Compound Interest
Calculator

Compound interest means earning interest on both your principal and the interest you've already earned. Unlike simple interest, compound interest grows exponentially — each period's interest becomes part of the base for the next calculation.

For example, if you invest $10,000 at 8% annual interest, you earn $800 in year one. In year two, you earn interest on $10,800 — giving you $864 instead of $800. That extra $64 is compounding in action.

The core formula is: A = P(1 + r/n)^nt

Where:
- A = Final amount
- P = Initial principal
- r = Annual interest rate
- n = Number of compounding periods per year
- t = Number of years

Our compound interest calculator is designed to be simple and instant. Just four inputs give you a full investment projection:

1. Enter your initial investment
This is your starting principal. Even a small amount can grow significantly over time.

2. Set a monthly contribution
Regular contributions are a powerful growth accelerator. A consistent monthly deposit dramatically increases your final balance.

3. Choose an annual interest rate
Select a rate that reflects your investment strategy. Conservative investments typically return 3–5%, balanced portfolios 6–8%, and growth-oriented strategies 10% or more.

4. Set your investment period
Time is compound interest's greatest ally. The longer your money compounds, the more powerful the effect.

The quote — "Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it" — is often attributed to Einstein. Whether or not he said it, the idea holds perfectly true.

The power of starting early
If you invest $1,000/month starting at age 25 at 8% annual return, you'll have approximately $3.5 million by age 65. Start at 35 instead, and that number drops to around $1.5 million — a difference of $2 million from just 10 extra years.

The Rule of 72
A simple mental math trick: divide 72 by your annual return rate to estimate how many years it takes to double your money. At 8%, your investment doubles roughly every 9 years.

Key takeaways
- Start investing as early as possible — time is your greatest asset
- Invest consistently to benefit from dollar-cost averaging
- Stay patient and let the compounding snowball grow
- Keep costs low to avoid eroding your compound returns