Retirement advice tends to collapse into one generic number — "save 15% of your income" — repeated regardless of your age, goals, or current savings. That's a reasonable starting rule of thumb, but the actual math behind a retirement projection is more specific, and more useful, than a flat percentage.
A retirement projection combines three things: your current savings growing with compound interest, your ongoing monthly contributions also growing with compound interest, and the number of years until you plan to retire. The result is your projected nest egg — and from there, a "safe withdrawal rate" (commonly 4%, based on research designed to make savings last roughly 30 years) tells you how much annual income that nest egg can sustainably support.
How the Projection Is Calculated
Your current savings compound forward using the standard compound interest formula for however many years remain until retirement. Your monthly contributions are calculated similarly, with each contribution compounding for its own remaining time until retirement (a contribution made in year 1 compounds for nearly the full period; one made in the final year barely compounds at all). Add both pieces together for your projected nest egg, then multiply by your target withdrawal rate to estimate sustainable annual retirement income.
A Worked Example
Starting at age 30 with $20,000 saved, contributing $500/month, earning 7% annually, and retiring at 65 (35 years), your projected nest egg comes out to roughly $984,000. Applying a 4% safe withdrawal rate, that supports about $39,000 in sustainable annual income. Delay starting by just 10 years — beginning at 40 instead of 30 with the same contribution — and the projected nest egg drops to around $455,000, less than half, purely because of the lost decade of compounding.
Common Mistakes to Avoid
- Underestimating the cost of delaying: the math above shows why — the earliest years of contribution do disproportionately more work than later ones.
- Ignoring employer match: if your employer matches 401(k) contributions, that's an immediate, guaranteed return that should be captured before any other savings goal.
- Using an unrealistic return assumption: 10-12% (the S&P 500's raw historical average) tends to overstate real outcomes; 6-8% is a more conservative, planning-friendly range.
- Treating the 4% rule as absolute: it's a widely used starting point, but some planners now suggest 3-3.5% for extra safety given longer life expectancies and market uncertainty.
Bottom Line
A specific number beats a generic percentage every time. Use a Retirement Calculator with your actual age, savings, and contribution rate to see your real projected nest egg and sustainable retirement income, rather than relying on someone else's rule of thumb.