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Free Compound Interest Calculator

See how your investments grow over time with compound interest and regular monthly contributions.

Investment Details
7.0%
0%20%
20 years
1 year50 years

Eindbedrag

$300,851

Totale inleg

$130,000

Totale rente verdiend

$170,851

Specificatie

Understanding Compound Interest

Compound interest is interest earned on both your original principal and on previously accumulated interest. It's the difference between linear growth and exponential growth — and over long time periods, the difference is enormous.

The formula is: A = P(1 + r/n)^(nt), where P is principal, r is annual interest rate, n is compounding frequency per year, and t is time in years. With monthly contributions, each deposit starts compounding from the month it's added.

Here's a concrete example: $10,000 at 7% annual return compounded monthly for 30 years becomes $76,123 — without adding a single dollar. Add $500/month and it becomes $657,307. That's $180,000 you contributed and $477,307 in pure compound growth. The money your money earned also earned money.

Einstein allegedly called compound interest the "eighth wonder of the world." He probably didn't actually say that, but the math is genuinely remarkable. The key insight: time matters more than amount. Starting 10 years earlier with less money often beats starting later with more.

When to Use This Calculator

Planning retirement savings

See how your 401(k) or IRA will grow over 20–40 years. Adjust the monthly contribution to find the amount that gets you to your retirement goal. Even small increases ($50/month more) compound dramatically over decades.

Comparing investment options

A savings account at 4.5% vs an index fund averaging 7% vs bonds at 5% — run each scenario and see the difference over your time horizon. The gap between 5% and 7% over 30 years is often hundreds of thousands of dollars.

Understanding the cost of waiting

Calculate the difference between starting to invest at 25 vs 35. With the same monthly contribution and return rate, the 10-year head start typically results in 50–100% more money at retirement.

Setting savings goals for major purchases

Need $50,000 for a down payment in 5 years? Work backwards to find the monthly contribution needed at your expected return rate.

Investment Calculation Tips

1.

Use realistic return rates

The S&P 500 has averaged about 10% nominal (7% after inflation) over the long term. High-yield savings accounts offer 4–5% currently. Don't use 12% unless you have a specific reason — overly optimistic projections lead to under-saving.

2.

Account for inflation

A million dollars in 30 years won't buy what a million buys today. Subtract 2–3% from your return rate to see "real" (inflation-adjusted) growth. $1M in 2055 at 3% inflation is worth about $412,000 in today's purchasing power.

3.

Compounding frequency matters less than you think

The difference between monthly and daily compounding on a 7% return is tiny — about 0.02% per year. What matters far more is the rate itself and how long you stay invested. Don't chase daily compounding; focus on return rate and time.

Features

  • Calculate compound growth with any principal, rate, and time period
  • Add regular monthly contributions to see total accumulation
  • Compare compounding frequencies: daily, monthly, quarterly, annually
  • Visual growth chart showing principal vs interest over time
  • Year-by-year breakdown table
  • Adjust for inflation to see real purchasing power
  • No signup required — runs entirely in your browser

Frequently Asked Questions

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 original amount), compound interest creates exponential growth because your earnings generate their own earnings.

How does compounding frequency affect returns?

More frequent compounding produces slightly higher returns because interest starts earning interest sooner. At 7% annual rate on $10,000 over 10 years: annual compounding gives $19,672, monthly gives $20,097, daily gives $20,138. The difference between monthly and daily is negligible for most purposes.

What is the Rule of 72?

The Rule of 72 is a quick mental math shortcut: divide 72 by your annual return rate to estimate how many years it takes to double your money. At 7% return: 72 ÷ 7 ≈ 10.3 years to double. At 10%: about 7.2 years. It's an approximation but surprisingly accurate for rates between 4–12%.

Should I use nominal or real (inflation-adjusted) returns?

Both are useful. Nominal returns (not adjusted for inflation) tell you the actual dollar amount you'll have. Real returns (subtract ~2-3% for inflation) tell you the purchasing power of that money. For retirement planning, real returns give a more honest picture of your future lifestyle.

How much should I save monthly for retirement?

A common guideline is 15% of gross income (including employer match). But the real answer depends on when you start, your target retirement age, and desired lifestyle. Use this calculator to model different scenarios — start with your goal amount and work backwards to find the monthly contribution needed.

Tips & Related Workflows