What is this calculator for?
You're 28, you just contributed $6,500 to a Roth IRA for the first time, and you want to know what that single deposit is worth at 65 if you never touch it. Or you're a 45-year-old who finally has $50,000 saved and wants to know whether $500 a month is enough to retire on. Or you're explaining to a 22-year-old niece why starting now matters more than waiting until she "makes more money." The compound interest calculator is the math behind all of those conversations.
Compound interest is interest earning interest. Each compounding period, the interest your money earned gets added to the principal, and the next period's interest is calculated on the bigger total. Over short windows it looks linear and unimpressive; over long windows it bends sharply upward. The S&P 500's roughly 10% nominal annual return means $1 today is worth about $1.10 in a year, $2.59 in ten years, and $45 in forty years — almost all of that final $44 of growth happens in the last two decades.
This calculator handles both the lump-sum case (you deposited X once and let it sit) and the recurring-contribution case (you add Y each month to the principal). It's the right tool for retirement projections, college savings, "what if I had invested instead of bought" thought experiments, and any back-of-envelope question about how money grows over decades.
How to use this calculator
Enter your initial investment as the dollar amount you start with today. If you're modeling a brand-new account with no balance yet, set this to $0 and use the monthly contribution field to model deposits going forward.
Monthly contribution is what you'll add each month. Realistic numbers matter here. The 2025 401(k) employee contribution limit is $23,500 ($30,500 if you're 50+) which works out to about $1,958/month maxed out. The 2025 Roth IRA limit is $7,000/year ($8,000 if you're 50+), or about $583/month. Don't model contributions you can't sustain — the most common error in projections is "what if I save $2,000/month" when your actual budget is $400.
Annual interest rate is the expected return. For long-term US stock index funds, financial planners typically use 7% real (after inflation) or 10% nominal. For high-yield savings, 4-5% in 2024-2026. For Treasury bonds, 4-5%. For a balanced 60/40 portfolio, 6-7% nominal. Be skeptical of any projection using 12%+ over decades — that overstates expected returns by 2-4 percentage points.
Compounding frequency rarely matters much for investments — monthly versus annually changes long-run balances by less than 1%. It matters more for credit card debt (where daily compounding at 24% APR is brutal). Time period is your investment horizon in years. Past 40 years the math becomes more theoretical than predictive — markets, tax law, and personal circumstances all shift.
Understanding your results
The headline numbers: final balance is what your account is projected to be worth at the end of the period. Total interest earned is the growth from compounding alone — your money's money. Total contributions is what you actually put in out of pocket. Interest as % of final balance tells you how much of your end-of-life balance is your work versus the market's work.
Reading the breakdown: the year-by-year snapshot shows the curve. For a 40-year horizon with $200/month at 7%, the first ten years look depressingly linear — most of your balance is your contributions, not growth. Years 20-30 the curve starts bending. Years 30-40 the slope is almost vertical. This is why financial planners are obsessive about starting early: not because the early years grow fast, but because the late years grow fast and they need the early principal to grow into.
Two reality checks. First, the numbers are nominal — they don't account for inflation. A $1.2M projected balance in 2065 dollars is worth maybe $400K in today's purchasing power at 3% inflation. For real-dollar projections, subtract your inflation assumption from your rate of return (use 7% instead of 10% if you assume 3% inflation). Second, the calculator assumes a constant rate of return. Real markets are volatile — 2008 saw a 37% loss; 2024 saw a 25% gain. The long-run average works out, but any specific year can deviate dramatically. Don't budget retirement around an exact dollar projection 40 years out.
The Rule of 72 is a quick sanity check: divide 72 by your annual return to estimate doubling time. At 7%, money doubles every ~10 years. At 10%, every ~7.2 years. If your tool result shows your money doubling in 4 years at 5%, the math is wrong somewhere.
A worked example
Maya is 25, just landed her first salaried job at $58,000/year in Denver, and decides to open a Roth IRA. She can afford $400/month — that's $4,800/year, below the $7,000 cap. She picks a target-date 2065 index fund with a roughly 7% real expected return. She plans to keep contributing for 40 years until she retires at 65.
The math: starting balance $0, monthly contribution $400, annual return 7%, monthly compounding, 40 years. Final balance: about $1.05 million. Total contributions: $192,000. Total interest: ~$858,000. Of her final balance, 82% is investment growth, 18% is what she actually put in. Her $400/month, sustained for four decades, becomes nearly a million dollars of buying power (in real terms — meaning today's purchasing power, since we used the 7% real rate).
The instructive contrast: suppose Maya waits ten years and starts contributing $400/month at age 35 instead of 25. Same 7% real return, 30 years instead of 40. Final balance: about $490,000 — less than half. The $48,000 of contributions she skipped in years 1-10 cost her roughly $560,000 of final balance. That's the time-versus-money tradeoff: the dollars she contributed in her twenties did more compounding work than any dollars she could ever contribute later.
One more variation: Maya contributes $400/month from 25 to 35 (ten years, $48,000 total), then stops completely and lets the balance ride at 7% until 65. Final balance at 65: about $463,000. She contributed for ten years and got almost as much as the 35-year-old who contributed for thirty. That asymmetry — the absurd power of the first decade of compounding — is the entire argument for starting an IRA before 30 even if you can only afford small amounts.
Related resources
For tax-advantaged retirement-specific projections, see the 401(k) Planner with employer match logic and the Social Security Estimator for benefit projections. For modeling long-horizon purchasing power, the Inflation Calculator converts nominal projections to real dollars. For investment returns on specific lump-sum decisions (a home purchase, a stock buy, a marketing campaign), the ROI Calculator handles single-period and annualized return math. The SEC's investor.gov compound interest calculator is the federal regulator's own version of this tool — equivalent math, different presentation.