Compound interest is interest earned on both your principal and previously earned interest — creating exponential wealth growth. A $10,000 investment earning 7% annually grows to $19,672 in 10 years through daily compounding. This calculator shows the exact growth curve with any frequency (daily, monthly, quarterly, annual) and models regular contributions so you can forecast real retirement savings and investment goals.
Compound Interest Calculator
Calculate the power of compound interest. Model different frequencies and see how your savings or investments grow exponentially over time.
How Compound Interest Is Calculated
Compound interest is calculated by applying the interest rate to the principal PLUS all previously earned interest. The more frequently interest compounds (daily vs. monthly vs. annually), the more you earn. Banks use daily compounding to maximize your returns (on savings accounts) or their returns (on credit cards).
| Variable | Definition |
|---|---|
| A | Final amount (principal + all interest earned) |
| P | Principal — your initial investment or savings |
| r | Annual interest rate (as a decimal, e.g., 7% = 0.07) |
| n | Compounding frequency per year (365 for daily, 12 for monthly, 1 for annual) |
| t | Time in years |
Example: $10,000 at 7% APR with daily compounding for 10 years grows to $19,861 (vs. only $17,000 with simple interest). The extra $2,861 is pure compound interest — money you didn't contribute. High-yield savings accounts at 4.5% compound daily; this calculator shows exactly how much daily compounding beats monthly or annual.
💡 Expert Pro-Tip
Time Is Worth More Than Money — Start 10 Years Earlier, Not Twice as Much: A 25-year-old investing $5,000/year for 10 years (retiring at 35) and earning 7% average annual returns ends up with $1.2M by age 65 — zero contributions after age 35. A 35-year-old investing $5,000/year for 30 years (retiring at 65) accumulates only $845K. The 25-year-old won more than a 40% return with 1/3 the total contributions, purely from compounding time. This is why "I'm too young to start investing" is the most expensive mistake you can make. Your first $50,000 invested at 25 will become more than your first $500,000 invested at 45.
Compound Interest FAQ
What average return should I expect from investments?
Stock market historical average: ~10% annually (before inflation). 60/40 portfolio (60% stocks, 40% bonds): ~7% annually. Bond index funds: 4–5% annually. High-yield savings: 4.5% in 2026, no risk. This calculator defaults to 7% as a conservative stock return. Running your own projections at 5%, 7%, and 10% shows how powerful even 2% differences become over decades.
Does compounding frequency really matter?
Yes. On $10,000 at 7% for 20 years: Annual compounding = $38,696. Monthly compounding = $40,766. Daily compounding = $40,936. Daily wins by ~$240 — more free money just for frequency. For large amounts or long timelines, daily compounding over annual can mean tens of thousands in extra gains. High-yield savings accounts use daily compounding; regular savings use monthly or quarterly. Always choose daily compounding when available.
What's the "Rule of 72" for doubling money?
Divide 72 by your return rate to estimate doubling time. At 7% returns: 72 ÷ 7 = ~10 years to double. At 10%: 72 ÷ 10 = ~7 years. At 4% (savings account): 72 ÷ 4 = 18 years. This approximation is quick and surprisingly accurate. Knowing your doubling time changes your perspective: 7% returns double your money every decade — $50k becomes $100k, then $200k, then $400k by retirement. Slow money (4%) takes 18 years per doubling — nearly half your working years.
How much do I need to invest monthly to hit my goal?
This calculator works backwards: enter your goal ($500k), expected return (7%), timeline (20 years), and initial investment ($0), and it shows the monthly contribution needed (~$815/month). This is far more useful than asking "how much will $500/month grow?" — most people work backwards from a goal, not forward from a guess.
Is 7% return realistic? What if markets crash?
7% is the 100-year historical average for stocks, but you'll have -20% years (2022), +30% years (2024), and everything in between. Using a conservative 5–6% return in your projections accounts for sequence of returns risk (bad markets early in retirement hurt more than bad markets mid-accumulation). If you can't stomach -20% declines, target 5% (bonds + stocks) instead of 7% (full stocks). Lower return beats panic-selling during crashes.
Should I be investing or paying down debt?
If you're earning 5–7% in investments and owing 3% on a mortgage: invest. If you're owing 20% on credit cards: pay debt first — guaranteed 20% "return" by reducing interest. If you're owing 7% on student loans and can earn 7% investing: it's a toss-up psychologically, but mathematically equivalent (debt payoff just feels better). Rule: Guaranteed returns (debt payoff) beat risky returns (investing) unless the risky return is significantly higher.