CalcSmart › Compound Interest Calculator

Compound Interest Calculator — Watch Your Money Grow

See how your savings or investments grow with compound interest. Add a monthly contribution to see the full power of consistent saving over time. Free, no login, runs privately in your browser.

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yrs
Future Value
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Total Contributed—
Interest Earned—

What is Compound Interest?

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which only earns on the principal), compound interest earns interest on interest — causing your money to grow exponentially over time.

Albert Einstein reportedly called compound interest the "eighth wonder of the world." Whether or not he actually said it, the sentiment is accurate: the longer your money compounds, the more powerful the effect becomes.

The Formula

For a lump sum: A = P × (1 + r/n)^(nt) where P is principal, r is annual rate, n is compounds per year, and t is time in years. This calculator compounds monthly (n=12) which is standard for most savings accounts and investments.

The Rule of 72

A quick mental shortcut: divide 72 by your annual return rate to estimate how many years it takes your money to double. At 7%, money doubles every ~10 years. At 10%, every ~7 years. This rule works surprisingly well for rates between 6% and 10%.

Why Monthly Contributions Matter

Adding even a small amount monthly dramatically accelerates growth. Starting with $10,000 at 7% for 20 years gives you ~$38,700. But adding $500/month turns that into over $300,000 — almost 8 times more — because each contribution also compounds over the remaining time.

Frequently Asked Questions

What is a realistic annual return rate to use?+
For long-term stock market investments (index funds like S&P 500), 7% is a commonly used estimate after inflation. The historical nominal return of the S&P 500 is around 10%, but adjusting for inflation brings it closer to 7%. For savings accounts or bonds, use a much lower rate (1–5%). Always use a conservative estimate for planning purposes.
How often does compound interest compound?+
Compounding frequency matters — the more often interest compounds, the faster your money grows. Common frequencies: annually, quarterly, monthly, daily. Most savings accounts compound daily but report interest monthly. This calculator lets you choose your compounding frequency — monthly is the default and most common for savings accounts, but you can select daily, quarterly, semi-annual, or annual to match your actual account terms.
When should I start investing?+
As early as possible — time is the most powerful variable in compound interest. $10,000 invested at age 25 at 7% becomes ~$149,700 by age 65. The same $10,000 invested at age 35 only grows to ~$76,100. Starting 10 years earlier almost doubles the outcome. Even small amounts invested early outperform larger amounts invested late.
Does compound interest work against me too?+
Yes — compound interest works both ways. On credit card debt with 20%+ annual rates, unpaid balances grow exponentially. A $5,000 balance at 20% APR that you only pay minimums on can take 20+ years to pay off and cost over $15,000 in interest. Always pay off high-interest debt before investing.

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