Compound Interest Savings Calculator

About compound interest savings

Compound interest means interest earned in one period is added to the principal, so future interest is calculated on a growing balance rather than the original amount alone. This tool calculates the maturity value of a savings plan combining an initial deposit plus regular monthly contributions, compounded monthly, over a term you choose — showing both pre-tax and after-tax interest.

How the maturity amount is calculated

The initial deposit grows using the standard compound interest formula, and each monthly contribution is treated as its own deposit that compounds for the remaining months until maturity, with all these amounts summed to the final total. After-tax interest applies South Korea's standard 15.4% withholding tax rate by default (adjustable to tax-free or a custom rate), reflecting how bank savings interest is commonly taxed there.

Frequently asked questions

Why is the after-tax amount lower than the pre-tax amount?

Interest income from bank deposits is typically subject to withholding tax (15.4% is the standard combined rate in South Korea for regular taxable accounts), which is deducted from the interest earned — the principal itself isn't taxed, only the interest portion.

Does the calculator account for changes in interest rate over the term?

No — it assumes a single fixed annual interest rate for the entire term you enter, which matches how most fixed-rate savings and deposit products work, but won't reflect variable-rate products where the rate can change.

What does the stacked bar chart show?

Each bar represents your balance at a point during the term, split into the principal you've contributed (dark segment) and the interest earned so far (colored segment), so you can visually see how compounding accelerates growth over time.