Investment Growth Calculator
Project how your portfolio grows with compound interest and regular contributions. See the power of time in the market with a year-by-year breakdown.
Current investment portfolio value. Enter 0 if starting fresh.
Regular monthly additions (401k, IRA, brokerage, etc.).
Nominal (before inflation). Historical S&P 500 avg ~10%.
Years until you need the money (e.g., until retirement).
The Power of Compound Interest in Retirement Investing
Compound interest is one of the most powerful forces in long-term investing. Unlike simple interest (which only earns returns on the original principal), compound interest earns returns on both the principal and all previously accumulated growth. Over decades, this creates exponential growth that can turn relatively modest contributions into substantial wealth.
The Rule of 72
A quick mental shortcut: divide 72 by your annual return to estimate how many years it takes to double your money. At 6% returns, your portfolio doubles in roughly 12 years (72 ÷ 6 = 12). At 8%, it doubles in about 9 years. At a 4% post-retirement return, doubling takes 18 years. This rule illustrates why even a 1-2% difference in investment fees or returns has a massive compounding impact over a 30-year retirement.
Why Early Contributions Matter Most
Because of compounding, early contributions have far more time to grow. A single $10,000 investment at age 25 growing at 7% annually becomes approximately $149,745 by age 65 — nearly 15× the original amount. The same $10,000 invested at age 45 only grows to about $38,697 by 65. This is the "time in market" advantage that no market timing strategy can consistently replicate.
For a deeper dive into how compound growth interacts with your retirement projection, see the Retirement Planning Guide. The SEC also maintains a useful Compound Interest Calculator with educational material on how compounding works.
Account Type Matters for Growth
Not all investment accounts compound equally in practice, because tax drag differs:
- Roth IRA / Roth 401k: grows completely tax-free; qualified withdrawals have zero federal tax. Best for assets with the highest expected growth. See the Roth Guide.
- Traditional 401k / IRA: tax-deferred growth — no annual tax drag, but withdrawals are taxed as ordinary income. Great for high earners who expect lower income in retirement.
- Taxable brokerage: dividends and realized gains taxed annually, reducing net compounding. Best for assets held long-term (capital gains rates are lower than ordinary income rates).
SmartRetireCalc models all three account types separately in the retirement projection calculator, applying different tax treatments to each. Asset location — putting high-growth assets in Roth and income-producing assets in tax-deferred accounts — is a key strategy for maximizing after-tax wealth.
See how investment growth integrates with your full retirement plan: Run the retirement calculator → or learn about Roth vs. traditional account strategies.