Compound Interest Calculator

Compound Interest Calculator overview

This calculator estimates how a starting balance and regular monthly deposits could grow when interest is added to the balance and earns interest in later periods. It separates the amount contributed from the estimated growth so the result is easier to check.

The result is a projection, not a forecast. It assumes the same annual return and monthly deposit for the full period. Real savings rates and investment returns can change, and fees, tax and inflation are not included.

How this calculator works

The annual rate is divided by 12 and applied monthly. At the end of each month, the regular deposit is added to the balance. This means the calculator treats deposits as end-of-month contributions and compounds the accumulated balance monthly.

Use the same currency for the starting amount and monthly deposit. Enter the expected rate as an annual percentage, not a monthly rate. If you are comparing products, use rates on the same basis and account for fees separately.

Compound interest formula explanation

For a starting lump sum, the standard formula is FV = P × (1 + r ÷ n)^n×t, where P is the starting amount, r is the annual rate as a decimal, n is the number of compounding periods each year and t is the number of years.

Regular deposits require an additional annuity calculation. This calculator handles that month by month so it can show the combined future balance, total cash contributed and the difference attributed to compound growth.

Step-by-step instructions

1. Enter the amount already saved or invested. Use zero if there is no starting balance.
2. Enter the amount you expect to add at the end of each month.
3. Enter an annual return or interest rate and the number of years.
4. Calculate the result, then compare the final balance with the contributed amount.
5. Run a lower-rate or lower-deposit scenario before relying on the result.

Practical examples

$1,000 plus $200 per month

With a $1,000 starting balance, $200 deposited at the end of each month, a constant 6% annual return and monthly compounding for 10 years, the calculator estimates a final balance of about $34,595. Of that, $25,000 is contributed cash and about $9,595 is estimated growth.

$10,000 with no extra deposits

A $10,000 starting amount at 5% a year for five years grows to about $12,834 when compounded monthly, before fees, tax and inflation.

Compare time as well as rate

A longer time period can make a large difference because earlier interest remains in the balance. Compare several realistic time periods instead of assuming a higher return is the only way to improve the result.

How to interpret the result

The final balance is useful only when the assumptions are realistic. A steady 6% entry does not mean the balance will rise smoothly by 6% every year. Savings accounts can change their rate, and investments can produce positive or negative returns.

Use a range. A conservative scenario can use a lower rate or missed deposits, while a stronger scenario can use the amount you could consistently contribute without relying on perfect behaviour.

Common mistakes to avoid

• Entering a monthly rate in the annual-rate field.
• Treating an investment return as guaranteed.
• Ignoring account fees, investment fees, tax and inflation.
• Assuming deposits will never be missed or reduced.
• Comparing a promotional savings rate with a long-term rate without checking its conditions.

Benefits and limitations

The benefit of the Compound Interest Calculator is speed with structure.

Benefits

• Separates contributed cash from estimated growth.
• Includes a starting amount and regular monthly deposits.
• Makes it quick to compare different rates, deposits and time periods.

Limitations

• Assumes a constant rate and constant monthly deposit.
• Uses monthly compounding and end-of-month deposits.
• Does not include tax, fees, inflation, withdrawals or changing returns.
• Does not model investment volatility or guarantee a future balance.

Summary

Use the calculator to estimate a future balance from a starting amount, monthly deposits, an annual rate and time. Compare the estimated growth with the cash contributed, then rerun the calculation with more conservative assumptions.

This calculator is general information only and is not financial, tax or legal advice.