Why a Lump Sum and Monthly Savings Earn Different Interest
A lump sum starts earning interest at once. Monthly deposits arrive later and therefore have less time to earn it. The examples below use simple interest with regular beginning-of-period deposits. They are hypothetical calculations, not product offers.
Compare two ways to deposit 6,000,000 units
A 6,000,000-unit lump sum at 3.5% for 12 months earns 210,000 units before tax. Twelve monthly deposits of 500,000 units under the stated model earn 113,750 units despite the same total deposited. Only the first installment stays for 12 months; the last stays for one month.
Calculate the time of each installment
Adding installment durations from 12 months down to one gives 78 months. The interest is 500,000 × 0.035 ÷ 12 × 78 = 113,750. Multiplying the final 6,000,000 balance by 3.5% treats every deposit as if it arrived at the start.
Separate tax from interest
The Korean tool’s ordinary-tax option applies 15.4% to interest, with its stated whole-KRW rounding. That is a Korean assumption, not an international tax rule. The English compound-interest tool excludes taxes and supports a different compounding model. Do not compare its projected net with a tax-adjusted simple-interest result.
Match the product’s actual model
Check simple versus compound interest, actual deposit dates and day-count rules. Late contributions or early withdrawal can change the model and rate. An advertised maximum rate is not necessarily the rate applicable to your deposits.
Try the relevant tools
Links to Korean tools retain their Korean rules. Use the English arithmetic tools for country-neutral calculations.
예금이자 계산기적금이자 계산기Compound Interest CalculatorSources and scope
Examples use fictional inputs. Relevant Korean policy assumptions were checked on October 10, 2026. Actual contracts and institution rules may differ.
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