Calculation method & accuracy

Check a mortgage payment by hand

Use a $300,000 balance, a 6.5% annual interest rate, 30 years remaining and $100 extra each month. Set yearly and one-time extra payments to zero. The regular principal-and-interest payment is $1,896.20, so the total sent each month is $1,996.20.

The monthly interest rate is 0.065 divided by 12. For the first payment:

1. Interest: $300,000 × 0.065 ÷ 12 = $1,625.00.
2. Principal within the regular payment: $1,896.20 − $1,625.00 = $271.20.
3. Add the extra $100, for a total balance reduction of $371.20.
4. New balance: $300,000 − $371.20 = $299,628.80.

The next month's interest uses that smaller balance. Round each month's interest to cents before subtracting it from the payment. The first three rows should look like this:

Repeating these steps gives 312 payments and $321,640.90 in total interest. The final payment is reduced to clear the remaining balance. Changing the first payment month changes the calendar dates, but not these totals when the only extra is the same amount every month.

Open this example in the mortgage calculator → Choose “By month” in the amortization schedule to compare the rows.

Choose the right model

Mortgage payoff: a remaining-term estimate

The mortgage calculator derives the payment from your current balance, fixed interest rate and years left. An existing loan's contractual payment can differ if you have made earlier extra payments, recast the loan or rounded the remaining term. If you need to model a known principal-and-interest payment, the loan calculator accepts that payment directly. Do not enter the escrow portion of your mortgage bill as a loan payment.

Monthly interest is rounded to the nearest cent, with half cents rounded up for positive amounts. Extra payments reduce the balance after that month's regular payment. The last scheduled payment clears any rounding remainder; over very long terms or at high rates, that final adjustment can be more than a few cents.

Loan payoff: your payment controls the schedule

The loan calculator can start with either a known payment or a number of payments left. If a regular payment does not cover interest, unpaid interest increases the balance before extra money is applied. For example, on $10,000 at 12%, a $50 regular payment plus $100 extra sends $150 in total: $100 covers the first month's interest and $50 reduces the balance. Under this model, that plan finishes in 111 payments with $6,561.68 in interest.

When you enter payments left, the last scheduled payment clears the remainder. With a known payment and no specified term, a remaining amount under $1 is folded into the final payment. The target-date payment rounds up to a whole cent; that calculation does not include a one-time payment. Results use monthly interest, so daily-interest loans, precomputed-interest loans, changing rates and fees can produce different outcomes.

Biweekly: payment handling matters

The biweekly calculator offers two models. “Applied as each half arrives” charges the annual rate divided by 26 each period and dates payments every 14 days, beginning on the first of the selected month. This is a periodic-interest approximation, not a simulation of a servicer's daily accrual.

“Held, extra sent once a year” models monthly payments plus one extra full payment after every 12 monthly payments. It does not reproduce every possible third-party withdrawal or remittance calendar. Ask your servicer when partial payments are credited, then choose the closest model. Program fees reduce net savings; they are not added to the loan balance.

PMI: a milestone is not an approval

The PMI calculator estimates conventional-loan milestones. Requested cancellation uses the 80% threshold; automatic termination uses the original schedule's 78% threshold or the month after the term's midpoint, whichever applies first. Extra payments can advance the request milestone, but they do not rewrite the original schedule used for automatic termination.

The CFPB explains the eligibility conditions, including being current, payment history and evidence of property value for requested cancellation. FHA, VA and lender-paid insurance follow different rules. A higher-value appraisal also depends on the loan investor's requirements. Confirm eligibility and the actual cancellation date with your servicer.

How results are checked

The calculators share one calculation engine. A separate reference implementation uses Python decimal and fraction arithmetic to check the JavaScript results. Both implementations are maintained as part of this project; this is an internal cross-check, not an independent professional certification.

• Payment-level checks: compare dates, interest, principal, extras and closing balances across more than two hundred schedules.
• Page-level checks: enter more than a thousand scenarios into the pages' JavaScript and compare the displayed payoff dates, interest savings, target payments and PMI milestones with the reference.
• Published examples: recalculate the numerical examples in the guides, including the benchmark on this page, and compare visible FAQ answers with their structured data.
• Boundary cases: cover zero interest, a zero balance, payments below interest, future lump sums, precise rates and the calculation range limit.

Agreement with the reference checks the stated model. It cannot establish that your lender uses that same model. The CFPB's explanation of payoff amounts describes why a dated payoff quote can differ from the balance on a statement.

Correction history

The October 14, 2026 calculator audit led to the following corrections. These describe changes to our calculations and interface, not changes to lending rules.

• Interest shortfalls: extra payments now cover unpaid interest before reducing principal. The calculator no longer assumes money that the user did not enter.
• PMI termination: remaining premiums and savings stop at the applicable earlier termination date. An explicit zero balance is treated as paid off.
• Input and scenario handling: malformed numbers are rejected, interest-rate precision is preserved, and the homepage passes the entered scenario to the full mortgage calculator.
• Payment timing and rounding: future lump sums are evaluated even after an interest-only first payment; half-cent rounding and final-payment counts were corrected. Loans beyond the calculation range are distinguished from payments that never reduce the balance.
• Worked-example timing: the loan guide's lump-sum example was aligned with the payment month used by the calculator.

Report a discrepancy

Send a correction report with the calculator name, the inputs needed to reproduce it, the result you expected and the result shown. Use rounded or fictional amounts if you prefer. Please do not send account numbers, login details or unredacted loan statements. A reproducible example helps us determine whether the issue is our math, an assumption or a lender-specific posting rule.