How to Use an Amortization Calculator

An amortization calculator saves you money by letting you test changes before you make them: enter your loan, add an extra payment, and read the new payoff time and interest next to the original. On a $340,000 loan at an assumed 6.25% over 30 years, adding $150 a month cuts the term to 25 years and saves $81,011.53 of interest. This guide shows the workflow, what each result means, and how to compare scenarios fairly. For how to read the schedule row by row, see the dedicated schedule guide; this post is about using the tool.

Step 1: enter the loan correctly

1. Loan amount: for a new loan, the amount borrowed; for a loan in progress, your current principal balance from the latest statement.
2. Annual interest rate: the note rate, not the APR.
3. Term: choose years or months. For a loan in progress, switch to months and enter the payments you have left.
4. Extra monthly payment and extra annual payment: leave at zero for the baseline run.

The baseline for the example, $340,000 at 6.25% for 360 months, is a payment of $2,093.44 and total interest of $413,637.85. In month one, interest is $340,000 x 0.0625 / 12 = $1,770.83, so only $322.61 of the payment reduces the balance. The calculator works out the payment itself, so it may differ by a few cents from the rounded figure on your statement.

Step 2: change one input at a time

The annual amount in this calculator is applied with every twelfth payment, starting with payment 13. The savings are not additive: $150 a month alone saves $81,011.53 and $3,000 a year alone saves $112,945.97, but together they save $155,185.39, not $193,957.50, because the second source of extra money works on a balance the first has already lowered.

Step 3: compare with the alternative of a different loan

Run the same loan with a shorter term to see whether extra payments are doing something a different loan could do. A 25-year loan at 6.25% has a payment of $2,242.88 and costs $332,862.77 in interest. Paying the 30-year loan's $2,093.44 plus $150 gives $2,243.44 a month and costs $332,626.32 in interest, only $236 less. The two are almost the same, with one difference: the extra payment is voluntary. If money gets tight, you can drop back to $2,093.44, which a 25-year loan would not allow. A 20-year loan costs $391.72 more a month than the 30-year one at the same rate (payment $2,485.16) and its interest is $256,437.41. Real shorter loans may also carry different rates, so enter the rate you are quoted.

Step 4: test rate and balance changes

Raising the rate to 6.75% on the same loan lifts the payment to $2,205.23 and total interest to $453,884.07. Adding $150 there shortens the term to 24 years 10 months and saves $92,077.26. A higher rate makes each extra dollar worth more, because it avoids more interest. To model a loan already in progress, balance and months remaining are enough. After 48 payments the balance here is $322,459.00 with 312 payments left. Entering that balance with 312 months and an extra $200 a month gives 254 further payments, which with the 48 already made is 302 in total, the same as running the whole loan with $200 extra starting at payment 49.

Step 5: pick the plan you can keep

A saving that you stop paying after a year is worth nothing. Rounding the payment up to $2,100, only $6.56 above the required amount, saves $4,511.29 of interest and ends the loan three months early. Choose an amount that fits your budget in a bad month, automate it, and re-run the calculator once a year.

Common mistakes

• Entering the original loan amount for a loan in progress. Use the current balance and the months remaining.
• Not specifying that extra money goes to principal. Confirm with your lender how to label it and check the next statement.
• Comparing runs that change several inputs at once. Change one thing per run.
• Forgetting prepayment terms. Some loans, especially non-mortgage loans, may charge a fee or compute interest in ways that reduce the savings. Read the loan agreement.
• Ignoring other uses of the money. Interest saved equals your loan rate, a certain return, but other uses such as an emergency fund or higher-rate debt may come first.

When the calculator's answer will differ from your statement

The tool assumes a fixed rate, monthly interest on the remaining balance and payments made on schedule. Adjustable-rate loans, daily-interest loans, escrow changes, fees and skipped payments all move real figures. Use the results as a comparison between scenarios, which is where the calculator is most reliable, and treat your lender's statement as the authority on the exact balance.