The Illusion Created by Nominal Simulations
When setting a goal like "build 50 million yen in assets over 30 years," most people calculate in nominal terms (face value). However, if inflation runs at 2% per year for 30 years, the purchasing power of 50 million yen drops to the equivalent of roughly 27.6 million yen in today's money. In other words, even if you hit the nominal 50 million yen target, its real value is only about half of what you intended. To avoid this illusion, all calculations should be performed in real terms (inflation-adjusted).
Real return can be approximated as "nominal return minus inflation rate." If the nominal return is 7% per year and inflation is 2%, the real return is approximately 5%. Future asset values calculated at this real return represent amounts converted to today's purchasing power, making them far more intuitive. Investing 50,000 yen per month at a 5% real return for 30 years yields approximately 41.61 million yen in real terms - an amount with the same purchasing power as 41.61 million yen today.
How the Inflation Rate Assumption Dramatically Changes Results
The inflation rate used in a simulation must be chosen carefully. Japan's Consumer Price Index (CPI) averaged about 0.5% per year from 2013 to 2021, but surged to 3-4% from 2022 onward. Whether you use the 30-year average or the most recent 5-year average produces vastly different results. For conservative simulations, it is important to run multiple inflation scenarios in parallel (low: 1%, medium: 2%, high: 3%) and develop a range of projections.
Some expense categories, such as healthcare and education, rise faster than the general inflation rate. Books on rising prices and household budgeting cover the differences in inflation rates by expense category and how to build financial plans that account for them.
Asset Erosion Simulation by Inflation Rate
Let's calculate the real erosion from inflation if you hold 30 million yen in savings without investing. At 1% inflation, the real value after 20 years is approximately 24.56 million yen (18% decline). At 2%, it drops to about 20.14 million yen (33% decline). At 3%, it falls to roughly 16.51 million yen (45% decline). In an environment where deposit interest rates are around 0.1%, the gap between the deposit rate and inflation translates directly into a real loss.
Conversely, investing at a return that exceeds inflation increases purchasing power. At 5% annual return with 2% inflation, the real return is approximately 3%, and 30 million yen grows to a real value of about 54.18 million yen after 20 years. Quantifying the "risk of doing nothing" is the strongest motivation to begin investing.
Setting Real-Term Goals to Improve Planning Accuracy
If you estimate needing 250,000 yen per month in living expenses during retirement, the nominal amount required 30 years from now would be over 450,000 yen per month (at 2% inflation). Planning in real terms lets you work with the straightforward goal of "250,000 yen per month in today's value." Designing withdrawals to increase in line with inflation ensures that purchasing power is maintained throughout retirement.
Pension benefits in Japan are partially adjusted for inflation through the price-slide system, but the macro-economic slide mechanism means they don't fully keep pace. You need to bridge this gap through your own efforts. Books explaining the pension system and macro-economic slide will help you understand the system and accurately estimate the shortfall. Try varying the inflation rate in our simulator to test multiple patterns and build an asset plan that suits your situation.