Simple Interest and Compound Interest: Concepts, Formula & Examples

In the previous chapter, we covered Profit, Loss & Discount in detail. Now it’s time to learn Simple Interest (SI) and Compound Interest (CI), two of the most important topics in quantitative aptitude.

This chapter introduces the basic concepts of Simple Interest and Compound Interest.

You will learn how interest is calculated, understand the difference between SI and CI, and build the foundation required to solve exam-level questions. In the following pages, we’ll cover SI & CI formulas in detail with worked examples and step-by-step solutions.

A strong understanding of these concepts, combined with regular practice, will help you solve questions quickly and accurately in the examination.

You can further strengthen your preparation by practicing questions from our Quantitative Aptitude Practice Questions and Quantitative Aptitude Previous Year Questions (PYQs) sections. You can also explore our Quantitative Aptitude Notes to revise concepts and our Quantitative Aptitude Formulas section for quick formula revision across all maths topics.

Let’s begin by understanding what Simple Interest and Compound Interest actually mean.

What is Simple Interest?

As the name suggests, Simple Interest is the simplest way of calculating interest. In Simple Interest, the interest is calculated on the principal amount only throughout the period of loan. The principal never changes during the entire period, so the interest earned or paid in each time period remains the same.

Example of Simple Interest

Suppose a shopkeeper borrows ₹5,000 from a lender at an interest rate of 5% per month for 6 months.

Every month, the interest will be calculated only on ₹5,000, not on the previous month’s interest.

Interest for one month:

₹5,000 × 5% = ₹250

Since the monthly interest remains the same:

Month 1 = ₹250
Month 2 = ₹250
Month 3 = ₹250
Month 4 = ₹250
Month 5 = ₹250
Month 6 = ₹250

Total Simple Interest = ₹250 × 6 = ₹1,500

This is called Simple Interest because the interest is calculated only on the original principal amount throughout the loan period.

From the above explanation, we can understand that Simple Interest depends on three factors—Principal Amount, Rate of Interest, and Time Period. In simple terms, the formula for Simple Interest is:

SI = (P × R × T) ÷ 100

where P is the Principal Amount, R is the Rate of Interest, and T is the Time Period.

In the next chapter, “Simple Interest & Compound Interest Formula with Examples and Explanations,” we’ll learn how this formula is derived, why it works, and how to apply it to different types of SI and CI questions asked in competitive exams.

What is Compound Interest?

As the name suggests, Compound means combined together.In Compound Interest, the interest earned in the first period is added to the original principal amount. In the next period, this new amount (Original Principal + Interest Earned) becomes the new principal for calculating interest. The same process continues in every subsequent period.

Example

Suppose the same shopkeeper borrows ₹5,000 at a compound interest rate of 5% per month for 6 months.

Month 1

Principal = ₹5,000

Interest = ₹5,000 × 5% = ₹250

New Amount = ₹5,000 + ₹250 = ₹5,250

Month 2

Interest = ₹5,250 × 5% = ₹262.50

New Amount = ₹5,250 + ₹262.50 = ₹5,512.50

Month 3

Interest = ₹5,512.50 × 5% = ₹275.63

New Amount = ₹5,512.50 + ₹275.63 = ₹5,788.13

Month 4

Interest = ₹5,788.13 × 5% = ₹289.41

New Amount = ₹5,788.13 + ₹289.41 = ₹6,077.54

Month 5

Interest = ₹6,077.54 × 5% = ₹303.88

New Amount = ₹6,077.54 + ₹303.88 = ₹6,381.42

Month 6

Interest = ₹6,381.42 × 5% = ₹319.07

Final Amount = ₹6,700.49

Notice that the interest increases every month because it is calculated on the new amount, not just on the original ₹5,000. This is the basic idea behind Compound Interest.

In simple terms, the formula for calculating the Amount in Compound Interest is:

A = P × (1 + R/100)^T

where:

A = Amount (Principal + Compound Interest) or Total Amount Payable/Receivable
P = Principal Amount
R = Rate of Interest
T = Time Period

The Compound Interest can then be calculated as:

Compound Interest CI = A − P

The complete explanation of this formula, including its derivation and applications in different types of Compound Interest questions, is covered in our “Simple Interest & Compound Interest Formula with Examples and Explanations” page.

Difference Between Simple and Compound Interest

Although both Simple Interest and Compound Interest are used to calculate interest, they differ in the way the interest is calculated. The table below highlights the key differences between them.

Simple Interest	Compound Interest
Calculated only on the principal amount.	Calculated on the principal plus accumulated interest.
Interest remains the same in every period.	Interest increases with each compounding period.
Formula: SI = (P × R × T) ÷ 100	Formula: A = P × (1 + R/100)^T, CI = A − P
Used for short-term loans in some cases.	Commonly used in banks, investments, fixed deposits, and loans.

Applications of Simple Interest and Compound Interest

Instead of definitions, explain where students see them in real life.

Simple Interest

Short-term personal loans
Some educational examples
Certain government schemes
Basic lending calculations

Compound Interest

Bank Fixed Deposits (FDs)
Recurring Deposits (RDs)
Mutual Funds
SIP Investments
Home Loans
Education Loans
Credit Card Outstanding Balance

Simple Interest & Compound Interest Topics Covered

Importance of Simple Interest & Compound Interest in Competitive Exams

Simple Interest and Compound Interest are among the most important topics in quantitative aptitude and are frequently asked in SSC CGL, SSC CHSL, SSC MTS, RRB NTPC, RRB Group D, RRB JE, IBPS PO, SBI Clerk, RBI Assistant, Insurance, State PSC, Defence, and other competitive exams.

Many questions also combine SI & CI with Percentage, Profit, Loss & Discount, and Ratio & Proportion concepts, making a strong understanding of these topics essential for scoring well in the quantitative aptitude section.

Tips to Master Simple Interest & Compound Interest

Understand the concepts first. Instead of memorizing formulas, try to understand how they are derived. A strong conceptual foundation makes solving questions much easier.
Start with basic questions. Solve plenty of easy questions to understand how the formulas are applied in different situations.
Practice NCERT Maths questions. Once you are comfortable with the basics, solve NCERT questions without looking at the answers or solutions.
Solve Previous Year Questions (PYQs). After sufficient practice, attempt SSC, RRB, Banking, Defence, and other competitive exam PYQs to become familiar with the types of questions asked in actual exams.
Avoid shortcuts in the beginning. Don’t search for tricks and shortcuts until you have a strong understanding of Simple Interest and Compound Interest concepts.
Shortcuts come naturally with practice. After solving enough practice questions and PYQs, you’ll begin to recognize faster methods and exam-oriented shortcuts on your own.
Don’t give up on a question too quickly. Spend time thinking about a question and try different approaches before looking at the solution. This habit improves your problem-solving ability and confidence.

Frequently Asked Questions on Simple Interest & Compound Interest

1. What is the difference between Simple Interest and Compound Interest?

Simple Interest is calculated only on the principal amount, whereas Compound Interest is calculated on the principal plus the accumulated interest from previous periods.

2. Where can I learn the derivation of SI and CI formulas?

Our Simple Interest & Compound Interest Formula with Examples and Explanations page explains how the formulas are derived, why they work, and how to apply them in different types of questions.

3. How can I improve my speed in Simple Interest and Compound Interest questions?

Build a strong conceptual foundation, practice basic questions first, and then solve previous year questions regularly. Speed improves naturally with consistent practice.

4. Which competitive exams ask Simple Interest and Compound Interest questions?

Questions on SI and CI are commonly asked in SSC, RRB, Banking, Insurance, Defence, State PSC, and many other competitive examinations.

5. Which topic should I study after learning the SI & CI formulas?

After understanding the formulas, start with topic-wise Simple Interest and Compound Interest questions, then move on to practice sets and previous year questions.

6. Are Simple Interest and Compound Interest connected to other aptitude topics?

Yes. SI and CI questions often require concepts from Percentage, Profit & Loss, and Ratio & Proportion, making these topics closely related in competitive exams.

Simple Interest & Compound Interest