How Is a Monthly Mortgage Payment Calculated?

Step 1: identify the amount you are actually borrowing

Start with the loan balance, not the property's full price. For example, a purchase price of 300,000 minus a deposit or down payment of 60,000 leaves 240,000 to borrow, before any fees added to the mortgage.

For an existing mortgage, use the outstanding balance and remaining repayment period. If a fee is financed, include it in the balance. A fee paid separately is a cash expense rather than part of the principal used in this calculation.

Step 2: convert the rate and repayment period

This example assumes the annual nominal interest rate is divided by 12 to obtain the monthly rate:

Monthly rate = 6 / 100 / 12 = 0.005

That is 0.5% per month, expressed as 0.005 in the formula. Next, convert the full repayment period into monthly payments:

Number of payments = 30 × 12 = 360

Use the borrowing interest rate, not a fee-inclusive APR or APRC unless the tool specifically requests it. Also check the compounding convention: dividing by 12 is not the correct conversion for every quoted annual rate. If the contract provides an effective annual interest rate, its equivalent monthly rate is (1 + annual effective rate)1/12 − 1, with the annual rate expressed as a decimal. This conversion concerns interest compounding, not the inclusion of fees in a disclosure rate.

Step 3: apply the monthly mortgage payment formula

For equal payments at the end of each month and a constant monthly interest rate:

M = P × r / (1 − (1 + r)−n)

• M: monthly principal-and-interest payment.
• P: loan balance, here 240,000.
• r: monthly rate as a decimal, here 0.005.
• n: payment count, here 360.

Substituting the example figures gives:

M = 240,000 × 0.005 / (1 − 1.005−360)

The result is approximately 1,438.92 per month. This is the loan repayment before separate taxes, insurance and other property costs.

You can check your own figures with our mortgage calculator. Enter the full repayment period, rather than just the length of an introductory fixed-rate deal.

Step 4: split the payment into interest and principal

The first month's interest is 240,000 × 0.005 = 1,200. Subtracting that from the payment leaves approximately 238.92 to reduce the loan. The next month's interest is calculated on the smaller balance.

Calculations retain full precision internally and round each displayed amount separately, so displayed components can differ by 0.01 when added or subtracted. A lender may round at different stages and adjust the final payment.

The payment stays level while the interest portion falls and the principal portion grows. Repeating this process produces an amortisation schedule: a record of how each payment changes the balance.

Why dividing the loan by 360 gives the wrong repayment

Dividing 240,000 by 360 gives approximately 666.67 in principal per month, before interest. In an equal-principal schedule, you would add each month's interest on the remaining balance to that principal instalment. The first payment would be approximately 1,866.67, with later payments falling as the balance decreases. That is a different repayment structure from the level-payment example above.

At a genuinely zero interest rate, dividing the balance by the payment count is sufficient. The standard formula needs that separate zero-rate case because its denominator would otherwise be zero.

Step 5: add costs outside principal and interest

The payment calculated above is not necessarily the amount collected by the lender or the full cost of owning the home. Depending on the country and product, you may also pay property taxes, buildings or homeowners insurance, mortgage insurance, association fees or service charges.

Imagine annual property tax of 2,400, annual insurance of 1,200 and a separate monthly property charge of 75. These illustrative expenses add 200 + 100 + 75 = 375 to the monthly budget. Together with the repayment, the budget estimate becomes 1,813.92 per month.

Converting annual expenses to monthly amounts helps with budgeting; it does not mean they are billed monthly or collected by your lender. The US Consumer Financial Protection Bureau's explanation of how lenders calculate mortgage payments also distinguishes the loan repayment from additional costs. Collection arrangements differ internationally.

What happens when the mortgage rate changes?

Where a product recalculates the payment after a rate change, the relevant inputs are the balance still owed, the new rate and the remaining repayment period. Reusing the original balance and original term would model a new loan rather than the current mortgage.

For a deal with an introductory fixed period, the initial calculation can use the full repayment schedule without guaranteeing that rate for the entire schedule. The contract determines when rates or payments may change. Some variable-rate products handle changes differently, so check the actual terms.

If the example rate stayed at 6% for all 360 payments, total repayments would be approximately 518,011.65, including 278,011.65 in interest. That is a constant-rate scenario, not a forecast of what a loan with future rate changes will cost.

Interest-only payments use a different calculation

Under the same simple monthly-rate assumption, an interest-only payment is P × r. The example would therefore cost 1,200 per month in interest, while the 240,000 principal remains outstanding unless separately repaid.

A smaller interest-only payment does not mean the whole debt is being cleared. Likewise, a balloon-payment arrangement may leave a substantial amount due at the end. The level-payment formula above describes a fully amortising loan only when payments continue for the complete repayment schedule at the assumed rate.

Check these details before relying on the result

• Use the outstanding loan balance and the correct remaining repayment period.
• Match the rate conversion and payment frequency to the calculator's assumptions.
• Separate the principal-and-interest payment from other monthly housing expenses.
• Account for financed fees, overpayments and possible future rate changes.
• Compare the estimate with the lender's illustration and your household budget.

A formula can estimate the payment for a specified loan; it cannot determine whether a lender will approve it or whether it fits all your commitments. Our mortgage affordability calculator offers a separate starting point for exploring borrowing, alongside your actual spending and savings needs.