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Private browser-side finance math

Interest Calculator

Model simple interest, compound growth, monthly savings, and after-tax results without sending anything to a server.

Inputs

Mode
Tax preset

Result

$32.100

Final amount

Gross interest$4100
Tax on interest$0
Net interest$4100
Total contributions$18.000
Effective return14,64%

Calculation mode: Compound. Tax rate: 0%. Contributions are added monthly at period end.

Source labels

  • Interest income withholding14%National Tax Service source recorded in D-001 source-evidence.md
  • Local income tax on withholding10% of national withholdingNational Tax Service source recorded in D-001 source-evidence.md

Formula

Compound mode applies the selected annual compounding cadence. Tax is applied only to gross interest, not 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 Korea preset is 14% national withholding plus 1.4% local income tax.
  • All calculations run locally in your browser.

Calculator only

This is an educational calculator. It does not compare bank products and is not financial, tax, or investment advice.