Adjustable-Rate Mortgage (ARM) Calculator
Estimate the monthly instalments, outstanding balance and total interest of an adjustable-rate mortgage (ARM) across the initial fixed-rate period and after the rate reset.
輸入資料
計算結果
重點速覽：An Adjustable-Rate Mortgage (ARM) is a mortgage that is 'fixed initially, then floating' — for example, a 5/1 ARM fixes the rate for the first 5 years and then adjusts with the market. This calculator uses a simplified two-stage model: the initial period applies one rate, and the post-reset period applies another until the end of the term. The initial instalment is computed by amortisation using the initial rate over the full term; the loan is amortised to the end of the fixed period to derive the outstanding balance; the post-reset instalment is recomputed on that balance at the adjusted rate. Example: a HK$3,000,000 mortgage over 30 years, initial rate 3%, fixed for 5 years, then reset to 5%, gives an initial monthly payment of about HK$12,648.12; after 5 years, when the rate rises to 5%, the payment for the remaining 25 years rises to about HK$15,592.13. The initial ARM payment is usually lower than a fixed-rate mortgage, but can rise significantly when rates climb. WARNING: This model assumes a single reset that then stays fixed; it does not account for rate caps, multiple resets or index spreads. Actual payments are subject to the lender's contract and market rates. Hong Kong mortgages are mostly linked to HIBOR / Prime rate, which is similar in nature.
計算公式
初期供款：以初期年利率、全期期數 (年期 × 12) 按等額本息計算。
逐月攤還初期固定利率期，得出固定期屆滿時的剩餘本金。
調整後供款：以調整後年利率、剩餘期數就剩餘本金重新計算。
總還款額 = 初期供款 × 固定期月數 + 調整後供款 × 剩餘月數;總利息 = 總還款額 − 本金。
$$M_{\text{init}} = P \times \dfrac{r_1(1+r_1)^n}{(1+r_1)^n - 1}$$$$B = P(1+r_1)^{k} - M_{\text{init}}\dfrac{(1+r_1)^{k}-1}{r_1}$$$$M_{\text{adj}} = B \times \dfrac{r_2(1+r_2)^{n-k}}{(1+r_2)^{n-k} - 1}$$使用說明
- Enter the mortgage principal and total loan term.
- Enter the initial annual rate, the initial fixed period and the adjusted annual rate.
- View the initial and adjusted monthly payments, the outstanding balance at reset, total repayment and total interest.
以按揭 HK$3,000,000、30 年期、初期利率 3%、固定 5 年為例，不同調整後利率對應的初期與調整後每月供款對照表 (固定期屆滿剩餘本金約 HK$2,667,191)
| 調整後利率 | 初期每月供款 | 調整後每月供款 | 供款升幅 |
|---|---|---|---|
| 3% (不變) | HK$12,648.12 | HK$12,648.12 | +HK$0 |
| 5% | HK$12,648.12 | HK$15,592.13 | +HK$2,944 |
| 7% | HK$12,648.12 | HK$18,851.15 | +HK$6,203 |
理財情境案例
利率重設的衝擊:同一按揭、不同調整後利率
李小姐承造 HK$3,000,000、30 年期的 5/1 ARM，初期利率 3%，初期每月供款約 HK$12,648.12。固定 5 年後剩餘本金約 HK$2,667,191。
情境一:利率維持 3% → 月供不變 (HK$12,648.12);情境二:升至 5% → 月供升至約 HK$15,592.13 (每月多約 HK$2,944);情境三:升至 7% → 月供升至約 HK$18,851.15 (每月多約 HK$6,203)。
同一份按揭，僅因 5 年後利率不同，月供差距可達每月數千元。
啟示:承造 ARM 前，務必用『壓力測試』的較高利率試算重設後的供款，確認家庭現金流能承受最壞情況。
誰適合 ARM？短期持有 vs 長期自住
陳先生打算 3 至 4 年後轉工移居、屆時會賣樓，看中 ARM 初期利率較低、初期供款較輕，能在固定期內享受低息、離場前不受重設影響 — 這正是 ARM 的理想使用情境。
反觀打算長住 20 年以上的家庭，固定期一過就要面對利率重設的長期不確定性，若期間加息，供款壓力會持續多年。
香港按揭多為浮息 (與 HIBOR 或 P 掛鈎)，加上『封頂利率 (cap rate)』機制，性質與 ARM 相近，同樣要留意加息週期的影響。
原則:ARM 適合有明確短期退出計劃、或預期收入將上升的借款人;長期自住者宜審慎評估重設風險。
常見問題
What does a 5/1 ARM mean?
A 5/1 ARM fixes the rate for the first 5 years, then adjusts with the market every year thereafter. This calculator uses a simplified two-stage model: the fixed period uses the initial rate, then a single adjusted rate applies until the end of the term, so you can see directly how a rate rise affects payments. A real ARM may reset multiple times.
Why does the payment change after reset?
During the fixed period you pay at the initial rate, leaving an outstanding balance at the end. Thereafter a new (usually higher) rate is applied to that balance over the remaining term, so payments rise if rates go up. If the adjusted rate equals the initial rate, the payment stays the same.
Who is an ARM suitable for?
The initial ARM payment is usually lower than a fixed-rate mortgage, making it suitable for borrowers who expect to hold the property short-term, or whose income will rise and who can absorb higher payments later. But if rates rise sharply, the burden increases significantly — you should stress-test with a higher reset rate before deciding. In Hong Kong most mortgages are floating (linked to HIBOR or Prime), which is similar in nature.
Do Hong Kong's H-plan and P-plan count as ARMs?
Broadly, yes. Hong Kong residential mortgages are mostly floating, split between the H-plan (HIBOR + spread) and the P-plan (Prime - discount), both moving with the market — consistent with an ARM's 'rate adjusts with the market' nature. The difference is that Hong Kong's H-plan usually has a 'cap rate' (capped at the P-plan level) that protects against rises, whereas an ARM typically uses a 'fixed period then floating' structure. This calculator's two-stage model can illustrate how a floating mortgage's payment changes after a rate rise.
What stress test should I note before taking a floating mortgage?
The Hong Kong Monetary Authority (HKMA) requires a mortgage stress test — banks assess affordability at an assumed rate several percentage points above the prevailing rate to confirm the debt-servicing ratio (DSR) stays within limits even if rates rise. Even after approval, you should stress-test with a more conservative rate (e.g. 2-3 percentage points higher) and keep a cash buffer to avoid losing control during a rising-rate cycle. Refer to the latest HKMA guidance and bank policies for the exact requirements.
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參考資料
內容審核：香港計算器財經團隊。計算邏輯與公式參考香港金融管理局（HKMA）及投資者及理財教育委員會（IFEC）之個人理財計算指引，結果僅供參考，實際以相關機構公佈為準。