Compound Interest Calculator

What this calculation means

• Compounding means that a later return is applied to both contributed principal and growth already accumulated. The long-run result does not come from the return assumption alone: it also depends on how much can actually be contributed each month and how long those contributions remain invested.
• This calculator adds the monthly contribution before applying a return at the selected monthly, quarterly, or yearly boundary. It therefore models beginning-of-month contributions. A product that invests at month-end, values assets daily, or receives irregular deposits will follow a different path even when the stated annual return is the same.
• The after-tax balance applies the editable tax percentage once to positive growth. Changing the display currency does not select a residence, account type, or tax jurisdiction, so the estimate should not be read as a tax filing result or as the amount an investor will necessarily receive.
• Affordability is easier to judge by reading contributed principal, the lower-return case, fee drag, and inflation-adjusted purchasing power together. If the monthly amount would crowd out ordinary expenses or an emergency reserve, changing the duration or target is more informative than raising the assumed return until the result fits.

Formula and calculation order

At a compounding boundary: B_m = (B_(m-1) + C) × (1 + r / n)

1. Begin with the starting principal and add C before each monthly return step.
2. At a selected monthly, quarterly, or yearly boundary, apply r / n to the balance at that point.
3. Subtract total contributed principal from the ending balance and apply τ only to positive growth.
4. Rerun the same inputs at r - f for the fee comparison, and divide the nominal after-tax balance by (1 + i)^t for purchasing power.
5. Calculate the three scenario rows at the entered return minus two percentage points, unchanged, and plus two percentage points.

Worked examples

Costs and conditions to check

• A nominal balance is the number of currency units at the future date. Real purchasing power discounts that balance by the entered inflation rate. Treating the two as equivalent can overstate how much a long-term target could buy.
• The annual fee comparison assumes that the fee reduces the annual return each year. Actual products can charge management, trading, foreign-exchange, or performance costs at different times, so the product documents remain necessary.
• The tax percentage is a one-step assumption applied to positive growth. It does not model realization dates, loss offsets, allowances, account wrappers, product rules, or residence, and it cannot replace an actual tax calculation.
• Inflation is held constant for the whole duration. Actual inflation changes over time, and the prices most relevant to one household may not move with a broad consumer-price measure.
• Every monthly contribution is added before the return step. Month-end deposits, skipped months, one-off additions, and withdrawals change how long each amount participates in the result.
• The one-year delay comparison shortens the contribution schedule while holding the monthly amount constant. It does not assign any return or inflation effect to cash held elsewhere during the omitted year.

Questions for each scenario

• First check whether the monthly contribution leaves room for ordinary expenses and an emergency reserve, then ask whether the duration remains workable in the downside case. A higher assumed return cannot make an unaffordable contribution schedule sustainable.
• Use the distance between the downside, base, and upside rows as a sensitivity signal. A wide range means the endpoint depends heavily on the return assumption, which can justify a wider target margin or another contribution and duration comparison.
• The delay table quantifies the modelled cost of waiting, but it is not a direction to begin with an amount that cannot be maintained. Choose an affordable contribution first, then compare start dates using that same amount.

Nominal balance, purchasing power, and fee-adjusted outcome

Starting with the baseline after-tax nominal balance, the table separately applies the 2% inflation conversion and the rerun using a return reduced by the 0.3% fee. They answer different questions and are not combined into one deduction.

Displayed amounts are rounded to whole won, while the engine retains its unrounded values. The table does not describe any particular product fee or tax rule.

Opportunity cost of starting now versus waiting one year

The starting principal, monthly contribution, return, and tax assumption stay fixed while the contribution period falls from 20 years to 19. This isolates the modeled cost of time; it does not assume what happens to the first-year cash elsewhere.

The modeled one-year difference is 15,500,395 KRW. It includes both twelve fewer contributions and less time for those amounts to participate in growth.

Downside, base, and upside return range

The calculator compares the baseline contribution, duration, and tax inputs at two percentage points below the 5% base return, at the base return, and two points above it. The upside row is a sensitivity boundary, not a target or prediction.

All three rows hold the annual return constant in every year. They omit return sequencing, volatility, contribution interruptions, and withdrawals.

Common mistakes

• Reading the expected annual return as a rate that repeats with certainty every year
• Treating a future nominal balance and inflation-adjusted purchasing power as the same amount
• Assuming the annual fee input includes every charge made by a real product
• Expecting a display-currency change to apply that country’s tax rules
• Ignoring the calculator’s beginning-of-month contribution sequence when deposits occur at another time
• Planning only from the base result without testing a lower return or a contribution interruption

Questions to consider

Calculation scope and evidence to check

• The U.S. Securities and Exchange Commission’s Investor.gov compound-interest calculator is an educational source for how starting principal, monthly contributions, time, return, and compounding assumptions affect money growth.
• The Japan Financial Services Agency explains the basics of long-term, regular, and diversified investing. Both official sources support education, not country-specific tax treatment. The 15.4% default remains an editable simplified assumption.
• Every displayed amount comes from the stated inputs and calculation sequence: contributions enter before the monthly return step, the simplified tax rate applies only to positive growth, and the fee, inflation, delay, and return-range comparisons use the same calculation basis.
• This global calculator embeds no official country tax rule, product return, or inflation forecast. Before acting, check the selected account and product’s current fee documents, the relevant tax authority’s guidance, and the actual contribution record independently.