Compound Interest Calculator
FAQ
Your questions answered.
What rate of return should I use?
The S&P 500 has returned approximately 10% annually in nominal terms over the long run, and approximately 10.3% since expanding to 500 stocks in 1957. A more conservative estimate of 7% adjusts for inflation and reflects a balanced portfolio. Use 10% for historical comparisons, 7% for inflation-adjusted planning. Adjust downward if your portfolio includes bonds or cash.
Does the calculator account for taxes?
No — the calculator models pre-tax growth. For tax-advantaged accounts like a 401(k) or IRA, these figures reflect your actual compounding since no taxes are taken during the growth phase. For taxable brokerage accounts, actual after-tax results will be lower depending on your tax rate and how often you realize gains.
How does monthly compounding differ from annual compounding?
Monthly compounding applies returns every month rather than once per year, producing slightly higher results. The difference between monthly and annual compounding on $100,000 at 7% over 30 years is approximately $14,600 — meaningful but far less important than the rate of return itself. This calculator uses monthly compounding, which is standard for investment accounts.
What is the difference between nominal and real returns?
A nominal return is the raw percentage gain before adjusting for inflation. A real return accounts for inflation, preserving purchasing power. Historically, the S&P 500 has returned approximately 10% nominally and 7% in real terms. For long-term planning, real returns are the more honest figure — $1 million in 30 years buys less than $1 million today.
Can I use this calculator for home equity growth?
Yes. Home equity appreciation can be modeled the same way. Enter your current equity as the starting balance, estimate an annual appreciation rate (the Case-Shiller long-term national average is approximately 4%), and set the time horizon. The calculator shows how your equity compounds over time — useful for comparing home appreciation to other investment returns.
How to Use This Calculator
Starting Balance ($): The amount you are investing today, or your current portfolio value.
Conservative (%): Your lower-bound expected annual return. A common conservative estimate is 7%, which reflects the S&P 500's long-term real (inflation-adjusted) return.
Optimistic (%): Your upper-bound expected annual return. 10–15% reflects the S&P 500's long-term nominal return or a higher-growth portfolio scenario.
Compound Frequency: Choose Monthly or Annually. Monthly compounding applies returns every month rather than once per year, producing slightly higher results over long periods.
Years of Growth: How long you plan to invest. Use the slider or type directly. This is the most powerful variable in the calculator — small changes in time horizon produce large changes in outcome.
Regular Contribution ($): The amount you add on a recurring basis.
Contribution Frequency: Choose Monthly or Annually to match how you actually invest.
The calculator returns a Conservative Estimate with Upside Potential, a Growth Over Time chart showing both scenarios, and The Split — the breakdown between your total contributions and the interest earned.
The Split: Contributions vs. Interest Earned
Below the chart, The Split shows how much of your final balance came from your own contributions versus interest earned through compounding.
In the early years, contributions dominate. Over longer time horizons, interest earned overtakes contributions — often dramatically. On a $100,000 starting balance with $10,000 in annual contributions over 20 years at a 10% Conservative rate, the calculator shows $300,000 contributed and $945,500 in interest earned. The compounding does more than three times the work of the contributions themselves.
This ratio is the core argument for starting early and staying invested.
How Compound Frequency Affects Your Results
Monthly compounding applies your return rate twelve times per year rather than once, allowing each month's gains to compound in the following month. The difference versus annual compounding on the same inputs is real but modest — the more important variables are your return rate and your time horizon.
If you contribute monthly, set Contribution Frequency to Monthly. If you invest a lump sum annually — such as a year-end bonus or annual IRA contribution — set it to Annually. Matching the Contribution Frequency to how you actually invest will give you the most accurate projection.
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