Compound Interest Calculator

See how your money grows with compound interest, monthly contributions, and time.

Investment Details

Initial Investment ($)

Monthly Contribution ($)

Annual Interest Rate (%)

Investment Period (years)

Compounding Frequency

Results

Final Amount

$345741.64

Total Contributions

$130000

Total Interest Earned

$215742

Interest as % of Total

62.4%

Contributions ($130000)Interest ($215742)

A = P(1+r/n)^(nt) + PMT x [((1+r/n)^(nt)-1) / (r/n)]

P = $10000, PMT = $500/mo, r = 8%, n = 12, t = 20

= $345741.64

How Compound Interest Works

Compound interest means your money earns returns on both the original principal and the interest already accumulated. In other words, your earnings start generating their own earnings. The base formula is A = P(1 + r/n)^(nt), where P is the starting amount, r is the annual rate, n is the compounding frequency, and t is time in years. When you add monthly contributions, the growth becomes even more powerful because every new contribution also starts compounding.

This calculator is designed for practical planning questions: How much will my money grow in 10, 20, or 30 years? How much difference do monthly contributions make? What share of my final balance comes from my own deposits versus compound growth? The year-by-year breakdown is especially useful because it shows when the compounding curve starts to accelerate.

A quick mental shortcut is the Rule of 72: divide 72 by the annual return rate to estimate how long it takes to double your money. At 6% your money doubles in about 12 years; at 8% it doubles in about 9 years; at 12% it doubles in about 6 years. This is not exact, but it is a useful way to compare savings and investment scenarios quickly.

Compounding frequency matters, but not as much as starting earlier, earning a better rate, and contributing consistently. The difference between annual and monthly compounding is usually small compared with the effect of an extra 10 years of investing or an extra $200 per month. If you want a basic primer, read Compound Interest Explained. If you want real-world debt and retirement examples, see Compound Interest in Real Life.

Remember that this calculator shows nominal growth before taxes and inflation. Real returns can be lower in taxable accounts or in high-inflation periods. For risk-free comparisons, use the CD calculator. For broader long-term planning, compare with the investment calculator.

Frequently Asked Questions

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all accumulated interest. Over time, compound interest grows exponentially while simple interest grows linearly.

How much does compounding frequency matter?

For most practical purposes, the difference between monthly and daily compounding is small. The biggest factor is the interest rate and time. However, more frequent compounding does produce slightly higher returns, especially at higher rates and over longer periods.

What is the Rule of 72?

The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by the annual interest rate. For example, at 8% return, your money doubles in approximately 72/8 = 9 years.

Does this calculator account for taxes and inflation?

No. This calculator shows nominal returns before taxes and inflation. Your real (inflation-adjusted) returns will be lower. For tax-advantaged accounts like 401(k) or IRA, taxes are deferred or eliminated, making the nominal calculation more representative.

See how compound interest silently drains your wallet through credit card debt and supercharges your retirement savings. Real numbers, real scenarios, real impact.

Compound Interest Explained: Formula, Examples and How Your Money Grows

Understand how compound interest works, how to calculate it, and why time matters so much. Includes the formula, examples, and a free compound interest calculator.