The Mortgage Math Every Borrower Should Understand

At a 2.50% mortgage rate, around 47% of your very first P&I payment already goes toward principal. For a borrower with a 10.00% interest rate, that share drops to just 5.0% on the first payment

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Every fixed-rate mortgage payment has a split between principal and interest (P&I), but the ratio isn't fixed. It shifts every month as your balance shrinks. This is due to amortization: because interest is charged on your remaining balance, and your balance shrinks with every payment, the dollar amount of interest owed shrinks too, leaving a growing share of each fixed payment to go toward principal. And how fast that shift happens depends heavily on your interest rate.

At a 2.50% mortgage rate, more than 47% of your very first P&I payment already goes toward principal, and within just over two years, principal payments overtake interest. At 6.50% mortgage rate, around today's ballpark rate, principal starts out at just 14% of the payment. Push the rate to 18.00%, a level not seen since 1981, and principal makes up 1% of your first payment.

Due to the underlying math/amortization, the pattern isn't quite linear. Small rate increases at the low end delay the crossover point dramatically, while equivalent increases at the high end barely move it. The basic math: at low rates, interest accrues slowly enough that principal chips away quickly. At high rates, interest dominates the balance for most of the loan's life, no matter how much higher rates climb from there.

This dynamic is one reason more borrowers in weaker pockets of the Sun Belt—where home prices have fallen from their Pandemic Housing Boom peaks—haven't ended up underwater: many locked in 2% and 3% mortgage rates before the mortgage rate shock. With fixed mortgage rates around 2% to 3%, a larger share of each monthly payment went toward principal from the very beginning. As a result, those borrowers have been paying down their loan balances much more aggressively than they would have under higher-rate mortgages.

Even in the Austin, Texas, metro area—where home prices have fallen more than 20% from their Pandemic Housing Boom peak in mid-2022—only 6.6% of mortgage borrowers in the Austin, TX metro are actually underwater (i.e., have negative equity).