Amortization is the schedule that splits each fixed loan payment into two parts, the interest you owe the lender and the principal that pays down the balance, with the mix shifting toward principal over the life of the loan. Your payment stays the same every month, but in the early years most of it is interest, and only near the end does most of it go to principal.
In practice
Say you borrow $300,000 at 7% on a 30-year fixed loan. The level payment works out to about $1,996 a month. In month one, interest is the balance times the monthly rate: $300,000 × (0.07 ÷ 12), which is $1,750. That leaves only $246 going to principal. The next month interest is charged on a balance of $299,754, so it drops a few cents and a few more dollars go to principal. That drift repeats 360 times.
The effect is slow at the start and steep at the end. In year one you pay roughly $23,950 in payments and knock about $3,050 off the balance. You will not cross the point where principal outweighs interest in a single payment until somewhere around year 18. By the final year, almost every dollar is principal. The total interest over 30 years on that loan is about $418,500, more than the amount you borrowed.
Why it matters to a small landlord
Amortization decides how fast you build ownership and how much you can deduct. Only the interest portion of a mortgage payment is a deductible rental expense. The principal portion is not a deduction at all, it is you buying the asset, so it never belongs in your operating numbers. Mixing the two is the most common bookkeeping error I see, and it overstates both your write-offs and your expenses. The same logic keeps principal out of net operating income: NOI ignores financing entirely, while principal still leaves your bank account as real cash. If you want the deductible split done correctly, see tracking mortgage interest, principal, and escrow.
rents.ai keeps a per-loan amortization schedule and auto-splits each recurring mortgage payment into deductible interest, principal paydown, and escrow, though it does not connect to your lender, so you enter the loan terms yourself.
Amortization is also why your equity grows on its own schedule, since every principal dollar is equity you did not have to bring in cash. It is worth checking whether your loan even fully amortizes: some carry a balloon payment that leaves a large balance due years before the table runs out. And when you total a monthly payment, remember the schedule only covers principal and interest, not the taxes and insurance bundled into PITI.