Private browser-side finance math
Interest Calculator
Model simple interest, compound growth, monthly savings, and after-tax results in your browser. Calculation inputs and values are not sent to our app backend or analytics.
Inputs
Result · A
Final amount
Starting amount10,000
Gross interest4,099.6594
Tax on interest0
Net interest4,099.6594
Total contributions18,000
Total-period net return14.64%
- Calculation mode
- Compound interest
- Period
- 60 months
- Compounding frequency
- Monthly
- Tax rate
- 0%
- Rate convention
- Nominal annual rate. Daily compounding uses 365 days, and the selected compounding cadence is converted to an equivalent monthly rate.
- Period and rounding
- The period is rounded to the nearest whole month. Monetary results use half-up rounding to 4 decimals, and a required target contribution is rounded upward.
- Contribution timing
- Contributions are added at each month end. The selected compounding cadence is converted to an equivalent monthly rate.
Result · A / BEstimate only
- Starting amount10,000 · 31.15%
- Total contributions18,000 · 56.08%
- Net interest4,099.6594 · 12.77%
- Starting amount10,000 · 30.09%
- Total contributions18,000 · 54.16%
- Net interest5,235.4116 · 15.75%
- Calculation mode
- Compound interest
- Compounding frequency
- Monthly
- Contribution timing
- Contributions are added at each month end. The selected compounding cadence is converted to an equivalent monthly rate.
| Period | Starting amount | Total contributions | A · Gross interest | A · Tax on interest | A · Net interest | A · Final amount | B · Gross interest | B · Tax on interest | B · Net interest | B · Final amount |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 months | 10,000 | 0 | 0 | 0 | 0 | 10,000 | 0 | 0 | 0 | 10,000 |
| 12 months | 10,000 | 3,600 | 474.1543 | 0 | 474.1543 | 14,074.1543 | 595.2756 | 0 | 595.2756 | 14,195.2756 |
| 24 months | 10,000 | 7,200 | 1,114.2959 | 0 | 1,114.2959 | 18,314.2959 | 1,405.1895 | 0 | 1,405.1895 | 18,605.1895 |
| 36 months | 10,000 | 10,800 | 1,927.1875 | 0 | 1,927.1875 | 22,727.1875 | 2,440.723 | 0 | 2,440.723 | 23,240.723 |
| 48 months | 10,000 | 14,400 | 2,919.867 | 0 | 2,919.867 | 27,319.867 | 3,713.4191 | 0 | 3,713.4191 | 28,113.4191 |
| 60 months | 10,000 | 18,000 | 4,099.6594 | 0 | 4,099.6594 | 32,099.6594 | 5,235.4116 | 0 | 5,235.4116 | 33,235.4116 |
This is an educational calculator. It does not compare bank products and is not financial, tax, or investment advice.
Official tax references
Reviewed 2026-07-15; next review due 2026-10-15.
- Interest income withholding14%Income Tax Act, Article 129
- Local income tax on withholding10% of national withholdingLocal Tax Act, Article 103-13
Formula
Simple interest uses P × r × t. Compound mode grows principal at the selected cadence and adds recurring contributions at each month end. The period is rounded to the nearest whole month. Tax is estimated once at the end and applies only to gross interest, never to principal or contributions.
Useful checks
- Compare a lump sum with monthly saving.
- Switch between no-tax and Korea 15.4% tax assumptions.
- Enter a target amount to estimate the needed monthly contribution.
Assumptions
- No live bank rates or product recommendations are used.
- The South Korea preset estimates general resident interest income as 14% national withholding plus 1.4% local income tax; special, nonresident, and product-specific rules are excluded.
- The period is rounded to the nearest whole month, and tax is estimated once at the end of the calculation.
- Total-period net return equals net interest divided by the sum of the starting amount and total contributions.
- Every amount stays in the same unit entered by the user; no currency conversion is performed.
- Calculation inputs and values stay in your browser and are not sent to our app backend or analytics. Anonymous usage analytics start by default and can be turned off at any time in Analytics settings.
Calculator only
This is an educational calculator. It does not compare bank products and is not financial, tax, or investment advice.
How to calculate interest
- Enter principal Type the starting amount and optional monthly contribution.
- Set rate and period Choose simple or compound mode, rate, years, and compounding cadence.
- Review the result Check final amount, gross interest, tax, net interest, and assumptions.
Frequently asked questions
Does this calculator use live bank rates?
No. You enter the interest rate yourself, and no bank or product data is fetched.
How is the Korea 15.4% tax preset applied?
The preset estimates a general South Korean resident's interest income by applying 15.4% to gross interest once at the end. Special, nonresident, and product-specific rules are excluded.
Is my data sent to a server?
Calculation inputs and values stay in your browser and are not sent to our app backend or analytics. Anonymous usage analytics start by default and can be turned off at any time in Analytics settings.