買樓的利息計算?

P rate = Prime Rate = 最優惠利率
P-2.5% =最優惠利率減兩厘半
HIBOR rate = Hong Kong Inter-bank Overdraft Rate = 香港銀行同業拆息

P rate is the normal rate used by banks to lend money to customers. It would only be adjusted when the US revises its interest rate.
HIBOR is the rate used by banks to lend money to other banks or financial institution. It would fluctuate on real time basis (every second is not the same).

HIBOR is normally much lower than P-rate offer.
The market could offer HIBOR+0.65% ≈ 0.8%, while the 0.65% is the profit margin of the bank.
P-2.5% = 2.5% >> HIBOR+0.65%

As such, the HIBOR rate could provide a much lower interest rate for mortgage. However, as it could fluctuate unstably (could up to 200% in 1997), the risk of using HIBOR is higher. Somehow, bank would offer a cap on the interest rate, say P-1.5% = 3.5% > P-2.5%. The cap must be higher than the P-rate offer.

Example:
Premises price = $2.98million
Downpayment = $298,000
Insurance premium = 3.55%
Total Mortgage Loan = $2.98million x 90% x (1 + 3.55%) = $2,777,211
Loan Tenor = 30yrs
For P-2.5% = 2.5%
Monthly Repayment = $10,932.67, while the total interest expense = $10,932.67 x 360 - $2,777,211 = $1,158,550.48
For HIBOR+0.65%=0.8%
Monthly Repayment = $8,676.11, while the total interest expense = $8,676.11 x 360 - $2,777,211 = $346,188.37
For HIBOR+0.65% at CAP = P-1.5% = 3.5%
Monthly Repayment = $12,385.93, while the total interest expense = $12,385.93 x 360 - $2,777,211 = $1,681,724.34

From the figure, you could find the HIBOR rate offer could reduce the interest expense by approximate 60%. However, if the market climate changes and the HIBOR rate goes up, you may have to take the risk of using the CAP=P-1.5%. Depending on the 罰息期, you could now choose HIBOR rate offer. If the HIBOR goes up, you can then change your plan with P-rate provided by another bank.