Compound Interest Lab
An interactive lesson in how time, contributions, fees, and inflation shape a hypothetical portfolio. Nothing here is a forecast or advice.
You put in￥9,300,000
Projected value￥17,512,063
In today's money￥12,069,416
Lost to fees￥1,397,615
What this shows
Time does the heavy lifting
The gap between the two lines is growth. It widens slowly at first, then accelerates — starting earlier usually beats contributing more later.
Fees are not small
A 1% fee looks tiny but compounds against you every year. Compare the "Lost to fees" figure to your contributions.
Inflation is real
"In today's money" strips out inflation so the number reflects what it could actually buy.
A straight line is not reality
Real markets rise and fall. A smooth curve hides volatility and the sequence of good and bad years.
Assumptions
- Contributions are added at the end of each month; growth compounds monthly.
- The return, fee, and inflation you enter are held constant for the whole period.
- Taxes are not modelled here — inside a NISA, qualifying gains are tax-free; outside it, gains are generally taxed.
Hypothetical illustration, not a forecast, guarantee, or investment advice. Verify current rules with official sources.