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Compound Interest Calculator
See how your money grows over time with compound interest. Add monthly contributions and watch the long-term impact.
Sources & Methodology
By Sean Baldwin · Last reviewed July 2026
Frequently Asked Questions
What is compound interest?
Compound interest means you earn interest on your interest, not just your original principal. Over time, this creates exponential growth, the longer you invest, the more powerful it becomes.
How often is interest compounded in this calculator?
This calculator compounds monthly, which is standard for most savings accounts and investment accounts.
What annual return rate should I use?
The S&P 500 has historically returned around 10% annually before inflation (about 7% after inflation). High-yield savings accounts currently offer 4–5%. Use the rate that matches your investment type.
Why does starting early matter so much?
Because of compounding, money invested early has exponentially more time to grow. $1,000 invested at age 25 vs. age 35 can result in double the final balance by retirement.
How does the Worth It Score work?
The score is based on your growth multiple, how many times your total contributions you end up with. A 3x multiple or higher scores very well; under 1.5x scores lower.
Why compound interest is the most powerful force in personal finance
Compound interest means you earn returns on your previous returns, not just your original principal. This creates exponential rather than linear growth. $10,000 invested at 7% for 30 years grows to $76,123, a 7.6x multiple where $66,123 of the final balance is interest earned on interest, not money you put in. The critical insight: time matters more than rate. $10,000 for 40 years at 7% becomes $149,745. Adding 10 more years nearly doubles the outcome without any additional contribution. This is why starting at 25 vs. 35 is worth tens of thousands of dollars, even with identical contributions.
Compounding frequency: daily vs. monthly vs. annual, how much does it matter?
Compounding frequency is how often interest is calculated and added to your balance. Daily compounding is slightly better than monthly, which is slightly better than annual, but the differences are smaller than most people think. On $10,000 at 5% for 10 years: annual compounding gives $16,289; monthly gives $16,470; daily gives $16,487. The $198 difference between annual and daily compounding over 10 years matters far less than whether you actually invest and at what rate. Don't let compounding frequency distract from the two numbers that matter most: contribution amount and time horizon.
What annual return rate to use for different investment types
Historical averages by asset class (before inflation): S&P 500 index funds ~10%, balanced stock/bond portfolio ~7–8%, high-yield savings accounts currently 4–5%, CDs 4–5%, bonds 3–5%, money market accounts 4–5%. Inflation has historically run 2–3%, so your real (inflation-adjusted) return is about 3 percentage points lower. For long-term planning, using 7% for a stock-heavy portfolio and adjusting down for more conservative mixes is a reasonable middle-ground assumption. Actual returns will vary widely year to year, the long-run average is what matters for projections.
The Rule of 72: quick mental math for doubling time
Divide 72 by your annual return rate to estimate how many years it takes to double your money. At 6%: 72/6 = 12 years to double. At 9%: 72/9 = 8 years. At 4%: 72/4 = 18 years. This is surprisingly accurate across a wide range of rates. It also works in reverse: if you want your money to double in 10 years, you need roughly 7.2% annual returns. The Rule of 72 is useful for quickly evaluating whether an investment opportunity is realistic, if someone promises to double your money in 3 years (implied 24% annual return), that's an enormous red flag.
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How We Calculate Your Score
The Worth It Score is based on your growth multiple — how many times your total contributions your final balance equals. A higher multiple means compound interest is doing more of the work for you. The score rewards long time horizons and strong return rates because those produce the highest multiples.
- · Growth multiple 4x or more (final balance is 4× what you put in) → 95
- · Growth multiple 3x or more → 88
- · Growth multiple 2x or more → 78
- · Growth multiple 1.5x or more → 65
- · Growth multiple below 1.5x → 45
Growth multiple = total final balance ÷ total amount contributed. A multiple below 1.5 means your time horizon or rate is too low for compounding to significantly outpace your contributions — consider increasing either.
Cite this calculator: Worth It Calculators, "How Much Will Your Money Actually Be Worth in 10, 20, 30 Years? (2026)," worthitcalculators.com/compound-interest/ (updated July 2026).