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Compound Interest Calculator
Einstein supposedly called compound interest the eighth wonder of the world. Enter your starting amount, monthly contribution, expected return, and time — and watch time do the heavy lifting.
For education only — not financial advice. Past returns never guarantee future results.
How compound interest works
Simple interest pays you only on your original deposit. Compound interest pays you on your deposit plus every dollar of interest already earned — so your money starts earning its own money, which earns its own money. The calculator compounds monthly, matching how most real accounts and funds accrue growth.
Run a classic example: $5,000 to start, $400 a month, an 8% average annual return, 30 years. You contribute $149,000 total — and end up with roughly $651,000. About $502,000 of that is pure growth: money your money earned while you were sleeping. That is compounding. Now run it at 20 years instead of 30: the result drops to about $260,000. Same monthly habit, but ten fewer years costs you nearly $390,000. Time is the ingredient most people underestimate. We unpack the full idea in Compound Interest: The Most Powerful Force in Money.
What your numbers mean
Future value is your projected balance at the end — the headline number. Treat it as a direction, not a promise: the calculator assumes a smooth, constant return, while real markets zigzag. A range of 6–10% for a stock-heavy portfolio is a common planning assumption; 4–5% for a high-yield savings account.
Total contributions is what you actually put in. Compare it against future value to see who did the work — you or your money. In long projections, growth usually contributes more than you do, which is exactly why starting beats waiting for "enough" money.
Interest earned is the payoff of patience: future value minus contributions. Experiment with the inputs to feel the levers. Raising your monthly contribution by $100 matters, but adding five years matters more. And nudging the return from 6% to 8% over 30 years can nearly double the outcome — which is why fees matter: a 1% fee is a 1% drag on this same engine, year after year.
Someone who invests $300 a month from age 25 to 35 and then stops can end up with more at 65 than someone who starts at 35 and invests $300 a month for 30 straight years. That is compounding rewarding the early starter — money's most unfair, most wonderful trick.
Mistakes that slow your growth
1. Waiting for the perfect moment. The biggest compounding mistake is delay. Every year you wait is a year of growth you can never buy back — run the calculator at 30 vs. 25 years with your own numbers and feel the difference.
2. Ignoring fees. A fund charging 1% annually instead of 0.1% quietly eats roughly a quarter of your gains over 40 years. Check every account's expense ratio; boring index funds usually win.
3. Raiding the account. Compounding only works on money that stays invested. Cashing out a 401(k) early costs you taxes, penalties, and decades of growth. Keep a separate emergency fund so market dips never force you to sell.
4. Timing the market. Missing just the ten best market days in a decade can cut long-term returns nearly in half. Consistent monthly contributions — the exact habit this calculator models — beat clever timing for almost everyone.
5. Forgetting inflation. Your $651,000 in 30 years buys less than $651,000 today. Mentally subtract ~3% annual inflation when judging the number, or compare scenarios relative to each other rather than treating any projection as exact purchasing power. Pair this with our budget planner to lock in the monthly contribution automatically.
Frequently Asked Questions
What is a realistic annual return to use?
For US stocks, the long-run average is around 10% before inflation (~7% after). Planners commonly use 7–8% for stock-heavy portfolios, 4–5% for balanced ones, and 1–5% for savings accounts. Be conservative: planning at 7% and getting 9% is a happy surprise; the reverse is a crisis.
How often does the interest compound here?
Monthly — each month's growth is added to the balance before the next month's growth is calculated, and your monthly contribution lands at the end of each month. Most real accounts compound daily or monthly, so this closely matches reality.
Is it better to invest a lump sum or monthly?
Mathematically, investing a lump sum immediately usually wins because markets rise more often than they fall — time in the market beats timing. But monthly contributions (dollar-cost averaging) are how most people actually invest from paychecks, and the discipline of automatic monthly investing beats a lump sum that never happens.
Does this account for taxes and inflation?
No — the projection is pre-tax and in nominal dollars. In a Roth IRA, qualified withdrawals are tax-free; in a traditional 401(k), you will owe income tax on withdrawals. And ~3% annual inflation means future dollars buy less. Use the numbers to compare strategies, not as a spending plan.
How much do I need to invest to reach $1 million?
It depends on return and time — which is exactly what this calculator answers. As a rule of thumb at 8%: about $300/month for 40 years, $700/month for 30 years, or $1,700/month for 20 years gets you there. Play with the inputs to find your own path.