Amortization is the schedule that splits each fixed mortgage payment between interest and principal. The payment never changes, but its composition does — and understanding that drift explains where your equity actually comes from and what a refinance really costs you.
The mechanics
Each month, interest is charged on the remaining balance; whatever's left of your payment reduces principal. Early on, the balance is large, so interest eats most of the payment. As the balance shrinks, interest shrinks with it and principal paydown accelerates. On a 30-year loan at 6.5%, roughly 85% of the first payment is interest; the crossover where principal exceeds interest doesn't arrive until around year 19.
Why it matters to a landlord
Principal paydown is real return your tenants fund — it just arrives as equity, not cash. A $240,000 loan at 6.5% builds about $2,700 of equity in year one and about $5,200 in year ten, from the identical payment. This is also the hidden cost of a refinance: a new 30-year loan resets you to the interest-heavy end of the curve, so compare total interest over your expected hold, not just monthly payments.
Reading your own schedule
Pull an amortization table for each loan (any calculator produces one from balance, rate, and term). Three numbers worth knowing per loan: current balance, principal paid per year at this point in the schedule, and total interest remaining. Extra principal payments jump you forward on the curve — a one-time $5,000 prepayment early in a 6.5% loan saves roughly $15,000 of interest over the life.
Worked example
$240,000 at 6.5%, 30 years: payment $1,517. Month one: $1,300 interest, $217 principal. Year 10: balance ≈ $203,000, and the split is now ≈ $1,096 interest, $421 principal. Same payment, twice the equity build — this is why long-held rentals quietly outperform their early-year cash flow.
When this rule of thumb breaks
Interest-only and balloon loans don't amortize — there is no equity build, so appreciation is your only value growth and the maturity date is a real risk. Aggressive prepayment isn't automatically smart either: money locked into a low-rate loan's principal may earn more deployed elsewhere, and it reduces your liquidity. And amortization math assumes you hold; if you sell in year three, you captured almost none of the curve's back-loaded benefits.