Calculate how many months earlier you can pay off your mortgage and how much interest you save by paying extra principal each month; compare the original and accelerated plans.
Input Data
Results
At a glance:Mortgage acceleration means paying extra principal regularly to retire the loan sooner and reduce total interest under a reducing-balance schedule.
Formula
basePayment = P × r·(1+r)^n / ((1+r)^n − 1) (r = annualRate%/12, n = years×12)
Apply the accelerated payment to an amortisation schedule to find monthsToPayoff and interestSaved.
How to Use
- Enter the mortgage principal.
- Enter the annual rate and original term.
- Enter the extra monthly principal.
- Review the payoff months and interest saved.
FAQ
How does paying extra actually save interest, and why is the effect so large?
The key is that mortgage interest is charged on the remaining principal each month. Every month the interest = current balance × monthly rate, so the faster the principal falls, the less interest you pay later, and the saved interest lets even more of each payment go to principal — a virtuous cycle. Early in a normal schedule the balance is large, so interest eats most of the fixed instalment and only a little repays principal; acceleration adds a chunk of pure principal each month that skips all the future interest that principal would otherwise have generated. Example: a HK$3,000,000 mortgage at 3% over 30 years has an original payment of about HK$12,648; adding just HK$2,000 a month pays it off in about 288 months instead of 360, saving about HK$344,276 and finishing 6 years earlier. Because what you save is the compounding long-term interest on the repaid principal, the effect grows with term and rate.
Is paying a little extra each month or one lump sum better?
Both save interest and shorten the term — the principle is the same, only the cash-flow pattern differs. Regular extra payments spread the burden evenly across the month and build a habit without needing a big lump; that is this calculator's model and suits people with steady monthly surpluses. A one-off lump sum (a bonus, inheritance, investment gain) slashes the balance and future interest immediately and is ideal when you receive intermittent large sums. In practice you can combine both. Before choosing, confirm two things: the prepayment terms (some mortgages cap the annual prepayable amount or charge a penalty within the lock-in period), and the opportunity cost/liquidity — money put into the mortgage cannot easily be taken out and forgoes other uses.
What should I watch out for when using this calculator and actually accelerating?
Several points. First, this tool assumes a fixed rate, but most Hong Kong mortgages float (H-Plan tracks HIBOR, P-Plan tracks Prime) and will move with the market, so treat results as an estimate at the current rate. Second, beware the penalty period: many mortgages charge a penalty (often a percentage of the loan or a clawback of the cash rebate) if you make large prepayments or refinance within the first 2–3 years. Third, ensure the extra payment is applied to reduce principal, not treated as a prepayment of future instalments — you usually must tell the bank your intent. Fourth, keep liquidity: once money goes into the mortgage it is hard to get back out without refinancing and a fresh stress test, so keep an emergency fund first. Fifth, the month-by-month simulation may differ slightly from the bank's actual billing (interest dates, rounding). This tool is for teaching and estimation, not mortgage advice.
Before accelerating, what must I confirm, and could it backfire?
Three things first, or it could cost you a penalty or sacrifice liquidity. One, the penalty period and prepayment terms: many mortgages charge a penalty or claw back the cash rebate for large prepayments within the first 2–3 years, and some cap the annually prepayable amount — read the contract (use the Mortgage Penalty Calculator to estimate the cost). Two, liquidity and emergency cash: keep 3–6 months of expenses aside before sinking spare cash into repayments. Three, opportunity cost: if your mortgage rate is low and you can earn a higher, reliable return elsewhere, or have more pressing financial goals, accelerating may not be urgent. Only when there is no penalty, your emergency fund is adequate and there is no better use for the money is acceleration a sound interest-saving choice.
How do I make sure the extra payment reduces principal rather than prepaying future instalments?
This is crucial: the saving only happens if the extra is counted as principal reduction, not as a prepayment of future instalments (which would not lower the balance and would save nothing). In practice: first, tell the bank explicitly that the extra is a partial prepayment to reduce the principal balance; second, confirm the post-payment arrangement — usually either 'shorten the term, same payment' (most saving) or 'lower the payment, same term' (more cash flow, less saving), and this calculator uses the former; third, after paying, check the statement to confirm the balance has indeed dropped; fourth, mind the penalty period and the annual fee-free limit. Bank procedures vary, so confirm with your lender before acting.
Content review: Calculatorism Finance Team. Results are for reference only; please refer to the relevant authorities for the official figures.