Mortgage Amortization Schedule

A mortgage amortization schedule is the breakdown of how each monthly payment splits between principal (money borrowed) and interest (cost of borrowing). Early in the loan, most of each payment covers interest; by the end, most covers principal. This front-loaded interest structure is why paying off a 30-year mortgage early saves substantial interest and builds equity faster.

The Math Behind the Split

The monthly payment on a fixed-rate mortgage is calculated using the amortization formula, which accounts for three variables: the loan amount (principal), the interest rate, and the loan term. The formula ensures that over the life of the loan, all principal and all accrued interest are paid off.

Each month, the calculation is straightforward:

1. Calculate interest owed for the month. Take the remaining balance on the loan and multiply by the annual interest rate, then divide by 12. On a $300,000 loan at 5% annual interest, the first month’s interest is ($300,000 × 0.05) ÷ 12 = $1,250.

2. The rest of the payment goes to principal. If the monthly payment (principal + interest combined) is $1,610.07 (a typical figure for these terms), then the principal portion is $1,610.07 − $1,250 = $360.07.

3. Subtract principal from the balance. The new balance is $300,000 − $360.07 = $299,639.93.

4. Repeat for the next month. Now the interest is calculated on $299,639.93. Because the balance is slightly lower, the interest owed is fractionally less, and the principal portion is fractionally more.

This pattern continues for 360 payments (30 years × 12 months). Early on, the gap between payment total and interest owed is small, so little principal is paid. As the balance shrinks, the same total payment now covers less interest and more principal, accelerating equity buildup late in the loan.

Why Early Payments Are Mostly Interest

The reason early payments are so interest-heavy is simple: the remaining balance is at its peak. Interest is calculated on the balance, so early on, the interest portion is large. The bank is front-loading its profit and collecting most of its interest upfront.

On a $300,000 loan at 5% over 30 years:

• Month 1: Interest = $1,250, Principal = $360, Balance = $299,640
• Month 12: Interest = $1,248, Principal = $362, Balance = $295,756
• Month 60 (year 5): Interest = $1,223, Principal = $387, Balance = $281,143
• Month 180 (year 15): Interest = $900, Principal = $710, Balance = $150,000
• Month 300 (year 25): Interest = $520, Principal = $1,090, Balance = $50,000
• Month 360 (year 30): Interest = $7, Principal = $1,603, Balance = $0

The shift is dramatic. In the final year, interest is negligible and nearly all the payment goes to principal.

The Amortization Schedule as a Table

Lenders provide a complete amortization schedule showing all 360 (or 180, or 420) payments. Here’s a sample of a $300,000 loan at 5% over 30 years:

Each payment is identical ($1,610.07), but the mix shifts over time.

Impact of Interest Rate and Term

The interest rate dramatically changes the shape of the amortization curve. A higher rate means more interest owed each month early on, so less principal is paid. A lower rate means less interest, so more principal is paid from the start.

The term (15 years vs. 30 years) also matters. A 15-year mortgage has larger monthly payments but much less total interest because the loan is paid off faster. On the same $300,000 at 5%:

• 30-year mortgage: ~$270,000 in total interest paid
• 15-year mortgage: ~$130,000 in total interest paid

The 15-year borrower saves nearly $140,000 in interest but pays roughly $300 more per month ($1,610 vs. $2,110).

Making Extra Principal Payments

Because of how amortization works, paying extra principal early in the loan saves substantial interest. If the borrower in our example pays an extra $100 per month toward principal:

• The balance drops faster, so subsequent interest calculations are on a smaller balance.
• Months are shaved off the end of the loan (instead of 360 months, perhaps 310 months).
• Total interest paid falls significantly.

If the extra $100 per month reduces the loan from 30 years to 25 years, the borrower saves roughly 5 years of interest—often $50,000 or more. This is why early prepayment is so powerful.

Amortization and Taxes

Homeowners who itemize deductions can deduct mortgage interest paid each year on their tax return (with limits depending on loan size and income). Because early payments are mostly interest, homeowners in their first 10 years of a mortgage can deduct a large portion of their payment. By year 20, when the principal portion dominates, the tax deduction is minimal.

Fixed-Rate vs. Adjustable-Rate Mortgages

The amortization schedule above assumes a fixed interest rate. On a fixed-rate mortgage, the payment and the split between interest and principal are predictable for the entire loan. On an adjustable-rate mortgage (ARM), the rate resets periodically, and the remaining balance is recalculated and re-amortized. If rates rise, the monthly payment rises and less principal is paid. Amortization still applies, but the schedule changes at each reset.

See also