Discount Rates, Inflation, and the Optimal Safety Bucket Length

A useful way to decide whether to use a shorter or longer safety bucket is to compare the discount rate (the rate used to discount future expenses when determining today's safety bucket) with expected inflation.

When the discount rate exceeds inflation, longer safety bucket periods (such as 10, 20, or even 30 years of expenses) become less burdensome. A higher discount rate reduces the present value of future spending liabilities, meaning less capital needs to be allocated to the safety bucket today. As a result, the debt allocation required to protect a given number of years of spending is lower than many investors might expect.

In fact, the greater the number of years of expenses held in the safety bucket, the greater the benefit, because the higher discount rate has more time to reduce the present value of future spending liabilities. Consequently, each additional year of spending protection requires progressively less additional debt allocation, making longer safety bucket periods increasingly economical.

The opposite occurs when expected inflation exceeds the discount rate. In this environment, the present value of future spending rises more rapidly as the number of years of expenses held in the safety bucket increases. As a result, the debt allocation required for longer bucket periods increases disproportionately. For example, in one of the scenarios modeled, increasing the safety bucket from 5 years to 20 years of expenses increased the initial debt allocation from approximately 16% to 68%—more than four times as much—even though the number of protected years increased by exactly four times.

The practical implication is that assumptions about the future macroeconomic environment should influence the choice of safety bucket length. If you expect inflation to exceed the discount rate over an extended period, shorter safety bucket periods are likely to be more efficient because the cost of extending the safety bucket increases rapidly. Conversely, if you expect the discount rate to remain above inflation, longer safety bucket periods become increasingly attractive because they provide substantially more years of spending protection without requiring a proportionate increase in debt allocation.