What is Simple Interest?
Simple Interest (S.I) is the method of calculating the interest amount for some principal amount of money. Have you ever borrowed money from your siblings when your pocket money is exhausted? Or lent him maybe? What happens when you borrow money? You use that money for the purpose you had borrowed it in the first place. After that, you return the money whenever you get the next month’s pocket money from your parents. This is how borrowing and lending work at home.
But in the real world, money is not free to borrow. You often have to borrow money from banks in the form of a loan. During payback, apart from the loan amount, you pay some more money that depends on the loan amount as well as the time for which you borrow. This is called simple interest. This term finds extensive usage in banking.
Simple Interest Formula
The formula for simple interest helps you find the interest amount if the principal amount, rate of interest and time periods are given.
Simple interest formula is given as:
Where SI = simple interest
P = principal
R = interest rate (in percentage)
T = time duration (in years)
In order to calculate the total amount, the following formula is used:
Amount (A) = Principal (P) + Interest (I)
Where,
Amount (A) is the total money paid back at the end of the time period for which it was borrowed.
The total amount formula in case of simple interest can also be written as:
A = P(1 + RT)
Here,
A = Total amount after the given time period
P = Principal amount or the initial loan amount
R = Rate of interest (per annum)
T = Time (in years)
Click here to get the simple interest calculator for quick computations.
Simple Interest Formula For Months
The formula to calculate the simple interest on a yearly basis has been given above. Now, let us see the formula to calculate the interest for months. Suppose P be the principal amount, R be the rate of interest per annum and n be the time (in months), then the formula can be written as:
Simple Interest for n months = (P × n × R)/ (12 ×100)
The list of formulas of simple interest for when the time period is given in years, months and days are tabulated below:
| Time | Simple interest Formula | Explanation |
| Years | PTR/100 | T = Number of years |
| Months | (P × n × R)/ (12 ×100) | n = Number of months |
| Days | (P × d × R)/ (365 ×100) | d = Number of days (non-leap year) |
Difference Between Simple Interest and Compound Interest
There is another type of interest called compound interest. The major difference between simple and compound interest is that simple interest is based on the principal amount of a deposit or a loan whereas compound interest is based on the principal amount and interest that accumulates in every period of time. Let’s see one simple example to understand the concept of simple interest.
Simple Interest Problems
Let us see some simple interest examples using the simple interest formula in maths.
Example 1:
Rishav takes a loan of Rs 10000 from a bank for a period of 1 year. The rate of interest is 10% per annum. Find the interest and the amount he has to pay at the end of a year.
Solution:
Here, the loan sum = P = Rs 10000
Rate of interest per year = R = 10%
Time for which it is borrowed = T = 1 year
Thus, simple interest for a year, SI = (P × R ×T) / 100 = (10000 × 10 ×1) / 100 = Rs 1000
Amount that Rishav has to pay to the bank at the end of the year = Principal + Interest = 10000 + 1000 = Rs 11,000
Example 2:
Namita borrowed Rs 50,000 for 3 years at the rate of 3.5% per annum. Find the interest accumulated at the end of 3 years.
Solution:
P = Rs 50,000
R = 3.5%
T = 3 years
SI = (P × R ×T) / 100 = (50,000× 3.5 ×3) / 100 = Rs 5250
Example 3:
Mohit pays Rs 9000 as an amount on the sum of Rs 7000 that he had borrowed for 2 years. Find the rate of interest.
Solution:
A = Rs 9000
P = Rs 7000
SI = A – P = 9000 – 7000 = Rs 2000
T = 2 years
R = ?
SI = (P × R ×T) / 100
R = (SI × 100) /(P× T)
R = (2000 × 100 /7000 × 2) =14.29 %
Practice Questions
- A sum fetched a total simple interest of Rs. 4016.25 at the rate of 9% per annum in 5 years. What is the sum?
- A sum of Rs. 725 is lent at the beginning of a year at a specific rate of interest. After eight months, a sum of Rs. 362.50 more is lent but at the rate twice the former. At the end of the year, Rs. 33.50 is earned as interest from both loans. What was the actual rate of interest?
- Simple interest on a certain sum is 16/25 of the sum. Find the rate of interest and time if both are numerically equal.
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