Getting Started 🧮💲 Compound Interest Calculator

Introducing the PortfolioGPT Compound Interest Calculator

The Compound Interest Calculator helps you visualize how your investments can grow over time when interest is reinvested—whether you're using a Fidelity SPAXX money market fund, Roth IRA, 401(k), or a taxable brokerage account. It shows how earning interest on both your original principal and previously earned interest leads to exponential growth over long horizons.

This tool is ideal for Gen Z and Millennial investors who want a simple way to see the long-term impact of interest rates, compounding frequency, and ongoing contributions on their wealth-building journey. Instead of guessing, you can run quick “what if” scenarios to compare accounts, yields, and contribution strategies side-by-side.

What the Calculator Takes into Account

The calculator models compound growth using a few intuitive inputs: your starting balance, expected annual interest rate or yield, compounding frequency, investment duration, and any recurring contributions you choose to add. These inputs feed a compound interest formula that grows your balance over each compounding period.

You can use it for a variety of goals—short-term savings in a high‑ yield cash account, multi-year Roth IRA contributions, or decades-long retirement investing. By adjusting rates and contributions, you'll see how even small changes in yield or monthly deposits can materially affect your future balance.

Key Outputs You'll See

After you run a calculation, the tool displays your projected future balance at the end of your chosen duration. It typically highlights your total contributions versus the growth generated by compound interest so you can clearly see how much of the outcome comes from your deposits versus market or interest-rate returns.

Many users treat this as a compound growth sandbox—adjusting interest rates, compounding frequency, and contribution amounts to understand the trade-offs between risk, yield, and time. Within PortfolioGPT, the same framework can be used to compare different yield-bearing strategies, such as cash-like products, bond funds, or diversified portfolios.

Step 1: 💵 Enter Your Starting Principal

Begin by entering your initial investment amount, also called the principal. This could be the current balance of your SPAXX position, Roth IRA, 401(k), or any other account balance you want to model. The calculator will treat this as your starting point for compounding.

Step 2: 📊 Set Your Interest Rate or Yield

Next, input the annual interest rate or yield you expect to earn. For a money market fund like SPAXX, this could be the current 7‑day yield; for long-term portfolios, you might use an estimated average annual return. Keeping this assumption realistic helps you avoid overly optimistic projections.

Step 3: 🔁 Choose Compounding Frequency

Select how often interest is compounded—annually, quarterly, monthly, or daily. More frequent compounding means interest is added to your balance more often, which slightly accelerates growth over long periods. This setting lets you align the calculator with how your specific account credits interest or dividends.

Step 4: ⏳ Set Your Investment Duration

Enter how long you plan to keep the money invested—this could be a few years for a short-term goal or several decades for retirement. Time is the key driver of compound growth, so extending your horizon often has a dramatic impact on your final balance.

Step 5: ➕ Add Recurring Contributions (Optional)

Optionally, specify a recurring contribution amount—monthly, quarterly, or annually. This models dollar-cost averaging into your account over time and shows how consistent deposits can significantly boost your ending balance compared to relying on growth from a one-time lump sum alone.

Step 6: 🚀 Run the Calculation and Compare Scenarios

Click the "Calculate" button to generate your results. The calculator will display your projected ending balance, break out your total contributions, and highlight how much growth came from compounding. You can then tweak rates, frequency, duration, and contributions to compare different strategies or accounts.

In PortfolioGPT, these outputs can feed directly into portfolio and income-planning tools—helping you see how cash yields, bond funds, or diversified portfolios might influence your overall long-term trajectory alongside retirement planning.

Ready to Explore the Power of Compounding?

The Compound Interest Calculator is an educational tool designed to help you understand how rate, time, and contributions work together to grow your wealth. Use it to set realistic expectations, compare account options, and design a savings pattern that fits your goals— then leverage PortfolioGPT's AI-driven portfolios to decide how to actually invest those growing balances.

💡 Try the Compound Interest Calculator today and see how consistent investing and the magic of compounding can transform your long-term financial outcomes.