The Compound Interest Calculator shows how an investment grows when interest is earned not just on your original principal, but also on all previously earned interest. It supports optional monthly contributions and multiple compounding frequencies.

How to Use This Calculator

1. Enter your Principal — the starting amount you’re investing.
2. Enter the Annual Interest Rate (e.g., 7 for 7%).
3. Enter the Time Period in years.
4. Choose how often interest compounds: annually, quarterly, monthly, or daily.
5. Optionally add a Monthly Contribution to model regular deposits.

Why Compounding Frequency Matters

Compounding frequency determines how often earned interest is added back to the principal. Daily compounding yields slightly more than annual compounding at the same nominal rate. For example, $10,000 at 8% for 10 years grows to $21,589 with annual compounding vs. $22,254 with daily compounding — a difference of $665 just from compounding more frequently.

Most savings accounts compound daily. Most bonds compound semi-annually. CDs can vary. Checking the frequency when comparing accounts gives you a more accurate apples-to-apples comparison.

The Power of Starting Early

The single most powerful variable in compound interest is time. Someone who invests $5,000/year from age 25 to 35 (10 years, then stops) ends up with more money at 65 than someone who starts at 35 and contributes every year until 65 — assuming the same 7% return. Starting even 5 years earlier can add tens of thousands of dollars to a retirement account.

Frequently Asked Questions

What is the Rule of 72?

Divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 8% interest, your money doubles in roughly 72 ÷ 8 = 9 years.

How does this differ from simple interest?

Simple interest is calculated only on the principal each period. Compound interest is calculated on the principal plus all accumulated interest. Over time, the difference becomes enormous — compound interest is why Albert Einstein reportedly called it the “eighth wonder of the world.”

Does this calculator account for inflation?

No — it shows nominal (not inflation-adjusted) growth. To estimate real purchasing power, subtract the expected inflation rate from the interest rate (e.g., 7% return − 3% inflation = 4% real return) and recalculate.