To calculate principal reduction on a mortgage, subtract one month’s interest charge from your monthly principal-and-interest payment. The interest charge is your annual interest rate divided by 12, multiplied by your current principal balance. On a $200,000 balance at 6% with a $1,500 monthly payment, one month’s interest is $1,000, so $500 goes to principal. That $500 is the only part of the payment shrinking what you owe.
What You Need Before You Start
Three numbers drive the whole calculation, and all three should appear on your most recent monthly statement or your servicer’s online portal. Federal rules require mortgage servicers to show how each payment was split among principal, interest, and escrow, so the figures are not hard to find.1eCFR. 12 CFR 1026.41 – Periodic Statements for Residential Mortgage Loans
- Your current principal balance. Look for “outstanding principal” or “unpaid principal balance” on the statement. This is what you owe before any future interest accrues.
- Your annual interest rate, not the APR. The APR bundles in fees like origination charges, which makes it useful for comparing loan offers but wrong for the monthly interest calculation.2Consumer Financial Protection Bureau. What Is the Difference Between a Loan Interest Rate and the APR?
- Your total monthly payment for principal and interest only. If your payment includes escrow for property taxes or insurance, strip those amounts out. You need the base payment that goes to the loan itself.
On a fixed-rate loan, these three figures are all you need for every month of the loan’s life. The math stays the same from the first payment to the last. For an adjustable-rate mortgage, redo the calculation each time the rate changes.
Calculate the Monthly Interest Charge
Convert the annual rate to a monthly rate by dividing by 12. A 6% annual rate becomes 0.06 ÷ 12 = 0.005, or half a percent per month. This monthly periodic rate is the standard basis for amortized residential mortgages and most consumer installment loans.
Multiply that monthly rate by your current principal balance. On a $200,000 balance at 0.005 per month, $200,000 × 0.005 = $1,000. That $1,000 is the interest for the month. The lender keeps it as the cost of borrowing; none of it reduces your debt.
Some lenders, particularly on auto loans and certain mortgage products, use daily interest accrual instead. For those loans, divide the annual rate by 365, multiply by the current balance, then multiply by the number of days since the last payment. The difference from the monthly method is usually small, but if your payment date shifts by a few days, the interest charge and your principal reduction shift with it.
Subtract to Find the Principal Portion
Take your total principal-and-interest payment and subtract the interest you just calculated. What’s left is your principal reduction for the month. With a $1,500 payment and $1,000 in interest, $500 goes to principal. After that payment posts, the balance drops from $200,000 to $199,500.
This is where amortization starts working in your favor. Next month, the interest calculation runs on $199,500 instead of $200,000. At the same 0.005 monthly rate, that comes to $997.50 in interest and $502.50 in principal reduction. The payment stays the same, but a slightly larger share hits the balance each month. Early in a 30-year mortgage, interest eats most of the payment. By the final years, the split flips almost entirely toward principal.
How Extra Payments Change the Number
Any money you send beyond the required monthly payment adds directly to that month’s principal reduction. If the calculation above gave you $500 in principal reduction and you send an additional $300, your balance drops by $800 that month. The extra $300 is a dollar-for-dollar decrease in the debt because it isn’t subject to any interest charge in the month it’s applied.
Label the extra payment correctly. If you simply overpay without instructions, some servicers will advance your due date rather than apply the surplus to the balance. Fannie Mae’s servicing guidelines require servicers to apply additional principal payments immediately, but only when the borrower identifies the payment as a principal curtailment.3Fannie Mae. Processing Additional Principal Payments Write “principal only” on the check, select the principal-only option in the online portal, or call the servicer and confirm how to designate it. If you’re behind on payments, extra funds will typically be applied to curing the delinquency before any surplus reaches the principal balance.
Check your loan documents for a prepayment penalty before sending a large lump sum. Under the Truth in Lending Act, a loan that isn’t a “qualified mortgage” cannot carry a prepayment penalty at all. For qualified mortgages that do allow penalties, the loan must have a fixed rate and cannot be a higher-priced loan, and the penalty is capped at 3% of the balance in the first year, 2% in the second, 1% in the third, and zero after that. Adjustable-rate mortgages with prepayment penalties are prohibited under the same statute.4Office of the Law Revision Counsel. 15 USC 1639c – Minimum Standards for Residential Mortgage Loans Most conventional and government-backed loans issued in the last decade carry no such penalty, but it costs nothing to look.
When the Standard Formula Doesn’t Fit
Adjustable-Rate Mortgages
The formula itself doesn’t change on an ARM, but the inputs do. When the rate resets, the monthly interest charge jumps or drops, and the share of the payment going to principal shifts with it. A move from 5% to 7% sends a much larger chunk of the same payment to interest, and principal reduction shrinks. Recalculate using the new rate after every adjustment period.
Negative Amortization
Some loan structures allow minimum payments that don’t even cover the monthly interest. The unpaid interest is added to the principal balance, so the debt grows instead of shrinking. The Consumer Financial Protection Bureau describes this as paying interest on interest, which dramatically raises the total cost of the loan.5Consumer Financial Protection Bureau. What Is Negative Amortization? If your statement shows the balance climbing despite regular payments, run the calculation from this article. The interest figure will exceed the payment amount, confirming negative amortization. Qualified mortgages are prohibited from allowing principal increases by design, so the issue is largely confined to older loans and certain non-qualified products.4Office of the Law Revision Counsel. 15 USC 1639c – Minimum Standards for Residential Mortgage Loans
Check the Number Against Your Statement
Federal regulations require your servicer to send a periodic statement showing how each payment was divided among principal, interest, escrow, and fees, with year-to-date totals for each category.1eCFR. 12 CFR 1026.41 – Periodic Statements for Residential Mortgage Loans Run the calculation, then compare it against the statement. If the servicer’s interest figure is higher than yours, the gap usually comes from one of three places: a daily interest accrual method rather than monthly, late fees deducted before principal, or an escrow shortage that raised the total payment.
Small rounding differences of a few cents are normal. Anything larger is worth a phone call. On a 30-year mortgage, even a $20-per-month misallocation compounds into thousands of dollars over the life of the loan. You are also entitled to request the full amortization schedule, which shows the projected principal and interest split for every remaining payment. Comparing the servicer’s schedule against your own numbers is the most reliable way to confirm the loan is being serviced correctly.