How this calculator works
Enter an initial amount, monthly contribution, annual return, years and optional inflation to compare different return and inflation scenarios.
Calculation method
The model compounds monthly and places each contribution at month-end. Model growth is ending value less total contributions. Today’s value discounts the future amount using the entered annual inflation rate.
Check before using the estimate
The page uses a constant monthly return and month-end contributions. Add volatility paths, taxes, product fees, contribution pauses, liquidity and irregular cash flows when analysing a specific product.
Worked example
Example input: initial ¥100,000, monthly ¥3,000, annual return 6.00%, 10 years, inflation 0%. Equivalent monthly rate ≈ 0.4868%.
| Item | Calculation | Amount |
|---|---|---|
| Total contributions | ¥100,000 + ¥3,000 × 120 | ¥460,000 |
| Ending value | Monthly compounding over 120 periods | ≈ ¥666,300 |
| Model growth | ¥666,300 − ¥460,000 | ≈ ¥206,300 |
| 0% return verification | ¥1,000 + ¥100 × 12 | ¥2,200, growth 0 |
At a 0% return rate, model growth is 0, confirming the calculation is correct.
Input variations
| Change | Ending value | Growth | vs default |
|---|---|---|---|
| Return 6% → 8% | ≈ ¥803,000 | ≈ ¥343,000 | Up ≈ ¥137,000 |
| Monthly contribution ¥3,000 → ¥5,000 | ≈ ¥863,000 | ≈ ¥283,000 | Contributions ¥580,000 |
| Years 10 → 20 | ≈ ¥1,735,000 | ≈ ¥915,000 | Time compounding is significant |
| Add 2% inflation (today's value) | ¥666,300 ÷ (1+2%)^10 | ≈ ¥544,300 | Purchasing-power loss ≈ ¥122,000 |
Time amplifies compound interest far more than small increases in return; inflation materially affects real purchasing power.
Differences from official documents
Differences from actual investment arise because: actual investment returns have a volatility sequence (early drawdowns and late rallies produce different results), while this tool uses a constant monthly rate; investment returns may include taxes (fund redemption fees, dividend tax) which this tool does not deduct; bank or fund product management, custody and sales-service fees are deducted from product NAV, which this tool does not model; actual contribution dates may be month-start or mid-month, while this tool assumes month-end; withdrawals or switches may trigger fees or taxes, which this tool does not compute.
Thresholds and limits
Initial amount ¥0–¥1,000,000,000; monthly contribution ¥0–¥10,000,000; annual return -20%–50% (required, no default); years 1–80; inflation 0%–20%. No default return — to avoid treating any specific historical average as a future guarantee. Negative rates in this tool are a constant negative-rate path, not an extreme drawdown in a volatility scenario.
Documents to check
Bank or fund product information sheet (showing fees, risk rating, past-performance-is-not-future-return warning); fund regular-savings plan agreement (verify debit date, amount, auto-renewal rules); bank app or fund company app holdings and transaction records; product annual or quarterly reports (verify actual management fees and past returns); tax filing records (verify dividend tax, fund redemption tax, etc.).
A steady return path is sensitivity analysis, not a forecast
One rate cannot represent volatility or sequence risk.
Contribution timing changes growth time
Starting money experiences every month; each month-end contribution has less time.
Nominal balance and purchasing power are separate
Inflation discounts today’s value but does not alter nominal growth; tax, fees and losses remain outside.