The Answer: 72,000 Years
Let us cut straight to the answer. At 0.001% annual interest, it takes approximately 72,000 years for 1 million yen to become 2 million yen. Using the Rule of 72 (72 ÷ interest rate = years to double): 72 ÷ 0.001 = 72,000 years. Homo sapiens emerged roughly 300,000 years ago, so this is about a quarter of all human history.
72,000 years ago, no humans had yet settled in the Japanese archipelago. Neanderthals were still living in Europe. Even if a Neanderthal had deposited 1 million yen at 0.001% interest, it still would not have doubled by 2026. That is the reality of ultra-low interest rates.
Years to Double at Different Interest Rates
Changing the interest rate transforms the doubling time dramatically. Savings account at 0.001%: 72,000 years. Time deposit at 0.1%: 720 years. Government bonds at 0.5%: 144 years. Index fund at 5%: about 14 years. Long-term stock average at 7%: about 10 years. The difference in doubling speed between a savings account and a stock index is more than 7,000-fold.
Of course, stocks carry the risk of declining in value, while deposits do not. But how much is the "safety" of "it will double if you wait 72,000 years" really worth? A human life spans at most about 100 years. Nobody can wait 72,000.a wealth-building primer lays out concrete alternatives beyond bank deposits.
Deposits Are for Storing Money, Not Growing It
There is nothing wrong with bank deposits. Keeping 3 to 6 months of living expenses in a savings account is an essential financial safety net. For emergencies - unexpected bills or a sudden loss of income - instantly accessible deposits are the best option.
The problem is leaving surplus funds beyond your emergency reserve sitting in a deposit account. Leave 1 million yen in a savings account for 30 years at near-zero rates and you earn about 300 yen in interest. Invest the same 1 million yen at 5% for 30 years and it grows to about 4,320,000 yen. The difference is 3,310,000 yen. Deposits are where you keep money safe; investments are where you grow money. Separating these roles is the fundamental strategy for putting compound interest to work.