How Compound Interest Calculator Works
The Compound Interest Calculator shows how your money grows over time with the power of compounding, including advanced features like tax drag, inflation adjustment, and contribution scheduling that basic calculators omit. It reveals the true after-tax, inflation-adjusted value of your investments.
Enter your starting principal, monthly or annual contribution amount, expected annual return rate, and investment time horizon. The calculator compounds your returns at the frequency you choose: annually, semi-annually, quarterly, monthly, or daily. A real-time chart shows your balance growing over time, with a clear visual split between principal contributed and interest earned.
The tax drag feature models how annual tax on dividends and capital gains distributions erodes returns. Select your account type (taxable brokerage, traditional IRA/401k, Roth IRA) and the tool adjusts growth accordingly. For taxable accounts, you set your dividend tax rate and expected turnover to model annual capital gains tax. Tax-deferred accounts grow without drag but show the tax due at withdrawal.
Inflation adjustment converts future nominal values to today's purchasing power using a configurable inflation rate (default 3%). This answers the critical question: "What will my $1 million actually buy in 30 years?" The answer—roughly $412,000 in today's dollars at 3% inflation—often surprises people.
The results table shows year-by-year balances with columns for contributions, interest earned, tax paid, inflation-adjusted value, and cumulative totals. Export the full amortization schedule as a CSV for use in personal financial planning spreadsheets.
Key Terms Explained
- Compound Interest
- Interest calculated on both the initial principal and all accumulated interest from prior periods—'interest on interest'—which creates exponential growth over time.
- Tax Drag
- The reduction in investment returns caused by annual taxes on dividends and realized capital gains. In a taxable account, tax drag can reduce effective returns by 1-2% annually.
- Real Return
- Investment return adjusted for inflation, representing the actual increase in purchasing power. If nominal return is 8% and inflation is 3%, real return is approximately 5%.
- Rule of 72
- A quick estimation method: divide 72 by your annual return rate to approximate years needed to double your money. At 8% returns, money doubles roughly every 9 years.
- Compounding Frequency
- How often interest is calculated and added to the balance. More frequent compounding (daily vs. annually) yields slightly higher returns due to interest-on-interest accruing sooner.
Who Needs This Tool
Just opened a Roth IRA at age 25 and wants to visualize how $500/month at 8% average returns grows to over $1.5 million by retirement.
Starting a 529 college savings plan for a newborn and needs to determine monthly contributions required to cover $200K in tuition in 18 years.
Pursuing FIRE and needs to model different savings rates and return scenarios to find when their portfolio crosses the 25x annual expenses threshold.
Comparing a taxable brokerage account vs. Roth IRA to quantify how much tax drag costs over 30 years on identical investments.
Methodology & Formulas
Compound interest formula: A = P(1 + r/n)^(nt) + PMT[((1 + r/n)^(nt) - 1) / (r/n)], where P = principal, r = annual rate, n = compounding frequency, t = years, PMT = periodic contribution. Tax drag reduces effective return by: r_eff = r - (r × dividend_yield × tax_rate) - (r × turnover × gains_rate). Inflation adjustment discounts future values: real_value = nominal / (1 + inflation)^t.
Pro Tips
- Always check the inflation-adjusted column—a $2M portfolio in 30 years has the purchasing power of roughly $825K today at 3% inflation.
- Compare tax-advantaged accounts (Roth, 401k) against taxable to see the true cost of tax drag—it often exceeds $100K+ over a career.
- Use conservative return estimates (6-7% for stocks) rather than historical highs to avoid planning around best-case scenarios.
- The biggest factor isn't return rate—it's time. Starting 10 years earlier often matters more than doubling your monthly contribution.
- Model multiple scenarios (bull, base, bear) with different return rates to understand your range of outcomes rather than a single projection.