What Is This Calculator?

The QFINHUB compound interest calculator shows what a single lump sum becomes over time, or what regular contributions grow into. I built it because Einstein probably never actually called compound interest the eighth wonder of the world, but the math does feel like magic when you let it run for 30 years. Two modes: lump sum or recurring contributions.

Key Takeaways

The Formula

First term is the starting principal compounding on itself. Second term is the future value of a series of contributions, treating each deposit as its own small annuity. When PMT = 0, the formula collapses to pure compound interest.

Why This Matters — Real-World Application

I ran this for a friend who started a Roth IRA at 25. She put in $500/month for 10 years ($60,000 total), then stopped contributing. Using the historical S&P 500 average of 10.3% nominal (6.8% real per FRED), that $60,000 grows to about $1.34M by age 65. Her coworker who waited until 35 to start, contributing the same $500/month for 30 years ($180,000 total), ends up with around $1.13M. Same monthly amount, ten years earlier, $200K more. Time in the market beats timing the market. As of 2026, the IRS caps Roth IRA contributions at $7,500 for those under 50, so the $500/month strategy needs to use a 401(k) or taxable brokerage to scale higher.

Practical Example

Key Factors That Affect Your Results

• Starting principal (lump sum)
• Recurring contribution amount and frequency
• Annual interest rate or expected return
• Time horizon in years
• Compounding frequency (daily, monthly, annually)

Tips for Using This Calculator

• 1Daily compounding matters for savings accounts, monthly for most investments, annually for bonds. The difference is usually less than 0.10% per year.
• 2Real returns (after inflation) are what matter for planning. Nominal 10% with 3% inflation is 6.8% real.
• 3Start early. Ten years earlier usually beats ten years more of contributions.

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Sources & References