Compound Interest

Compound Interest

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The Mechanics and Benefits of Compound Interest for Your Savings

When you deposit money into a savings account, you expect it to grow with time due to the accumulation of interest. This growth is a result of compound interest - a simple yet powerful concept.

Understanding Compound Interest

Compound interest is the gradual addition of interest to the principal amount of money.

The idea is that the interest earned on the principal amount is reinvested, resulting in interest being calculated on both the original principal and the accumulated interest. This process continues until the specified time period has passed. Let's visualize this with a compound interest graph.

  • Compound interest graph: As seen in the graph, the amount of money increases with time as a result of compound interest.

Calculating Compound Interest in Simple Steps

Solving compound interest problems involves determining the growth of money over a specific period of time due to the interest rate.

To calculate compound interest, we need to know three things:

  • The principal or initial amount: This is the original amount of money invested.
  • The percentage rate of compound interest: This is the rate at which the money will accumulate interest over time.
  • The time period: This is the length of time for which the money will be invested and accrue interest.

The Compound Interest Formula

There are two methods for calculating compound interest: using a table or using the compound interest formula. The formula is:


  • A: The final amount accumulated after interest has been added over time.
  • P: The original principal amount.
  • r: The interest rate per compounding period (usually per year).
  • n: The number of compounding periods (usually per year).

This formula provides us with the final amount of money earned after interest has been added over time.

An Alternative Method: Using a Table

The second method for calculating compound interest is by using a table. Here are the steps:

  1. Draw a table with two columns: one for the amount and one for the interest rate in percentage format.
  2. In the first row under the "amount" column, write the principal amount.
  3. Multiply the principal amount by the percentage rate under the "interest rate" column and write the product on the second row under the "amount" column.
  4. Add the principal amount to the interest amount on the second row and multiply it by the percentage rate on that row. Write the product on the third row under the "amount" column.
  5. Repeat this process until the specified time period has elapsed.

Using a table may take longer compared to the formula, but it is still an effective method for calculating compound interest.

Examples of Compound Interest Calculations

Let's see some examples of calculating compound interest using both the formula and the table method.

Example 1:

If you deposit £4000 in a bank for three years at an interest rate of 4% per annum, how much money will you have at the end of the three years?


We will first use the table method:

  1. Principal amount = £4000
  2. Interest rate = 4%

Using the formula, we get the same answer:

So, after three years, you will have £4520.16.

Example 2:

Suppose Jane deposits £800 in a bank with a 1% compound interest rate per annum. How much will she have after two years using both the table and formula methods?


Principal amount = £800

Using the table method:

Using the formula, we get the same answer:

So, after two years, Jane will have £816.08 in her account.

Example 3:

Let's say Ben takes a loan of £15000 from a bank with a 10% compound interest rate per annum.

How to Calculate Compound Interest: Understanding the Formula and Its Importance

When it comes to managing your finances, it's essential to understand the concept of compound interest. This refers to the accumulation of interest on a principal amount over time. By grasping the formula and calculations, you can better plan for your financial future.

The Formula for Compound Interest

The formula for calculating compound interest is:

Final Amount = Principal x (1 + Rate)^n

In this formula, the "Rate" represents the annual interest rate and "n" represents the number of compounding periods. It's essential to note that the more frequently interest is compounded, the higher the final amount will be.

Let's use an example to better understand this formula:

Ben deposits £15,000 in a savings account with a 5% annual interest rate that compounds monthly. After one month, the balance in his account will be £15,000 + (£15,000 x 0.05/12) = £15,012.50.

After two months, the balance will be £15,012.50 + (£15,012.50 x 0.05/12) = £15,025.06. This process continues, with each month adding a little more interest to the principal amount.

The Difference Between Compound and Simple Interest

Besides compound interest, there is also simple interest. The main difference between these two is that simple interest only applies to a one-time interest rate on the principal amount, while compound interest accumulates interest over time.

In simple terms, with simple interest, interest is only earned on the original principal amount. But with compound interest, interest is earned on the principal amount plus any previously earned interest.

For more information on simple interest, be sure to read our article on Understanding Simple Interest.

Calculating Compound Interest

There are two ways to calculate compound interest. The first is by using the formula mentioned earlier. The second is by using the table method. This involves creating a table with columns for the starting amount, interest earned, and final amount. The table is then used to calculate compound interest for different time periods.

Remember, the more often interest is compounded, the higher the final amount will be. This is because the interest earned from previous periods is added back into the principal amount, creating a larger starting amount for future calculations.

Key Points to Remember

  • Compound interest is the accumulation of interest on a principal amount over time.
  • You can calculate compound interest using a formula or a table method.
  • The main difference between compound and simple interest is that compound interest takes into account previous periods' interest, while simple interest does not.

In Conclusion

Having a grasp on compound interest is crucial for making informed financial decisions. By understanding the formula and how to calculate compound interest, you can better plan for your financial future. Use this knowledge to your advantage, and watch your money grow over time.

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