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Compound interest calculator

Model an initial amount and fixed periodic deposits at a constant nominal rate while retaining exact balances between periods.

Fixed nominal educational models, calculated locally. No live rates, recommendations, fees, taxes or legal APR calculation. Nothing is saved automatically.

The currency is a label only. Choose 0–3 decimal places; amounts with more places are rejected, including extra trailing zeros. Amounts: 0–1000000000000. No currency conversion.

Use nonnegative decimal numbers without spaces, grouping or exponents. Arabic and Persian digits and ٫ are supported. Each field accepts at most 32 UTF-16 units; line breaks are rejected.

The deposit frequency equals the compounding frequency. Enter 1–600 periods directly. Balances stay exact between periods; only display is rounded halfway away from zero. Displayed rounded rows may not add exactly.

Annual rate 0–100%, at most 6 decimal places. Numeric results ≤10^18; at most 600 periods, 65536-bit arithmetic and a 5-second worker deadline. Combined TXT/CSV output ≤256 KiB. These are work limits, not a total browser-memory guarantee.

How to use

Enter P, periodic contribution C and the nominal annual rate. Choose1,2,4 or12 periods per year; deposits use the same frequency. Enter the whole period count directly and choose beginning or end of period. Calculate, inspect the exact and rounded balance and table, then export TXT and CSV.

Try an example

P=100, C=10, annual rate10%, frequency1 and two periods give142.00 for end deposits and144.10 for beginning deposits. With0.01, no deposits, rate100%, frequency2 and two periods, the exact result is0.0225, displayed as0.02; it does not compound a rounded0.02 balance into0.03.

Support & limits

i=annual percent/(100×frequency), q=1+i. Balance=P×q^n+C×(q^n−1)/i; multiply the deposit part byq for beginning timing. At zero rate it becomes P+nC. No fractional-year rounding to a period count and no internal balance rounding; only display rounds half a minor unit away from zero. Rows show cumulative contributions and interest, whose displayed sums may differ due to rounding. This is a fixed educational model without taxes, fees or a market-return forecast.

Common questions

Does this interest represent an actual return or bank offer?

No. The same manual nominal rate applies every period and deposits remain fixed. An independent FV reference example uses500 initially,200 deposited,6% annual, frequency12 and ten beginning-timing periods, displaying2581.40. This demonstrates neither a guaranteed return nor fidelity to a real financial product.

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