How Extra Principal Payments Save You Thousands in Interest
Explore the compounding math behind principal prepayments. Understand how hitting your principal early prevents banks from charging years of interest.
Most consumers view loans as simple monthly bills, but underneath is a complex compounding interest engine. When you take out a loan, the bank calculates your interest monthly based on the outstanding principal balance. By making extra principal prepayments, you attack this calculation directly.
The Reducing Balance Math
Every standard installment payment is split: interest is paid first, and the remaining amount reduces the principal. Because interest is computed as: Interest = Remaining Balance × (Annual Rate / 12), reducing your balance directly reduces next month's interest cost.
Any extra dollar you contribute bypasses this split completely. It goes 100% to principal reduction, decreasing the base balance for all future interest calculations. This stops the compounding cycle early, resulting in exponential interest savings.
Early Prepayment vs. Late Prepayment
Because interest compounding is exponential, prepayments made early in your loan cycle yield significantly more savings than prepayments made near the end. Shaving $1,000 off your principal in Year 1 of a 30-year mortgage saves more than making a $5,000 prepayment in Year 28.
Model various prepayment scenarios using our general Loan Interest Calculator or the home-specific Mortgage Extra Principal Calculator.