Compound Interest Calculator

The tool runs a period-by-period simulation. For each compounding period (monthly, quarterly, etc.) it adds the per-period contribution, then multiplies the new balance by (1 + annual_rate / periods_per_year). This handles arbitrary compounding frequencies correctly. The per-period contribution is (monthly_contribution × 12 / periods_per_year), so the total annual contribution matches your monthly input regardless of the compounding frequency. The "Contribute at start of month" toggle controls whether the contribution is added before or after the interest accrues in that period.

The real value is what your nominal future value would buy in today's prices, accounting for inflation. The formula is real = nominal / (1 + inflation_rate)^years. So if you end up with Rp 1 billion after 20 years and inflation was 3% per year, the real value is Rp 1B / 1.03^20 = Rp 554 million in today's money. This tells you whether your investment is actually growing in purchasing power or just keeping up with rising prices. A 6% return with 3% inflation is a real 3% return, which doubles purchasing power in ~24 years.

Tax is applied once at the end, on the total interest earned (not on each period). The tool sums all interest over the lifetime of the investment, multiplies by your tax rate, and shows the tax owed and the net after-tax value. This is a simplification — actual tax rules may tax gains annually, allow cost-basis adjustments, or apply different rates to different holding periods. Use the tool for planning scenarios, not for tax filing. The tax field is optional; set to 0% for tax-advantaged accounts (401k, Roth IRA, Indonesian Reksa Dana).

Compounding frequency controls how often interest is added to the balance. Monthly means interest is calculated 12 times per year, quarterly 4 times, semi-annually 2 times, annually once. More frequent compounding produces a slightly higher final value because interest earns interest sooner. For a 6% annual rate over 20 years on Rp 10M with no contributions: monthly compounding gives Rp 33.1M, quarterly gives Rp 32.9M, annual gives Rp 32.1M. The difference is small for low rates and short periods, larger for high rates over decades. Most savings accounts and CDs compound monthly; most bonds compound semi-annually.

Yes. The math is pure JavaScript inlined in the HTML. There are no network calls, no API lookups for current rates, no CDN. The currency formatting is local JavaScript. The values you enter and the results you see never leave your device. You can use the tool to plan your personal finances, model retirement scenarios, or estimate loan costs without any data being transmitted.