How to use this compound interest calculator
- Pick a currency. This only changes the symbol and number format. Nothing is converted.
- Enter your starting amount. Use 0 if you are starting from nothing.
- Enter a regular contribution and how often you make it (daily, weekly, monthly, quarterly or yearly). Use 0 if you will not add more.
- Choose whether each contribution goes in at the start or the end of its period. Start-of-period contributions earn one extra period of interest.
- Enter the yearly interest rate you want to test and how often it compounds. The rate is your assumption, not a prediction.
- Enter the number of years and any extra months, up to 100 years.
- Optionally turn on inflation to see the balance in today’s money, or add a second scenario to compare two plans side by side.
What is compound interest?
Compound interest is interest earned on your original money and on the interest that has already been added. In the first year you earn interest only on what you put in. In the second year you earn interest on that amount plus the first year’s interest, and so on. The balance grows slowly at first and faster later, which is why starting early and staying invested matter.
The same idea works against you with debt: unpaid interest is added to what you owe, and the larger balance then charges more interest.
Compound interest formula
A = P × (1 + r/m)m×t
- A – the final balance.
- P – the starting amount (principal).
- r – the yearly nominal interest rate as a decimal (5% is 0.05).
- m – the number of times interest compounds each year (12 for monthly, 365 for daily).
- t – the time in years.
How this calculator treats regular contributions
Each contribution is grown separately from the date it is paid until the end of the period, using the same factor (1 + r/m)^(m×t) with t set to the time that deposit stays invested. That keeps one consistent model even when you contribute weekly but interest compounds monthly, for example. Nothing is treated as if it were deposited on day one.
- End of period: the first contribution is made after one contribution period and earns no interest for its own period.
- Start of period: the first contribution is made on day one and every contribution earns one extra period of interest.
- A year is 12 months, 52 weeks or 365 days. If the time is not a whole number of contribution periods (for example 1 year 1 month with weekly deposits), only complete periods receive a contribution.
- Because every pound or naira earns the same effective annual rate in this model, the “effective annual rate” shown in the results is also the annualized return of your cash flows.
How regular contributions affect growth
Regular contributions often matter more than the starting amount, especially over long periods. Each deposit starts its own compounding clock, so early deposits do the most work. Timing matters too, but modestly: paying at the start of each month instead of the end gives every deposit one more month of interest.
Example: with 10,000 to start, 100 a month, 5% interest compounded monthly and 10 years, paying at the end of each month gives 31,998.32, while paying at the start gives 32,063.02. Frequency matters less than the amount: what you contribute in total over a year usually counts for more than how it is split.
Compound interest examples
These examples use illustrative rates and were checked against the calculator’s own engine. Real rates differ by product and change over time.
| Assumptions | Total contributed | Estimated interest | Estimated final balance |
|---|---|---|---|
| USD: 10,000 start, no deposits, 5% compounded monthly, 10 years | $10,000.00 | $6,470.09 | $16,470.09 |
| NGN: ₦500,000 start, ₦20,000 added at the end of each month, 12% compounded monthly, 5 years | ₦1,700,000.00 | ₦841,741.75 | ₦2,541,741.75 |
You can reproduce both by entering the same figures above. The naira example uses 12% only as an illustration; check current rates from your bank, fund or treasury instrument.
Simple interest vs. compound interest
Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus earlier interest. With 10,000 at 5% for 10 years and no deposits:
- Simple interest: 10,000 × (1 + 0.05 × 10) = 15,000.00
- Compounded annually: 16,288.95
- Compounded monthly: 16,470.09
The gap widens with time: the longer the period, the more the compounding adds. Check which method a loan or deposit uses. Some products, such as certain short-term deposits, use simple interest.
How inflation affects your savings
A nominal balance is the number on your statement. Purchasing power is what that number can buy. When prices rise, the same balance buys less. If you assume a constant yearly inflation rate i, the balance in today’s money after t years is:
Real value = Future balance ÷ (1 + i)t
Example: 5,000 to start and 200 a month at 6% compounded monthly for 20 years reaches 108,959.20. With 3% yearly inflation that is worth about 60,328.07 in today’s money. This is a simplified estimate that uses one constant rate you choose. Real inflation varies from year to year and by country, and the calculator does not assume any country’s rate for you.
Limitations of compound interest calculations
- Rates change. Savings rates move with the market and policy, and the calculator uses one fixed rate for the whole period.
- Investments are volatile. Shares and funds do not earn a steady yearly rate, and a bad early year can matter more than the average suggests.
- Fees and taxes are not included. Management fees, account charges and tax on interest or gains reduce what you keep. Subtract them from the rate you enter if you want a rough allowance.
- Products have their own rules: crediting dates, minimum balances, lock-ins, penalties and day-count conventions can all change the real figure.
- Inflation is simplified into one constant yearly rate.
Frequently asked questions
How do I calculate compound interest?
For a single deposit, multiply the starting amount by (1 + r/m) raised to the power m × t, where r is the annual rate as a decimal, m is the number of compounding periods a year and t is the number of years. For example, 10,000 at 5% compounded monthly for 10 years is 10,000 × (1 + 0.05/12)^120 = 16,470.09. With regular deposits, each deposit is grown from the date it was made, which is what this calculator does for you.
Is daily compounding better than monthly compounding?
Slightly, at the same stated rate. More frequent compounding credits interest sooner, so the balance ends a little higher. On 10,000 at 5% for 10 years, monthly compounding gives 16,470.09 and daily gives 16,486.65. The gap is small compared with the effect of the rate, the time and how much you contribute. Compare products by their effective annual rate, not by the compounding label.
Can I calculate compound interest with monthly deposits?
Yes. Enter your regular contribution, choose Monthly, and pick whether the deposit is made at the start or the end of each month. You can also choose daily, weekly, quarterly or annual contributions, and the compounding frequency can differ from the contribution frequency.
How does inflation affect my savings?
Inflation reduces what a balance can buy. If prices rise 3% a year, a balance of 108,959.20 in 20 years buys about what 60,328.07 buys today. Turn on the inflation option to see the projected balance in today’s money. The calculator uses one constant inflation rate that you choose, which is a simplification.
Does compound interest guarantee investment growth?
No. The calculator applies the rate you type in every year. A savings account may change its rate, and investments such as shares or funds can rise and fall. A fixed rate in a calculator is an assumption for planning, not a promise or a forecast.
What is the difference between interest rate and annual percentage yield?
The interest rate (nominal rate) is the stated yearly rate before compounding. The annual percentage yield, also called the effective annual rate, is what you actually earn over a year once compounding is included. 5% compounded monthly has an effective annual rate of about 5.12%. The results panel shows the effective annual rate for your inputs.
Can I use this calculator for Nigerian naira?
Yes. Choose NGN in the currency list and enter your amounts in naira. Selecting a currency only changes the symbol and number format; it does not convert between currencies. Interest rates in Nigeria change often and differ by product, so use the rate quoted by your bank, fund manager or treasury instrument, and treat the result as an estimate.
How accurate are the results?
The arithmetic is exact for the model described on this page, and the year-by-year table adds up to the headline figures. How close the result is to real life depends on your assumptions. Real accounts may compound or pay interest on different dates, change their rates, charge fees or deduct tax, none of which are included unless you reflect them in the rate you enter.