Regular Investing Simulation: What ¥10,000, ¥30,000 or ¥50,000 a Month Grows To

How the amount is calculated

The future value of regular investing is determined by three things: the monthly contribution, the assumed return (annual) and the investing period. All figures in this article use the same assumptions as the asset-management simulation of Japan's Financial Services Agency: "contribute at the end of each month, monthly rate = annual rate ÷ 12, compounded monthly". When comparing with other simulators, first check that these assumptions match.

With monthly contribution P, monthly rate r (annual rate ÷ 12) and number of contributions n (years × 12), the future value is P × {(1 + r)^n − 1} ÷ r. For ¥30,000 a month at 3% for 20 years, that's a monthly rate of 0.25% over 240 contributions: 30,000 × (1.0025^240 − 1) ÷ 0.0025 ≈ ¥9.849 million. The principal you put in is ¥7.2 million, so about ¥2.65 million is growth from investing.

Quick-reference table: monthly contribution × years

These common combinations of contribution and period are calculated at 3% a year (a figure often used for balanced funds) and 5% a year (often used for global equities). The figures in parentheses are the total principal contributed.

Over 10 years the gap between principal and value isn't that large, but by 30 years it opens up sharply. At ¥10,000 a month and 5%, gains are about ¥350,000 after 10 years but about ¥4.72 million after 30, more than the ¥3.6 million principal. The table shows that regular investing works better the longer you keep going.

How much 1% of return changes the result after 20 years

Even with the same ¥30,000 a month for 20 years (principal ¥7.2 million), the result varies widely with the assumed return.

Each 1% step is worth about ¥880,000 going from 1% to 2%, but about ¥1.77 million going from 6% to 7%, so the gap widens as returns rise. On the other hand, higher-return products also swing more, so choosing one just because "7% reaches my goal" is risky. When planning, it's reassuring to also calculate at 1–2% below what you expect and confirm you'd still be fine.

The split between principal and gains reverses over time

This shows the share of gains in the total value by period, for ¥30,000 a month at 5%.

In the first 5 years, nearly all the value is money you put in yourself. Around 25 years, gains roughly equal principal, and by 30 years more than half the value is growth. This is the effect of compounding, where the gains earned so far earn further returns. How compounding works is explained in detail in compound vs simple interest.

Start early, or contribute more?

With age 65 as the same finish line, this compares combinations of starting age and monthly contribution (calculated at 5%).

¥20,000 a month from age 25 puts in ¥2.4 million less than ¥50,000 a month from age 45, yet ends up about ¥10 million higher at 65. Compounding works better over longer periods, so the benefit of starting early, even with a small amount, is hard to make up later by contributing more.

Even if you stop contributing partway through, your existing assets can stay invested. If you contribute ¥30,000 a month for 10 years (about ¥4.66 million) and then leave it for another 10 years without adding more, it reaches about ¥7.67 million at 5%. Even if you pause during a tight period, compounding keeps working as long as you hold rather than sell.

What fund fees take away

Investment funds charge a management fee that's deducted for as long as you hold them. For a simulation, entering the assumed return minus the fee rate gives a close result. Here's the comparison for ¥30,000 a month over 20 years.

A 1.5% annual fee looks small, but over 20 years the difference comes to about a quarter of the ¥7.2 million principal. If you're torn between products tracking the same index, the basic rule is to choose the one with the lower fee.

Simulate with your own numbers

1. 1

   Enter contribution, return and period

   On the "Build by saving" tab of the Savings & Compound Interest Simulator, enter the monthly contribution, assumed return and investing period. You can also type forms like "3万" or "100万". For the return, choose from presets: deposits 0.2%, balanced 3%, global equities 5%, US equities 7%.

2. 2

   Check the principal and gains breakdown

   You'll see the value after your chosen number of years and how much of it is principal and gains. The yearly trend appears in a chart and table, and the table can be pasted straight into a spreadsheet.

3. 3

   Compare terms with the quick-reference tables

   Compare 0–10% at once in the table by return, and use the contribution × years table to see how much sooner an extra ¥10,000 a month gets you there. If you already have a lump sum, enter it as the initial investment.

For how much tax NISA, Japan's tax-exempt investment account, can save, see NISA's tax-free limits vs a taxable account. To set a monthly contribution from a target, see the contributions needed to reach ¥20 million for retirement.

FAQ

How much will ¥30,000 a month grow to in 20 years?

The principal is ¥7.2 million. It reaches about ¥9.85 million at 3%, about ¥12.33 million at 5% and about ¥15.63 million at 7%. If you just save without investing, it stays at ¥7.2 million. A 2% difference in return changes the amount after 20 years by around ¥2.5 million.

Is investing just ¥10,000 a month worthwhile?

Yes. ¥10,000 a month at 5% for 30 years grows to about ¥8.32 million on ¥3.6 million of principal, so gains exceed principal. The period affects the result more than the amount, so starting early is worthwhile even with a small sum. When your budget has room, raise the contribution and the amount grows in proportion over the same period.

How much will ¥50,000 a month grow to in 30 years?

The principal is ¥18 million. It reaches about ¥29.14 million at 3% and about ¥41.61 million at 5%. ¥50,000 a month uses up NISA's ¥18 million lifetime limit in exactly 30 years, so if you invest in a NISA account, the gains of about ¥23.61 million at 5% are tax-free.

Will my money grow exactly as the simulation says?

No. A simulation assumes a constant return every year, while real markets rise and fall repeatedly. Even with the same average return, the order of the moves changes the final amount, and there can be periods below principal. When planning, it's a good idea to calculate at a lower return as well.