ask how to designate an extra payment as principal-only, whether the loan has a prepayment penalty, and whether a lump-sum payment changes the required payment. Request a written payoff quote for the exact date you intend to satisfy the loan. The CFPB explains why a payoff quote differs from the current balance.
Read Mortgage Payoff Planning before committing a large amount of cash.
The calculator is educational and does not retrieve loan data, contact a servicer, or guarantee interest savings under a specific contract.