The 30-Second Summary
A common way to estimate this is to start from the desired annual spending in retirement, calculate the portfolio size needed using a commonly referenced sustainable withdrawal rate (often cited around 4% per year), and then work out what consistent monthly contribution — at an assumed rate of return — would reach that figure within the available time horizon.
1. Starting From Desired Spending, Not Current Income
Rather than starting from how much someone earns today, many planning frameworks suggest estimating how much will actually need to be spent annually during retirement, then multiplying that figure by a factor (often 25, derived from a 4% withdrawal rate) to arrive at a target portfolio size.
| Desired Annual Retirement Spending | Approximate Target Portfolio (4% rule) |
|---|---|
| $20,000 | ~$500,000 |
| $40,000 | ~$1,000,000 |
| $60,000 | ~$1,500,000 |
*The 4% rule is a simplified reference point, not a guarantee; its reliability depends on the length of retirement, the portfolio's composition, and market conditions.
2. From the Total Target to a Monthly Contribution
Once a target portfolio size is defined, the next step is calculating what consistent monthly contribution, at an assumed rate of return, would reach that figure in the years available before retirement. The more years remaining, the smaller the monthly contribution can be, thanks to the effect of compound growth.
🧮 Variables That Change the Result the Most
- Years until retirement: More time allows for smaller monthly contributions to reach the same target.
- Assumed rate of return: A more conservative estimate requires larger contributions; a more optimistic one isn't guaranteed.
- Estimated annual retirement spending: Adjusting this number up or down directly changes the total target.
- Other retirement income sources: Pensions or public benefits can reduce the portfolio size needed.
3. Why the 4% Figure Is a Starting Point, Not a Rule
The commonly cited 4% withdrawal rate comes from historical research on how long a diversified portfolio has tended to last under various market conditions. It's frequently used as a simplified planning shortcut, but actual sustainable withdrawal rates can vary based on the length of retirement, the specific mix of investments held, and the sequence of market returns experienced early in retirement.
4. Revisiting the Number Over Time
Because both the target spending figure and the assumed rate of return are estimates, most planners recommend revisiting the calculation periodically — for example, every few years, or after a major life change like a move, a new dependent, or a significant change in income. A number calculated once in someone's twenties is unlikely to still be accurate decades later without adjustment.
5. Frequently Asked Questions
What if I don't know my future spending yet?
Many people start with a rough estimate based on current spending, adjusted for expected changes like a paid-off mortgage or reduced work-related costs, and refine it over time.
Does this calculation include other income like public benefits?
It can — some approaches subtract expected benefit income from the target spending figure before calculating the required portfolio size.
How often should I recalculate this number?
Revisiting it every few years, or after major income or life changes, is a common practice.
This article is educational and doesn't constitute personalized financial advice. Consider speaking with a financial advisor for a calculation tailored to your situation.