How to use this calculator
Enter your initial deposit amount, then specify the annual interest rate your investment earns. Set your investment timeframe in years and months. Next, choose your compounding frequency—daily, monthly, quarterly, or annual—which determines how often interest is calculated and added to your balance.
If you plan to add regular deposits, specify their amount and frequency (weekly, monthly, quarterly, or annual). You can also set whether deposits occur at the beginning or end of each period, and optionally model growing deposit amounts if your contributions are expected to increase over time.
The calculator instantly shows your final balance, total interest earned, and a detailed breakdown of how much came from the initial balance versus your additional deposits.
Understanding interest rates and compounding
An interest rate represents the cost of borrowing or the return on lending. When you deposit money in a savings account, you're effectively lending to the bank; the interest rate is your compensation. Interest rates are expressed as percentages of the principal.
Compounding frequency answers a critical question: how often is interest calculated and added to your balance each year? Annual compounding calculates interest once yearly. Quarterly means four times per year. Monthly compounds twelve times. Daily compounding (365 times yearly) accelerates growth significantly. More frequent compounding means each new calculation includes previously earned interest, creating an exponential effect over time.
The relationship between your periodic growth rate and annual growth rate is linked by this equation:
Annual Growth Rate + 1 = (1 + Periodic Growth Rate) ^ Payment Frequency
This means if you set a growth rate for your deposits, the calculator adjusts automatically based on how often you contribute.
The compound interest formula
The fundamental equation calculates your investment's future value by compounding the interest rate over your investment period. It accounts for the principal, annual rate, how often compounding occurs, and the total time invested.
FV = P × (1 + r/m)^(m×t)
FV— Future value (final balance)P— Principal (initial balance)r— Annual interest rate (as a decimal, e.g., 0.05 for 5%)m— Compounding frequency (number of times per year)t— Time in years
Simple versus compound interest
Simple interest calculates returns only on your original principal amount. It grows linearly and is rarely used in real-world banking or investments.
Compound interest calculates returns on both the principal and all previously accumulated interest. This creates exponential growth—each compounding period, you earn interest on a larger base. Over decades, this difference becomes dramatic. For example, £10,000 at 5% annually grows to £16,289 with annual compounding over 10 years, but only £15,000 with simple interest.
The magic of compounding intensifies with:
- Longer time horizons (more compounding periods)
- Higher interest rates
- More frequent compounding cycles
- Regular additional deposits
Key considerations when calculating compound interest
Avoid these pitfalls when forecasting investment growth:
- Nominal versus effective rates — A 12% annual rate compounded monthly delivers different returns than 12% annual compounded yearly. Always clarify the compounding frequency—it significantly impacts your actual yield. Many institutions highlight the nominal rate while burying compounding details.
- Inflation erodes real returns — A 5% interest rate might seem attractive until you factor in 3% inflation. Your real return—the purchasing power gain—is only about 2%. Savings accounts often fail to keep pace with inflation, especially during economic downturns.
- Tax implications — Interest earned is typically taxable income. If you're in a higher tax bracket, your after-tax return may be considerably lower than the calculator's gross figure. Tax-advantaged accounts (ISAs in the UK, 401(k)s in the US) can preserve more of your gains.
- Irregular deposits reduce effectiveness — Missing or delaying contributions disrupts the compounding schedule. Even small gaps compound over decades—a single month's missed deposit in year 5 of a 30-year plan costs you thousands in lost growth.