Compound Interest Calculator
Visualize the power of compound interest on your investments. Adjust principal, rate, duration and contributions.
Investment Parameters
Adjust the values below. The calculation updates automatically.
Investment Growth
Capital contributed vs interest earned over 10 years
Year-by-Year Breakdown
| Year | Opening Balance | Deposits | Withdrawals | Interest | Closing Balance |
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What is Compound Interest?
Compound interest is the mechanism by which interest earned on an investment generates its own interest in subsequent periods. Unlike simple interest, where only the original principal earns returns, compound interest creates a snowball effect: your money works for you, and the earnings from your money also work for you.
Albert Einstein reportedly called compound interest the "eighth wonder of the world." Whether the quote is authentic or not, the principle is powerful: a $10,000 investment at 7% over 30 years grows to $76,123 with compound interest, compared to just $31,000 with simple interest.
The Compound Interest Formula Explained
The basic compound interest formula is:
A = P × (1 + r/n)n×t
- A = future value of the investment
- P = principal (initial investment)
- r = annual interest rate (as a decimal, e.g., 7% = 0.07)
- n = number of compounding periods per year
- t = number of years
With regular contributions (PMT) added at the end of each compounding period, the formula becomes:
FV = P × (1 + r/n)n×t + PMT × [((1 + r/n)n×t - 1) / (r/n)]
This is the complete formula our calculator uses, accounting for both your contribution frequency and the compounding frequency you select.
The Snowball Effect: Time is Your Greatest Ally
The most powerful factor in compound interest isn't the rate or the initial amount — it's time. Growth is exponential, meaning the later years of an investment generate far more interest than the early years.
Real example: with a $10,000 initial investment and $200 monthly contributions at 7%:
- After 10 years: $53,832 (including $19,832 in interest)
- After 20 years: $134,814 (including $76,814 in interest)
- After 30 years: $283,382 (including $201,382 in interest)
The interest earned in the last 10 years exceeds the first 20 years combined. This is why starting early is the most important financial decision you can make.
Compounding Frequency: What's the Impact?
Compounding frequency determines how often interest is calculated and added to your principal. The higher the frequency, the stronger the compound effect:
- Annual (1x/year): interest calculated once per year
- Semi-annual (2x/year): interest calculated every 6 months
- Quarterly (4x/year): interest calculated every 3 months
- Monthly (12x/year): the most common for savings accounts
- Daily (365x/year): the practical maximum
On $10,000 at 7% over 10 years, annual compounding yields $19,672 while daily compounding yields $20,138. The $466 difference may seem modest, but it grows significantly with larger amounts and longer time horizons.
Simple vs Compound Interest
The fundamental difference:
- Simple interest: Interest = P × r × t. The calculation is always based on the original principal. A $10,000 investment at 5% for 20 years earns $10,000 in interest, totaling $20,000.
- Compound interest: The calculation is based on principal plus accumulated interest. The same investment at 5% compounded annually earns $16,533 in interest, totaling $26,533.
In other words, compound interest earns 65% more than simple interest in this example. The longer the duration, the wider the gap. Over 40 years, compound interest earns more than triple what simple interest does.
Frequently Asked Questions
How does compound interest work?
Compound interest calculates interest not only on your initial principal, but also on the accumulated interest from previous periods. Each compounding period, your interest earns interest, creating a snowball effect that accelerates the growth of your investment over time.
What is the compound interest formula?
The basic formula is A = P × (1 + r/n)^(n×t). P is the principal, r is the annual rate, n is the number of compounding periods per year, and t is the number of years. To include regular contributions (PMT), add PMT × [((1 + r/n)^(n×t) - 1) / (r/n)].
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal — you earn the same amount each year. Compound interest is calculated on the principal plus accumulated interest, making your investment grow exponentially rather than linearly. Over 20 years at 5%, compound interest earns 65% more than simple interest.
Does compounding frequency matter?
Yes. The more frequently interest is compounded, the greater your returns. Daily compounding earns slightly more than monthly, which earns more than annual. The gap increases with larger investment amounts and longer time horizons.
How to maximize returns with compound interest?
Three key factors: start as early as possible (time is your greatest ally), make regular contributions even if modest, and choose investments with higher compounding frequency. Even $200/month at 7% for 30 years grows to over $243,000.
What is the effective annual rate?
The effective annual rate (EAR) is the actual return after accounting for compounding. A 6% nominal rate compounded monthly yields an EAR of 6.17%. The formula is EAR = (1 + r/n)^n - 1. It allows fair comparison of investments with different compounding frequencies.
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