When you make your regular monthly mortgage payment, the lender allocates part to interest accrued that month and part to principal reduction. When you make an additional payment specifically designated as 'principal-only,' 100% of that extra amount reduces the remaining loan principal immediately.
Because interest for each subsequent month is calculated as (Remaining Principal x Monthly Interest Rate), reducing the principal balance early lowers the interest charged in every single future month for the rest of the loan term.
The compounding savings of extra payments
Consider a $300,000 30-year fixed mortgage at 6.5% interest. The standard monthly principal and interest payment is $1,896.20. Over 30 years, total payments equal $682,633, meaning you pay $382,633 in interest alone — significantly more than the original loan amount.
If you pay an extra $200 per month (total monthly payment $2,096.20), you cut the loan term from 30 years down to 23 years and 7 months — shaving over 6 years off your mortgage — and save $94,840 in total interest charges.
One-time lump sum vs recurring extra payments
A single lump-sum extra payment made early in the loan has a dramatic compounding effect. Paying a $5,000 lump sum on month 12 of the $300,000 mortgage above reduces total interest by $21,450 and shortens the loan by 10 months. The earlier in the loan term the lump sum is applied, the greater the total interest savings.
Whether to pay extra on your mortgage depends on alternative investments and interest rates. If your mortgage rate is 3% while high-yield savings or index funds offer 5-7%, investing may yield a higher net return; if your mortgage rate is 7%, paying down principal is a guaranteed risk-free 7% return on that capital.