The Simple Interest Guide to Growing Money

Created on 19 Oct 2023

Wraps up in 4 Min

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Simple interest

Welcome to the world of finance made simple! Today, we're going to dive headfirst into the concept of Simple Interest, the kind that won't make your head spin with complicated calculations. 

If you've ever wondered how money grows when lending or borrowing money, you're about to embark on an enlightening journey. In this friendly and conversational guide, we'll break down the core aspects of Simple Interest, teach you the easy-peasy formula, walk you through real-life examples, and even show you how it stacks up against its more complex cousin, Compound Interest

Before we get started, I am sure you must have had nimbu paani/ lemonade or will be familiar with it. Why? To understand this concept, I am going to open up an imaginary nimbu paani stall today.

So, grab a glass of lemonade, get comfy and read on.

Understanding Simple Interest and Opening up of the Lemonade Stand

So, I have decided that I want to open up a lemonade stand, let’s say, near Marine Drive, Mumbai. And now imagine you've got ₹1,000 stashed under your mattress. You're generous and lend it to me. In exchange, I agree to give you a little extra money for the favour. This extra money is the Simple Interest (S.I). 

Thus, Simple Interest is the amount of money you either earn (as a lender) or pay (as a borrower) to use someone else's money, more like a little bonus for lending money or a modest fee for borrowing. Now, you might wonder how Simple Interest works.

So, it is calculated based on three key components: the principal amount, the interest rate, and the time period for which you're either borrowing or lending money. Let me explain this using the formula.

Simple Interest Formula

Don't worry; we won't delve deep into complex mathematical equations. It's simple, remember? 😂

The formula for Simple Interest is as straightforward as it gets:

Let's break this down:

1. Principal Amount: This is the initial sum of money you either lend or borrow.

2. Rate of Interest: It's the percentage of interest you earn or owe annually. For instance, if the interest rate is 10%, you'd enter 10 in the formula above. This rate is determined by the lender.

3. Time: Time is the duration for which the money is lent or borrowed, usually in years. If it's less than a year, don't worry; we can adjust for that.

Example of Simple Interest

It's time to put this formula to work with a real-life example. I have borrowed ₹1,000 from you at an annual interest rate of 10%. And I will pay you back after 3 years. What's the Simple Interest?

Plug the numbers into the formula:

S.I = (1,000 × 10 × 3)/ 100

S.I = ₹300

In this case, I will pay you ₹300 in Simple Interest over three years.

Now, let’s see how the calculation will be if the time period is considered semi-annually and then monthly.

Semi-Annually (Twice a Year):

So, the time period is 3 years, which is 6 half-year periods.
Kese? Aise! 3 years * 2 semesters per year = 6, samjhe? 

Same for rate, it is 10% per annum, or 5% per half-year.

That is it; substitute in the formula and calculate! Likewise,


Rate of Interest (R): 10% per annum, or approximately 0.0083% per month
Time (t): 3 years, which is 36 months (3 years * 12 months per year).

Simple Interest vs. Compound Interest

Simple Interest, as you now know, is straightforward. You earn or owe interest only on the initial principal amount, and it doesn't grow over time, as explained above.

Compound Interest, on the other hand, is a bit more complex. It calculates interest on the principal amount and any interest that has already accumulated. As a result, your interest can snowball and grow over time. So, while Simple Interest remains steady, Compound Interest can increase substantially, making it an important concept in savings accounts and investments.

Take a look at the image below to understand things better. ⤵️

Simple Interest vs. Compound Interest

Now that we know both, moving on to the…

Limitations of Simple Interest

While Simple Interest is fantastic for its simplicity, it does have some limitations.

1. Inflation: Simple Interest doesn't consider the impact of inflation. Over time, the purchasing power of the interest earned may decrease due to rising prices.

2. Constant Interest: With Simple Interest, the amount remains the same throughout the loan or investment period. It doesn't account for the changing value of money due to inflation or fluctuating interest rates.

3. Not Ideal for Long-Term Investments: Compound Interest is often more advantageous for long-term investments, as it allows your money to grow faster. 

When it comes to long-term investments, there are more better options than simple interest. Have you ever explored the concept of Annual Percentage Yield (APY)? If not, you'll want to discover What is the Annual Percentage Yield and how is it calculated?

The Bottom Line

So, here's the scoop on Simple Interest. Remember the formula. Before you go, let's get back to my lemonade stand and the key takeaways:

1. The Principal Amount (₹1000): The lemon juice in your lemonade – it's the main ingredient, the star of the show. 🍋

2. Rate of Interest (10%): The sugar in your lemonade. It sweetens the deal, but too much can make it too syrupy! 

3. Time (3 years): Time is like the ice cubes in your glass of lemonade – it can change the flavour over a while, but it's essential to keep things cool. 🧊

With that ₹300 Simple Interest, maybe you can treat yourself to a refreshing glass of lemonade and call it even. Cheers! 🥂

Now that you've got the basics of Simple Interest in your pocket, remember to check Insider by Finology to dive into further details.

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Sakshi Dhakre

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Sakshi is an adventurous spirit who enjoys both the intellectual stimulation of Finance and the sensory experiences of good food and nature’s beauty. She has a passion for delving into complex financial topics and distilling them down into easy-to-understand insights. When she's not poring over financial reports, you might find her exploring a new corner of the city, trying out new restaurants and cuisines or admiring the beauty of the night sky.

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