How Mortgage Payments and Total Interest Are Calculated

Start with the amount actually borrowed

The property price is not normally the mortgage principal. Subtract the cash deposit or down payment first. A RM1,000,000 property with a 20% deposit leaves a principal of RM800,000. The distinction matters because interest is calculated on the outstanding loan balance, not on the property price.

A larger deposit lowers the starting balance and therefore usually lowers both the scheduled payment and lifetime interest. It also consumes more cash at the beginning, so the sensible comparison includes emergency savings, purchase costs and the opportunity cost of using that cash elsewhere.

• Property price: RM1,000,000
• Deposit: RM200,000 (20%)
• Starting mortgage principal: RM800,000

The fixed-payment amortisation formula

For a fully amortising fixed-rate loan, the monthly payment M is calculated as M = P × r × (1 + r)^n ÷ ((1 + r)^n − 1). P is principal, r is the monthly interest rate and n is the total number of monthly payments. An advertised annual rate of 4% becomes 0.04 ÷ 12 per month for this simplified model; 30 years becomes 360 payments.

The formula produces a level principal-and-interest payment when the rate and schedule stay unchanged. Real lender calculations may differ because of day-count conventions, rounding, rate resets, fees or payment timing. Treat a calculator result as a planning estimate, then compare it with the lender’s formal repayment schedule.

Worked example: RM800,000 at 4% for 30 years

Using the assumptions above, the estimated monthly principal-and-interest payment is RM3,819.32. Across 360 scheduled payments, the total paid is about RM1,374,956.05. Subtracting the RM800,000 principal gives estimated total interest of RM574,956.05.

This example shows why a manageable monthly figure can still carry a large lifetime cost. The result is not a prediction of future rates and does not include ownership or transaction costs.

Why early payments contain more interest

Interest for each period is based on the remaining principal. At the start, that balance is largest, so a greater share of the payment goes to interest. As principal falls, the interest charge normally falls and more of the same payment reduces principal. This changing split is the amortisation schedule.

For the example, the first month’s simplified interest is RM800,000 × 4% ÷ 12, or RM2,666.67. Only about RM1,152.66 of the RM3,819.32 payment reduces principal. The next month starts with a slightly smaller balance, so the interest component is slightly lower.

Costs the payment estimate may exclude

A mortgage calculator commonly estimates principal and interest only. Your housing budget may also need legal and valuation costs, lender charges, taxes or stamp duties, insurance, maintenance, service charges and changes in utility or commuting costs. Requirements differ by country, property and lender.

If the loan has a variable or adjustable rate, test more than one rate. A stress scenario helps show whether the payment would remain manageable if financing costs rise. Also check whether the quoted rate is nominal or effective and whether compulsory products affect the real cost.

• Compare the same loan amount and term across lenders.
• Check the effective rate or APR-equivalent disclosure, not only the headline rate.
• Ask whether extra payments reduce principal immediately and whether a prepayment penalty applies.
• Keep purchase costs and an emergency reserve outside the deposit calculation.

Compare rate and term sensitivity

A single estimate can create false confidence. Recalculate with the same RM800,000 principal at a rate one or two percentage points higher, particularly when a fixed-rate period is short or the product is variable. Record the payment increase in ringgit and as a share of reliable monthly income.

Then compare a shorter term at the expected rate. The payment will normally rise, but principal falls faster and fewer interest periods remain. This makes the trade-off visible: rate sensitivity tests resilience, while term sensitivity tests how much monthly flexibility you are exchanging for lower lifetime interest.

Do not use a low introductory rate for the entire term unless it is contractually fixed. If the loan can reset, model the post-introductory rate separately and confirm whether fees or refinancing assumptions are being hidden outside the calculation.

How to use the result responsibly

Run at least three scenarios: your expected deposit and rate, a higher-rate stress case, and a shorter term. Compare monthly payment, total interest and remaining cash—not one number in isolation. A shorter term usually raises the monthly commitment but reduces the number of interest-bearing periods.

Terbit Calculator provides an educational estimate, not affordability advice or a lender offer. Confirm the final payment, fees, insurance requirements and early-payment terms directly with the lender before making a commitment.