What this calculator does
Jurisdiction: Japan. This tool simulates a Japanese-style recurring savings or time-deposit plan where you pay in a fixed amount every month, add a larger top-up twice a year (the summer and winter bonus custom), and interest is credited and taxed at the end of each compounding period. Amounts are entered in units of 10,000 (man-en); the default tax rate of 20.315% is Japan's interest withholding tax since January 2013 (15.315% income tax including the Reconstruction Special Income Tax plus 5% inhabitant tax). Non-Japanese users can simply change the tax rate and read the amounts as their own currency.
How to use it
Enter the annual interest rate, the savings period in years, your monthly contribution, the bonus-month top-up, the existing principal, and choose how often interest compounds (monthly, semiannual, or annual). Pick whether contributions earn interest in the same period they are paid (beginning) or not (end), and whether interest is taxed each period or tax-exempt. The calculator runs a month-by-month simulation and shows the maturity total, total principal, after-tax interest, total tax, and a period-by-period table.
The formula explained
The model tracks a yen balance B starting from your existing principal. Each month it adds your monthly contribution, plus the bonus whenever the month aligns with the bonus start month and every 6 months after. At the close of each k-month period it computes interest as \( B \times \dfrac{r \times k}{12} \), rounds it to whole yen (truncate, round, or ceiling), withholds tax of \( \lfloor interest \times taxRate \rfloor \), and adds the net interest back to the balance. Because rounding happens every period, results can differ slightly from a single closed-form future-value formula — the simulation is authoritative.
$$ i_{period} = B \times \frac{r \times k}{12}, \quad net = i_{period} - \lfloor i_{period} \times t \rfloor $$
Worked example
Rate 3%, 1 year, 10 man-en monthly (100,000 yen), 40 man-en bonus, bonus start month 6, annual compounding, beginning timing, taxed at 20.315%. Contributions = \( 12 \times 100{,}000 + 2 \times 400{,}000 = 2{,}000{,}000 \) yen. One period closes at month 12 with base 2,000,000. Gross interest:
$$ 2{,}000{,}000 \times 0.03 = 60{,}000 \text{ yen} $$tax = \( \lfloor 60{,}000 \times 0.20315 \rfloor = 12{,}189 \); net = 47,811. Maturity = 2,047,811 yen.
FAQ
Why 20.315%? It is the standard Japanese withholding on interest income since 2013. Edit the field for other countries or tax-advantaged accounts.
What does beginning vs end timing change? Only whether the current period's contributions earn interest in that same period — "beginning" treats them as deposited at period start, "end" as deposited at period end.
Are real bank results identical? Rounding conventions vary by institution, so treat this as an estimate; your bank's figure may differ slightly.
