Deposit and Savings Interest in Depth: From Compounding to After-Tax Returns
Between the advertised annual rate you see at the bank and the money that actually lands in your account at maturity stand three gates: how interest accrues (simple vs compound), the 15.4% tax withheld from interest, and the monthly-installment structure peculiar to savings plans. Miss any of these and you'll sign up for a '4% savings plan' only to receive roughly half the interest you expected. This guide puts numbers on all of it — how fast compounding pulls ahead, how the tax is structured, how to convert a nominal rate into an after-tax effective yield, what installment-savings interest really is, and how deposit insurance limits work.
How Fast Simple and Compound Interest Diverge — the Rule of 72
Simple interest accrues only on the principal, so it grows in a straight line; compound interest earns interest on interest, so it grows along a curve. The gap is almost invisible at first and accelerates over time. The classic shortcut for judging that speed is the Rule of 72: years to double your principal ≈ 72 ÷ annual rate (%). At 4% compounded, 72 ÷ 4 = about 18 years; at 6%, about 12 years. With simple interest at the same 4%, doubling takes 100% ÷ 4% = 25 years.
| Period | Simple interest total | Compound total (annual) | Gap |
|---|
| 5 years | KRW 12.00M | approx. KRW 12.17M | approx. KRW 0.17M |
| 10 years | KRW 14.00M | approx. KRW 14.80M | approx. KRW 0.80M |
| 20 years | KRW 18.00M | approx. KRW 21.91M | approx. KRW 3.91M |
| 30 years | KRW 22.00M | approx. KRW 32.43M | approx. KRW 10.43M |
The table shows KRW 10 million growing at 4% per year. For a one-year deposit the simple/compound difference is practically nil, but past ten years the gap becomes hard to ignore. This calculator's compound option uses monthly compounding, so its results come out slightly higher than the annual-compounding figures above. Note that most Korean bank deposits and savings plans pay simple interest — to actually capture compounding you either roll the matured principal and interest into a new deposit (manual compounding) or look for genuinely monthly-compounding products such as CMA accounts or short-term notes issued by securities firms.
The Anatomy of the 15.4% Interest Tax
Interest from deposits and savings is taxed at a combined 15.4%, made up of 14% interest income tax plus 1.4% local income tax (10% of the 14%). The bank withholds it at source when paying your interest, so no separate filing is needed. However, if your combined annual financial income (interest plus dividends) exceeds KRW 20 million, the excess is added to your other income under comprehensive financial income taxation, where progressive rates of up to 49.5% (including local tax) can apply.
| Tax type | Rate | Typical cases |
|---|
| Standard | 15.4% | Most bank deposits and savings (14% income tax + 1.4% local tax) |
| Preferential | 1.4% | Member deposits at mutual-finance cooperatives (Shinhyup, Saemaul, etc.) — only the special tax for rural development applies (caps and sunset dates can change with tax-law revisions) |
| Tax-free | 0% | Tax-free comprehensive savings (age 65+, persons with disabilities, etc., up to KRW 50M principal per person), ISA tax-free allowances, youth-preferred products |
Used to its full limit, the tax-free scheme makes a difference you can count. Put KRW 50 million into a one-year deposit at 4% and the pre-tax interest is KRW 2,000,000 — under standard taxation only KRW 1,692,000 remains after tax, while tax-free comprehensive savings pays out the full KRW 2,000,000, leaving an extra KRW 308,000 every year. That is why, if anyone in your family qualifies, it pays to fill their tax-free limit first. You can switch this calculator's tax option (standard, preferential, tax-free) to see the after-tax difference directly.
Converting a Nominal Rate into an After-Tax Effective Yield
What really matters when comparing products is not the pre-tax advertised rate but the after-tax effective yield. Under standard taxation the conversion is simple: after-tax rate = nominal rate × (1 − 0.154) = nominal rate × 0.846. A 4.0% deposit yields about 3.38% after tax; 3.5% becomes about 2.96%. Working backward, if inflation runs at 2%, you need a pre-tax rate of at least 2 ÷ 0.846 ≈ about 2.36% just to preserve purchasing power. When comparing promotional rates across banks, multiplying everything by 0.846 and ranking on an after-tax basis is a habit that keeps you from being dazzled by complicated bonus-rate conditions.
The Installment-Savings Trap — Why a 4% Plan Pays Half That
Almost everyone checking their first installment-savings payout wonders, 'Why is the interest so small?' The reason is not that the bank cheated — it's the monthly deposit structure. The annual rate applies only to the time each installment actually sits in the bank. In a 12-month plan, the first month's deposit earns 12 months of interest, but the final month's deposit earns just one. Since the average deposit period is about 6.5 months, the perceived return on total principal drops to about 54% (13/24) of the advertised rate.
- Setup — KRW 1,000,000 per month × 12 months, 4% simple-interest savings plan (total principal KRW 12,000,000)
- Pre-tax interest = 1,000,000 × (4% ÷ 12) × (12 + 11 + … + 1) = 1,000,000 × 78 × 0.3333% = KRW 260,000 (about 2.17% of total principal)
- Interest tax = 260,000 × 15.4% = KRW 40,040
- After-tax interest = KRW 219,960 — about 1.83% of total principal
- Comparison — Put the same KRW 12,000,000 into a 4% one-year time deposit and you get KRW 480,000 pre-tax, KRW 406,080 after tax — about 1.8 times the savings plan.
So if you already have a lump sum, a time deposit is the right tool; if you're saving fresh money each month, an installment plan is. Even when a promotional savings rate looks higher than deposit rates, splitting an existing lump sum into monthly installments is often worse on an effective basis. The most accurate check is this calculator's comparison mode (deposit vs savings), which contrasts after-tax payouts side by side. And once an installment plan matures, don't let the lump sum sit idle — move it straight into a time deposit. 'Accumulate with savings, then grow with deposits' is the standard two-step strategy.
Deposit Insurance Limits and Spreading Your Money
No rate is high enough to matter if the institution fails. Under the Depositor Protection Act, even if a bank or savings bank goes bankrupt, up to KRW 100 million per person, per institution — principal and interest combined — is protected. This limit was raised from the previous KRW 50 million in September 2025 (Korea Deposit Insurance Corporation). Mutual-finance institutions such as Saemaul, Shinhyup, and Nonghyup community cooperatives are covered not by the KDIC but by their own central federations' funds (banks like NH Bank and Sh Suhyup Bank fall under the KDIC), with the same KRW 100 million limit.
- The limit counts principal plus interest combined. Factor in maturity interest and keep each institution comfortably below KRW 100 million.
- Multiple branches and accounts at the same bank are all added together. Diversification only counts across different financial institutions.
- The more you chase high promotional rates at savings banks, the more spreading within the limit matters. Protecting your principal outranks a 0.2–0.3%p rate difference.
- Deposits and principal-guaranteed products are covered, but investment products like funds, stocks, bonds, and RP-type CMA accounts are not.
The rates and limits in this guide reflect the rules as of the time of writing and may change with tax-law and regulatory revisions. Before signing up, confirm the latest terms with your financial institution and official bodies such as the Korea Deposit Insurance Corporation and the National Tax Service.
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