Mortgage Amortization Schedule Calculator

How an amortization schedule works

Every monthly instalment is split into two parts: interest on the current outstanding balance, and principal that pays that balance down. Because the interest charge shrinks as the balance shrinks, the principal share of a flat monthly payment grows every single month — this is what makes the schedule "front-loaded" with interest, even though the instalment itself never changes during a fixed-rate period.

Worked example

Take a $600,000 loan at a 3.00% rate over a 25-year tenure (n = 300 months). The monthly instalment works out to about $2,845. In month one, $1,500 of that is interest and $1,345 is principal. Because Singapore's current rates are relatively low, the principal portion overtakes the interest portion within roughly 24 months — well before the popular assumption that "the first half of the loan is mostly interest." Across the full 25 years you'd pay about $853,600 in total, of which roughly $253,600 is interest.

What changes the result

• Interest rate — a higher rate pushes the crossover point (where principal exceeds interest each month) much later in the schedule, and increases lifetime interest paid.
• Loan tenure — a longer tenure lowers the monthly instalment but spreads interest over more months, increasing total interest paid.
• Extra or lump-sum repayments — any amount paid above the scheduled instalment reduces the balance immediately, which compounds into lower interest on every future payment.
• Repricing or refinancing mid-way — switching packages generates a new amortization schedule based on the remaining balance, not the original loan amount.

Common mistakes to avoid

• Assuming principal and interest are split evenly every month — the split changes constantly.
• Comparing amortization schedules across two loans with different tenures without normalizing for that.
• Not accounting for the rate reset after a fixed-rate lock-in ends, which changes the schedule going forward.
• Ignoring how a partial repayment can be applied — check with your bank whether it shortens the tenure or reduces the instalment.

Want to see what happens if you make a lump-sum repayment or refinance partway through? Try the refinancing calculator to compare savings, or use the monthly instalment calculator to test different rates and tenures before committing.