Compound Growth Formula
The foundation of compound interest is that interest earned in one period generates its own interest in subsequent periods. When you know your initial balance, annual rate, compounding frequency, and time span, you can calculate your future balance using the formula below.
FV = PV × (1 + r/m)^(m×t)
CG = FV − PV
FV— Future value—the total amount you'll have after compound growthPV— Present value—your initial depositr— Annual interest rate (expressed as a decimal, e.g., 0.05 for 5%)m— Number of compounding periods per year (12 for monthly, 4 for quarterly, etc.)t— Time in yearsCG— Compound growth—the total interest earned (FV minus PV)
How to Use the Compound Growth Calculator
Enter your initial deposit, then specify the annual interest rate and how long you plan to keep the money invested. Select your compounding frequency—daily, weekly, monthly, quarterly, semi-annually, or annually—since interest accrues more frequently at higher frequencies, boosting your returns.
If you make regular deposits (weekly, monthly, quarterly, or annual), enter the deposit amount and how often you contribute. The calculator also lets you set whether deposits occur at the beginning or end of each period, which slightly affects the outcome. You can even model deposits that grow at a percentage rate annually, reflecting salary increases or inflation adjustments over time.
Once calculated, you'll see your final balance broken down into:
- Principal growth—interest earned on your initial deposit
- Deposit growth—interest earned on your periodic contributions
- Total compound growth—combined interest from all sources
A bar chart visualises how your balance evolves, making it easy to spot where most growth comes from.
Periodic and Annual Growth Rate Relationship
If your deposits themselves grow over time—say, you increase contributions by 3% annually as your income rises—the calculator links periodic growth rates to your deposit frequency automatically.
(1 + annual growth rate) = (1 + periodic growth rate)^(number of periods per year)
annual growth rate (g)— The percentage increase in deposit size per yearperiodic growth rate (g_p)— The percentage increase per deposit period (month, quarter, etc.)q— Number of deposit periods per year
Key Factors That Shape Your Final Balance
Several variables drive how much you accumulate:
- Compounding frequency—More frequent compounding (daily vs. annually) means interest earns interest more often, creating a snowball effect. At lower rates this difference is modest, but at higher rates or longer time horizons it compounds substantially.
- Interest rate—Even small differences in annual rate have outsized effects over 20+ years due to exponential growth.
- Time horizon—Longer time frames allow compound interest to work its magic. A 10-year investment grows far more than a 2-year one at the same rate.
- Periodic deposits—Regular contributions accelerate growth, especially if the deposit amount increases annually. Someone who saves £500 monthly for 20 years will see dramatically different results than someone who makes one lump sum deposit.
- Deposit timing—Beginning-of-period deposits compound slightly longer than end-of-period ones, creating a small but measurable advantage over decades.
Common Pitfalls in Compound Growth Planning
Avoid these frequent mistakes when projecting investment growth.
- Ignoring inflation on long horizons — A 5% interest rate looks good until inflation runs at 3% annually, leaving you with only 2% real growth. Over 30 years, that difference is enormous. Adjust your expectations for real purchasing power, not just nominal gains.
- Forgetting tax implications — Interest income and investment gains are often taxable. A calculator showing 6% annual growth may net you only 4–4.5% after taxes, depending on your jurisdiction and account type. Always factor in your after-tax returns.
- Assuming consistent rates — Interest rates rise and fall. A fixed-rate savings account may offer 4% today and 1% in two years. Use the calculator to model multiple scenarios—best case, worst case, and realistic middle—rather than relying on one static number.
- Underestimating the cost of early withdrawal — Many investment accounts or fixed-term deposits penalise early withdrawal, erasing months or years of interest. Check terms carefully before committing funds, especially if you may need liquidity sooner than planned.