Compound Interest Calculator

Compound interest calculator for monthly savings growth

This compound interest calculator shows how a starting balance combined with regular monthly contributions can grow over time at a fixed annual interest rate. It breaks the final result into three parts: your original starting amount, the total of your monthly contributions, and the total interest earned. This split is important because it shows clearly how much of your future balance you "put in" versus how much came from growth alone.

To use it, enter your starting amount (which can be zero if you are starting from scratch), your monthly contribution, the annual interest rate you expect, and the number of years you plan to invest. The calculator applies monthly compounding — interest is added to your balance each month and then earns interest itself in subsequent months — which is how most savings accounts, money market funds, and many investment products work in practice.

The core concept behind compound interest is that interest earns interest. With simple interest, a 8% annual rate on 10,000 always adds the same 800 each year, regardless of what has accumulated. With compound interest, the first year adds 800, but the second year's interest is calculated on 10,800, adding roughly 864, and the growth accelerates over time. Over many years and at reasonable rates, this compounding effect produces results that are substantially larger than simple interest would suggest — which is why "time in the market" is such a consistently emphasised principle in long-term savings advice.

Monthly contributions amplify the compounding effect further. Even a modest regular contribution — say, 200 per month — adds up to 24,000 in principal over ten years, but with compounding at a moderate rate that amount could grow significantly beyond what you contributed. The calculator makes this visible by separating your contributions from your interest earned, so you can see the real value of compounding over the period you choose.

The results here are illustrations, not guarantees. The calculator assumes a fixed interest rate for the entire period, which is rarely how real investments behave. Market returns vary, savings rates change, fees are not modelled, and tax on interest or returns is not deducted. For long-term planning purposes — retirement, education savings, major goals — use this calculator to understand the general shape of growth under different rate and contribution assumptions, then work with a qualified financial adviser for a plan based on real products, tax implications, and your personal circumstances.

Assumptions and how to use this calculator

• Interest compounds monthly. The annual rate is divided by 12 and applied to the balance at the end of each month.
• Monthly contributions are added at the end of each month before that month's interest is applied.
• The interest rate is fixed for the entire period and does not change from year to year.
• No tax, fees, platform charges, or transaction costs are deducted from the balance or growth.
• Starting amount can be zero. Monthly contribution can also be zero if you only want to model lump-sum growth.
• This calculator is for planning and illustration purposes. It is not financial advice.

Common questions

What is compound interest and how is it different from simple interest?

Simple interest pays a fixed amount based only on your original principal each period. Compound interest pays interest on your principal plus all interest that has already been added to the balance. Over time this means your balance grows faster with compound interest because each period's interest calculation is based on a larger number. The longer the period and the higher the rate, the bigger the difference between the two approaches.

Why does the calculator use monthly compounding?

Monthly compounding matches the way most retail savings accounts and many investment products operate — interest is credited to the account once per month. Using monthly compounding also aligns with monthly contributions, which is how most people save. Annual compounding would understate growth for someone making monthly deposits because those mid-year deposits would not earn any interest until the end of the year.

What interest rate should I enter?

Enter the annual interest rate you expect to earn, as a plain number (for example, 7 for 7%, not 0.07). For a savings account, use the current annual interest rate the account pays. For an investment portfolio, you might use a long-term historical average as a planning assumption — but remember that past returns do not guarantee future results and actual returns will vary year to year.

What happens if my interest rate is zero?

With a zero interest rate the calculator simply totals your starting amount and all monthly contributions, with no interest component. The future balance equals exactly what you put in. This is useful as a baseline comparison: you can compare saving in cash (zero rate) against investing at various assumed rates to see how much the growth component matters over your chosen time horizon.

Can I use this calculator for debt or loan calculations?

This calculator is designed for savings and investment scenarios where you are building a balance. For loans and debt, the dynamics are reversed — you are paying down a balance rather than building one, and the interest works against you rather than for you. Use a loan repayment or amortisation calculator for those scenarios, as it will show monthly repayments, total interest paid, and a balance payoff timeline.