Simple Interest Calculator
Calculate the interest and maturity value generated on a principal at simple interest.
輸入資料
計算結果
重點速覽：Simple Interest accrues only on the original principal; interest does not earn interest. The formula is I = P × r × t (principal × annual rate × term), and the maturity value A = P + I. It is commonly used for short-term loans, deposits, certain notes or simple deposit estimates, unlike compound interest which 'rolls over'.
計算公式
Simple interest: I = P × r × t, where P is principal, r is annual rate, t is term.
Maturity value: A = P + I.
$$I = P \times r \times t$$$$A = P + I = P(1 + r \times t)$$$$(P = \text{本金},\; r = \text{年利率},\; t = \text{年期})$$使用說明
- Enter the principal amount.
- Set the annual interest rate and term.
- The interest total and maturity value are shown instantly.
With a principal of 100,000, simple interest at various rates and terms, compared with compound interest (HK$).
| Annual rate | Term | Simple interest | Compound interest | Compound excess |
|---|---|---|---|---|
| 5% | 3 years | 15,000 | 15,763 | 763 |
| 5% | 5 years | 25,000 | 27,628 | 2,628 |
| 8% | 3 years | 24,000 | 25,971 | 1,971 |
| 8% | 5 years | 40,000 | 46,933 | 6,933 |
理財情境案例
Case 1: Interest on a short-term simple-interest loan
Mr Chan borrows 50,000 from a friend at a simple annual rate of 6%, repaid in full after 2 years.
Interest I = 50,000 × 6% × 2 = 6,000; maturity value A = 50,000 + 6,000 = 56,000.
Being simple interest, the two-year interest is just the sum of the annual interest with no compounding. This is common for private short-term loans, deposits or certain notes; when the amount and term are fixed, simple interest is the most straightforward.
Case 2: Simple vs compound interest — the gap after 10 years
May wants to compare 100,000 at 5% over 10 years under simple versus compound interest.
Simple: I = 100,000 × 5% × 10 = 50,000. Compound (annual): A = 100,000 × (1.05)^10 ≈ 162,889, interest ≈ 62,889.
Over 10 years compound interest (≈62,889) exceeds simple (50,000) by about 12,889. The longer and the higher the rate, the more pronounced compounding becomes — which is why long-term saving and investing emphasise compounding.
常見問題
How does simple interest differ from compound interest?
Simple interest is charged only on the original principal, so interest does not earn further interest. Compound interest rolls interest back into the principal. In Hong Kong most deposits and loans use compounding; simple interest is mostly for short-term or specific products.
Do Hong Kong deposits and loans use simple or compound interest?
In Hong Kong most deposits and loans are compound in nature; simple interest appears in specific short-term or flat-rate scenarios. Savings and time deposits credit interest on a periodic basis, so leaving interest in the account creates compounding. Mortgages, personal loans and revolving credit are generally calculated on a reducing-balance (monthly compounding) basis. Flat-rate quotes charge interest on the full original principal and usually imply a much higher effective APR. Use a compound / mortgage / loan calculator when rolling interest or reducing balance applies. Actual rates follow the bank's terms.
Why focus on compounding rather than simple interest for long-term planning?
Compounding produces 'interest on interest', and the longer the horizon the larger its advantage — often quoted as the 'eighth wonder of the world'. At 100,000 and 5%, simple interest yields a fixed 5,000 a year (50,000 over 10 years, 100,000 over 20); compounding yields about 62,889 over 10 years and about 165,330 over 20. Time and rate are the two key variables. Start early, let returns reinvest, and clear high-interest debt quickly. Returns are not guaranteed; this is illustrative only.
How do I invert the simple interest formula to find rate or term?
With I = P × r × t, knowing any three variables solves the fourth. Rate: r = I ÷ (P × t). Term: t = I ÷ (P × r). Principal: P = I ÷ (r × t). This is useful to back out the implied annual rate from a maturity amount. Note the derived 'rate' applies only to genuinely simple-interest cases; for compound or flat-rate borrowing it differs from the true APR, so compare loans by APR.
What is the 'flat rate' often seen in loan ads, and how does it relate to simple interest and APR?
A 'flat rate' computes interest as principal × flat annual rate × term — identical to the simple interest formula — but is applied to instalment loans where the principal is repaid monthly. Because you repay principal while interest is still charged on the original full amount, the true APR is roughly 1.8–2× the flat rate (e.g. 100,000 at 6% flat over 3 years implies about 11% APR). Regulation requires disclosure of the APR for this reason. Our personal-loan / EMI calculator uses the reducing-balance (APR) basis. Actual rates follow the lender's contract and statutory disclosure.
相關工具
參考資料
Reviewed by the Hong Kong Calculator finance team. Calculation logic and formulas follow the personal finance guidance of the Hong Kong Monetary Authority (HKMA) and the Investor and Financial Education Council (IFEC). Results are for reference only; please refer to the relevant authorities for the latest figures.