Savings Interest Calculator — Simple, Compound
Deposit and monthly savings interest, simple or compound, with optional tax
Runs in your browser · nothing is uploaded
Updated 2026-10-04Results are before tax unless you enter a rate here: leave it blank or 0 for no tax. Korea’s general rate on interest is 15.4% (14% income tax + 1.4% local income tax). Basis: 소득세법 제129조 제1항 제1호 라목 (이자소득 14%)·지방세법 제103조의13 (지방소득세 10%) (effective 2026-01-01).
Maturity
Enter an amount, a rate and a term to see interest before and after tax and the maturity value.
For reference only. Whether a product pays simple or compound interest varies, so check its terms. For the monthly plan, compounding assumes each deposit is made at the start of the month. Interest is rounded to the nearest cent. Bonus rates, early-withdrawal rates, rates after maturity, tax-free allowances and any other taxes are not included. Check the product terms for real figures. Amounts are shown without a currency symbol.
What it is
A savings calculator for the two most common products: a lump-sum deposit and a plan where you add the same amount every month. Pick the product and the interest method, enter the amount, the annual rate and the term, and it shows your total principal, interest before tax, tax, interest after tax, the maturity value both ways and the after-tax return for the whole term. The formula is printed with your own numbers so you can check it by hand. Amounts have no currency symbol, and tax is optional, so you can use it in any country. Everything runs in your browser.
How to use
- Choose lump-sum deposit or monthly savings plan, then simple interest or monthly compounding.
- Enter the deposit amount (or the monthly deposit) and the annual interest rate.
- Enter the term in months or years.
- Optionally enter a tax rate on interest. Blank means no tax.
- Read the maturity value and the formula below it.
How it works
- Deposit, simple: interest = principal × annual rate × months ÷ 12.
- Deposit, monthly compounding: interest = principal × ((1 + annual rate ÷ 12)^months − 1).
- Monthly plan, simple: interest = monthly deposit × n(n+1)/2 × annual rate ÷ 12.
- Monthly plan, compounding: interest = monthly deposit × Σ(1 + annual rate ÷ 12)^k for k = 1 to n, minus monthly deposit × n, with deposits at the start of each month.
- Tax: interest × your tax rate, rounded to the nearest cent. Interest is rounded to the nearest cent too.
Examples
| Plan | Interest before tax | Tax at 15.4% (example rate) |
|---|---|---|
| Deposit 10,000, 3%, 12 months, simple | 300.00 | 46.20 |
| Monthly 1,000, 3%, 12 months, simple | 195.00 | 30.03 |
| Monthly 1,000, 3%, 12 months, compounding | 196.80 | 30.31 |
| Deposit 10,000, 3%, 12 months, compounding | 304.16 | 46.84 |
For reference only — check the product terms for actual interest.
FAQ
How is simple interest on a monthly savings plan calculated?
Monthly deposit × n(n+1)/2 × annual rate ÷ 12, where n is the number of months. The first deposit earns n months of interest and the last one earns 1, so the interest-months add up to n(n+1)/2. A 1,000 monthly deposit for 12 months at 3% earns 1,000 × 78 × 0.03 ÷ 12 = 195.00.
How much more does monthly compounding pay?
On a 10,000 lump sum at 3% for 12 months, simple interest is 300.00 and monthly compounding is about 304.16. The gap widens with longer terms and higher rates. Many bank products pay simple interest, so check the product terms.
When are deposits made in the compounding plan?
The calculator assumes each deposit is made at the start of the month and compounds monthly until maturity. A real product may use other payment dates or interest rules.
How does the tax field work?
Leave it blank or 0 for no tax, or enter your own rate to see interest and maturity value after tax. In Korea the general rate on interest is 15.4% (14% income tax under Income Tax Act Article 129 plus 1.4% local income tax under Local Tax Act Article 103-13, effective 2026-01-01), and rules differ by country and account type, so nothing is prefilled in this version.
Are promotional rates or early withdrawal included?
No. One rate applies for the whole term. Bonus rates, early-withdrawal penalties, rates after maturity and tax-free allowances are not included.
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