Compound Interest & Monthly Savings Simulator
＋More features: work backwards from a target amount
Set the goal first — for example “20,000,000 yen by retirement”, which in Japan is usually written as 2,000 man-yen — and work backwards (the target amount and assumed return are shared by the two calculations below).
How much do I need to put aside each month?
When will I reach the target?
Just enter how much you save each month and an assumed return, and you see how much you would have in the future. It also handles a single lump sum invested at once. Amounts are in Japanese yen, so it fits straight into planning around Japanese savings and investment accounts (such as NISA) while you live in Japan.
How to use
- Enter your monthly contribution, the assumed return and the period — the future amount appears immediately.
- For a single lump sum, use the “Lump-sum investment” form below.
- It is worth calculating several times with different returns so you see a range rather than one number.
How it is calculated
- Monthly saving (monthly compounding, contribution at month end): future value = monthly amount × ((1+i)ⁿ − 1) ÷ i (i = annual rate ÷ 12, n = years × 12)
- Lump sum (annual compounding): future value = principal × (1 + annual rate)^years
- Example: 30,000 yen a month at 5% a year for 20 years → 7,200,000 yen of contributions grows to about 12,330,000 yen (about 5,130,000 yen of gains)
Sources and assumptions
- The calculation uses the standard mathematical formulas for compound interest (future value and the future value of an annuity), rounded to the nearest yen.
- Saving assumes “contribution at the end of each month with monthly compounding”. Simulators that assume a different contribution timing will differ by a few percent.
Assumptions and notes
- Past returns do not guarantee future ones. Check a conservative assumed return as well.
- Investment decisions are your own responsibility. For the details of a scheme (such as NISA) or a product, check the information from the financial institution or the relevant public body.
Frequently asked questions
What return should I assume?
Nobody knows future returns. The realistic approach is to calculate several optimistic and pessimistic cases (for example 1%, 3% and 5%) and think in terms of a range.
Is tax deducted?
This tool shows figures before tax. In a taxable account in Japan, gains are taxed at about 20% (it depends on whether you use a tax-free scheme).
What is the difference between compound and simple interest?
With simple interest only the principal earns interest; with compound interest “the interest earns interest” as well. The longer the period, the bigger the gap. This tool uses compound interest.