How Mortgage Amortization Works

The balance is what you owe; the payment is a cash flow

Principal is the amount borrowed that remains unpaid. Interest is the charge for borrowing that amount over a period. Amortization is the process of reducing the balance through scheduled payments until the loan is repaid.

For a fully amortizing fixed-rate loan, the scheduled principal-and-interest payment is calculated from the original loan amount, the note interest rate and the number of payments. The lender does not divide the original principal evenly and then add a constant interest charge. Instead, interest is calculated on the remaining balance. Whatever is left of the payment reduces principal.

A large payment does not necessarily mean a large reduction in debt. The interest portion must be accounted for first. Conversely, a late payment in the schedule can remove much more principal than an early payment of the same size.CFPB

Use the note rate and name the timing

The note rate determines interest under the loan contract. Mortgage APR is a broader annualized borrowing-cost measure that incorporates specified fees and charges. Substituting a disclosure APR for the note rate can produce the wrong principal-and-interest payment. MoneyBasis’s mortgage field asks for the annual note interest rate.

The planning model divides that annual rate by twelve. Each month, it applies interest to the opening balance and then subtracts the payment at month end. Real servicing statements can reflect different dates, rounding, escrow adjustments or contract terms. This is a monthly planning schedule, not a payoff quote.

At a zero note rate, there is no interest allocation: principal divided by the number of monthly payments gives the payment. If the down payment equals the purchase price, there is no mortgage balance to amortize. That does not remove property taxes, maintenance or insurance.CFPB

Worked example

Worked example: a $320,000 loan at 6.5%

Consider an illustrative $400,000 home with $80,000 down. The original loan is $320,000, the fixed note rate is 6.5%, and the term is 30 years, or 360 monthly payments. The calculated principal-and-interest payment is $2,022.62 when displayed to cents.

First-month interest is $320,000 × 0.065 ÷ 12 = $1,733.33. Subtracting that interest from the unrounded payment leaves $289.28 of principal. The next month begins with a smaller balance, so its interest charge is smaller too. No extra payment is needed for this change in composition; it follows from the scheduled amortization itself.

The annual snapshots below show payments made during each selected year, with the remaining balance measured after that year’s final payment. Total interest across the full term is $408,142.36 under these assumptions. Intermediate calculations retain precision; displaying every amount to cents can create small differences when rounded cells are added.

Escrow and ownership costs are separate

An escrow account collects money for bills such as property taxes and homeowners insurance. Those collections do not pay down loan principal. Even when principal and interest stay fixed, changes in those bills can change the total amount collected by the mortgage servicer.

For example, $4,800 of annual property tax and $2,400 of annual insurance add $600 per month to a planning estimate. Adding $100 of monthly HOA dues and $50 of monthly PMI makes the example’s entered monthly cost $2,772.62. HOA dues may be paid separately rather than through the servicer. Maintenance is another ownership cost and is not part of that mortgage payment estimate.

Private mortgage insurance protects the lender against specified loss; it is not insurance that pays the borrower’s mortgage. Its cost and cancellation conditions depend on the loan. MoneyBasis keeps an entered monthly PMI amount in the cost estimate; it does not automatically determine when PMI can end.CFPB · CFPB

Extra principal, recasting and refinancing do different things

An extra amount applied to principal reduces the balance sooner. With the scheduled payment otherwise unchanged, the loan can end earlier and incur less future interest. Sending extra money does not by itself establish that the lender will lower the required payment. Payment instructions and the loan contract determine how a real servicer applies it.

A recast generally recalculates scheduled payments over the remaining term after a principal reduction, while retaining the existing loan’s rate. Availability, minimum principal reduction and fees are lender-specific. Refinancing replaces the existing loan with a new one; the new rate, term and transaction costs change the comparison. Neither action is simulated by simply adding an extra payment to this amortization schedule.

A shorter term can also change both the payment and total interest. To isolate the term effect in the calculator, hold principal and note rate constant while comparing terms. Actual loan offers may quote different rates, so that controlled comparison is an explanation of the math rather than a prediction of available financing.CFPB · Fannie Mae

Read the schedule without confusing payment and cost

The principal column measures debt reduction. The interest column measures borrowing cost for that period. A year-end balance is a stock of outstanding debt, so it cannot be added to annual payments as though it were another annual expense.

The schedule assumes the stated fixed rate, timely monthly payments and no changes in the loan contract. It excludes tax benefits, sale expenses, investment opportunity costs and future changes in property expenses. Those omissions matter when comparing housing decisions, even though they do not change the principal-and-interest arithmetic shown here.