How Compound Interest Works
Compound interest allows your money to grow not only on the amount you originally invested or saved, but also on the interest you have already earned.
Over time, this can create a powerful compounding effect, where your money has the potential to grow faster the longer it remains invested or saved.
What Is Compound Interest?
Compound interest is interest calculated on both your original amount of money and the interest that has already accumulated.
This is different from simple interest, which is calculated only on the original amount.
For example, if you save or invest £1,000 and earn interest, compound interest means that future interest can also be earned on the interest already added to your balance.
How Compound Interest Is Calculated
Compound interest is calculated using the original amount, the interest rate, how often the interest is compounded, and how long the money remains invested or saved.
The basic formula is:
A = P (1 + r ÷ n)ⁿᵗ
Where:
A = Final amount
P = Starting amount (principal)
r = Annual interest rate
n = Number of times interest is compounded per year
t = Number of years
The more frequently interest is compounded and the longer the money remains invested or saved, the greater the potential compounding effect.
Compound Interest Example
Suppose you invest £5,000 at an annual interest rate of 5%, compounded once a year.
After the first year, you would have £5,250.
In the second year, the 5% interest is calculated on £5,250, not just the original £5,000. Your balance would then grow to £5,512.50.
After 10 years, without adding any more money, the original £5,000 would grow to approximately £8,144.47.
This demonstrates why time is such an important part of compound growth
Why Time Matters With Compound Interest
Compound interest becomes more powerful over longer periods because each round of interest can contribute to the amount that earns interest in the future.
This means starting earlier can make a significant difference, even when the amount you save or invest is relatively small.
The key factors that influence compound growth are your starting amount, interest rate, compounding frequency, additional contributions and time.
Compound Interest vs Simple Interest
The main difference is what the interest is calculated on.
With simple interest, interest is calculated only on the original amount.
With compound interest, interest can be calculated on the original amount plus the interest already earned.
Over a short period, the difference may appear small. But over many years, compounding can create a much larger difference in the final amount.
When estimating compound growth, common mistakes include:
Using the wrong interest rate or compounding frequency
Forgetting that fees or charges can reduce your actual returns
Assuming an interest rate will remain the same indefinitely
Ignoring the effect of inflation
Treating projected returns as guaranteed results
Compound interest calculations are useful for planning, but the final outcome will depend on the actual rates, costs and returns achieved over time.
Common Compound Interest Mistakes to Avoid
Calculate Compound Interest Instantly
You don't have to work through the compound interest formula manually. The ToolNestPro Compound Interest Calculator can estimate how your money could grow over time based on your starting amount, interest rate, contributions and investment period.
Use it to explore different scenarios and see how changes in time, contributions and interest rates can affect the potential result.