BEHIND THE NUMBERS
How it works—and some practical advice
A short explanation of the calculations, plus the ideas I use when thinking through retirement assumptions.
Project My Retirement
This calculator follows the information you enter and projects your portfolio from your current age through your life expectancy.
During your working years
Your annual contribution is your salary multiplied by your savings rate. Salary then grows by your annual salary-growth estimate.
The half-year of growth assumes that contributions are made gradually throughout the year.
During retirement
Retirement spending is entered in today's dollars. The calculator increases it for inflation before retirement and during each retirement year. Passive income starts at the amount entered and then grows with inflation.
If passive income is greater than spending, the withdrawal is $0; extra passive income is not added to the portfolio.
Find My Savings Rate
This calculator works backward. It tests different savings rates until it finds the estimated minimum percentage of salary needed to reach your desired balance at the start of your life expectancy age. It uses the same salary growth, contribution timing, inflation, spending, passive-income, and investment-return rules as the main calculator.
THE SPREADSHEET VERSION
Download the Excel calculator
If you prefer working directly in a spreadsheet, this is the updated Excel model behind the website. It includes the retirement calculations and the original notes that shaped the advice below.
Download the Excel calculator →Choosing reasonable assumptions
No single set of assumptions will predict the future. I like to start with roughly 3% inflation, 8% investment growth while working, and lower growth in retirement—then rerun the calculator with both more cautious and more optimistic numbers.
Salary growth depends heavily on your field and career path. A steady annual raise is useful for planning, but moving into a higher-paying role can matter far more than a small change in the assumed investment return.
Save consistently and collect the match
A useful starting goal is saving at least 10% of income for retirement, including an employer match. If that is not realistic today, start where you can and increase it as your income grows.
My general order is: build an emergency fund, collect the full employer match, work toward a solid overall savings rate, fund an HSA when it fits your situation, and then add more to tax-advantaged or low-fee investment accounts.
Plan for the life you actually want
Life expectancy and retirement spending are two of the hardest inputs. It is usually safer to test living longer than the average rather than planning only to the average age.
Spending can vary enormously. Someone with a paid-off home and modest plans may need much less than someone who wants to travel often and help family. Think about housing, healthcare, transportation, travel, and generosity—not just basic bills.
Use the accounts available to you
An HSA can be especially valuable for future healthcare costs. Contributions and growth can receive tax advantages, and withdrawals for qualified medical expenses can be tax-free under federal rules.
Traditional and Roth accounts tax money at different times. Traditional contributions can reduce taxable income now, while qualified Roth withdrawals can be tax-free later. Having some of each can give you more flexibility when managing taxes in retirement.
Be careful with Social Security estimates
Social Security rules and benefit levels can change over a long retirement-planning horizon. Instead of assuming either the full benefit or nothing at all, try the calculator with several passive-income amounts and see how much your plan depends on the benefit.
Keep the result in perspective
The calculator uses constant average returns and does not model taxes, fees, market swings, detailed Social Security rules, or every unexpected expense. Use it to compare choices and understand tradeoffs—not to predict an exact future.