The Three Numbers That Define a Mortgage
- Principal: The amount you borrow - the price of the home minus your deposit.
- Rate: The interest charged on that principal, expressed annually.
- Term: How long you have to repay - commonly 25 to 30 years, sometimes 15.
Change any one and the monthly payment moves. A longer term lowers the monthly payment but raises the total interest paid; a higher rate does the reverse of lowering it.
Fixed vs Variable Rate
This is the central choice, and the terminology differs by country (in the UK a fix lasts a few years then reverts; in the US a 30-year fixed holds for the whole term).
- Fixed: The rate is locked for a set period. Your payment is predictable, which makes budgeting easy. You pay a small premium for that certainty and you do not benefit if rates fall.
- Variable / adjustable: The rate moves with the market. You may start lower, but your payment can rise - sometimes sharply. Better suited to those who can absorb a payment increase or expect to move or refinance before rates climb.
The honest way to choose: fixed if a payment increase would strain your budget, variable only if you have genuine room to absorb one. This maps directly onto the good-debt framework - a mortgage is good debt only while you can comfortably service it.
Amortization: Why Early Payments Are Almost All Interest
Amortization is the schedule that splits each payment between interest and principal. Because interest is charged on the outstanding balance, and the balance is largest at the start, your early payments are mostly interest with only a sliver going to principal.
On a 30-year loan, it can take a decade before you are paying more principal than interest each month. The balance barely moves in the early years, then falls faster and faster as more of each payment attacks principal - the compounding dynamic from how interest works, running in reverse.
This is why the total interest on a long mortgage can approach or exceed the amount borrowed.
The Power of Extra Payments
Because early principal is so hard to chip away, any extra payment in the early years is disproportionately powerful - every dollar of principal you pay off now avoids years of future interest on that dollar.
Even modest overpayments can knock years off the term and save a large sum in interest. The trade-off: money used to overpay a low-rate mortgage is money not invested or used to clear higher-rate debt first. Clear the expensive debt before overpaying the cheap one - the ordering logic runs through the whole debt payoff hub.