Lump Sum Payoff Savings

How Lump Sum Payoff Savings works

When a borrower makes a one-time payment toward principal on an instalment loan, the outstanding balance drops immediately, but the monthly payment typically stays the same. Because each payment covers less interest on a smaller balance, more of each instalment goes toward principal, shortening the loan term and reducing total interest paid. This calculator quantifies that benefit by simulating both the original amortisation schedule and the revised schedule after the lump sum is applied.

The formula

The tool first calculates the fixed monthly payment M using the standard amortisation formula:

M = B × [r(1 + r)ⁿ] / [(1 + r)ⁿ − 1]

where B is the remaining balance, r is the monthly interest rate (APR ÷ 12 ÷ 100), and n is months remaining. Total interest without a lump sum is M × n − B. After subtracting the lump sum from B, the code runs a month-by-month simulation: each period, interest accrues as (balance × r), then the fixed payment M is applied. The simulation stops when the balance reaches zero, recording cumulative interest and the number of periods elapsed. Interest saved is the difference between the two totals.

Where this method is most accurate

The calculation assumes a fixed-rate simple-interest instalment loan with no prepayment penalties, and that the lender applies the lump sum entirely to principal. It does not account for any fees charged to process the early payment, changes in APR, or contractual clauses that recalculate the monthly payment when principal is reduced. Results are most reliable when the lump sum is applied at the start of a billing cycle.

What this tool does not do

This calculator does not incorporate early-payment fees, account maintenance charges, taxes, insurance premiums, or any jurisdiction-specific consumer-credit regulations. It does not evaluate whether making the lump sum payment is financially optimal compared to alternative uses of the funds, nor does it model scenarios in which the monthly payment is re-amortised (lowered) rather than held constant. The tool produces a mathematical estimate; actual savings depend on the lender's processing rules and loan contract terms.

Disclaimer

This calculator is an educational tool that applies standard amortisation arithmetic to user-supplied figures. It does not constitute financial advice, does not recommend any payment strategy, and does not guarantee results will match those reported by any lender. Users remain responsible for verifying loan terms and consulting contract documents or financial professionals before making payment decisions.