Simple Interest Calculator - A Tool for Savvy Borrowers

Simple Interest Calculator

Simple interest is a fundamental method for calculating the cost of borrowing money. It represents the fee borrowers pay lenders, expressed as a percentage of the principal amount. 

In the complex world of finance, simple interest stands out as a beacon of clarity. Whether considering a bridging loan for your next property investment or shopping for a new car, understanding simple interest can be key to making informed financial decisions.

Simple interest offers a typical approach to calculating interest (without factoring in compounding effects). Unlike compound interest, which increases over time as interest accumulates on previous interest, simple interest remains constant throughout the loan period. 

It’s calculated simply by multiplying the principal amount by the interest rate and the period. Borrowers benefit from simple interest because it lowers total interest expenses than compound interest. It’s typically used for short-term borrowings like car or bridging loans. But how does it work, what are the pros and cons, and what financial products use simple interest?

What is Simple interest?

Simple interest”, or “serviced interest”, is the simplest method for calculating the cost of borrowing money. Interest is the fee or charge the borrower pays the lender for the financed sum, typically expressed as a percentage of the principal amount.

Simple interest is calculated against the principal amount, which remains constant as long as payments are made on time. Unlike compound interest, which is calculated on a growing balance, simple interest does not involve compounding. The interest is solely based on the principal amount borrowed.

How does Simple interest work?

Simple interest is a simple method for calculating the cost of borrowing money. Here's how to calculate it in 4 simple steps.

  1. Identify the principal amount. This is the initial sum of money borrowed.
  2. Annual interest rate. This is the percentage of the principal that will be charged as interest each year.
  3. Calculate the period. The funds will be borrowed, typically annually.
  4. Use the formula. Multiply the principal amount by the annual interest rate and then by the period to find the total simple interest.

What is the formula for Simple interest?

What is the formula for Simple Interest?

The formula for calculating simple interest is:

Simple Interest = Principal × Rate × Time

Simple interest examples

If you borrow £10,000 at an annual interest rate of 5% for 3 years, the simple interest would be calculated as: 

Simple Interest = £10,000 × 0.05 × 3 = £1,500

So, you would pay £1,500 in interest over the 3 years.

Simple Interest Calculator

A simple interest calculator is a tool designed to compute the interest due on a loan or investment based on the principal amount, interest rate, and period. It simplifies determining how much interest will accrue over a specific duration.

  1. Rolled Up
    £0 per month
    Amount repayable at end of term:
    Total Interest: £0.00

    £0.00 per month
    Amount repayable at end of term:
    Total Interest: £0.00

Simple interest vs Compound interest

Simple Interest has a lower total interest cost than compound interest for the same loan term. Compound Interest accumulates more over time due to compounding, increasing the total amount payable, especially for long-term debt.


Simple Interest    

Compound Interest

Calculation Basis

Calculated only on the principal amount.

Calculated on the principal and acc


Interest = P x r x t

Interest = P(1+r/n?)nt

Interest Growth

The interest amount remains constant throughout the loan period.

Interest grows exponentially as it is calculated on the accumulated amount.


Commonly used for short-term loans and certain types of instalment loans.

Used in most savings accounts, credit cards, and long-term investments.

Simple vs compound interest example

Here are the differences between simple and compound interest over 3 years, with a principal of £5,000 and an interest rate of 5%.


Simple Interest

Compound Interest 

Loan amount 



Annual interest rate 



Loan term 

3 years 

3 years 

Compounding frequency



Interest calculation

£10,000 x 0.05 x 3 = £1,500

£10,000 x (1.05)3  - £10,000 = £1,576.25

Total interest paid



Total payable



Pros and cons of simple interest loans

Simple-interest loans are typically used for their usefulness, making them suitable for short-term borrowing. However, like any financial product, they have advantages and disadvantages.

Simple to understand. The formula for calculating simple interest is easy to understand. Simple interest calculations do not require knowledge of complex financial concepts, making them accessible to most people.

Predictability. The interest amount remains consistent throughout the loan, providing predictable payments. Borrowers can easily plan their finances, as they know the exact amount of interest they will pay over the life of the loan.

What are the drawbacks?

It may not accurately reflect loan costs. In some cases, particularly for loans with irregular payment schedules, simple interest may need to reflect the actual cost of borrowing accurately. Borrowers need to understand the full implications of simple interest, especially if payments are missed or delayed.

Potential for higher costs. If a borrower defers or misses payments, the total interest cost may increase as the interest accrues on the principal amount. Borrowers need to make timely payments to avoid additional interest costs.

Simple Interest vs. Amortised Debt 

Amortised debt involves loans paid off over time through regular payments that cover both principal and interest. Each payment reduces the loan balance until it is completely paid off at the end of the loan term.

Amortisation schedules determine the payment amount for interest and principal. The formula for the monthly payment on an amortised loan is. 

Amortisation formula.

  • M is the monthly payment
  • P is the principal loan amount
  • r is the monthly interest rate
  • n is the number of payments (loan term in months)

Amortised Debt Example

  • Principal (P) - £10,000
  • Annual Interest Rate - 5%
  • Loan Term (n) - 3 years

We need to convert the annual interest rate to a monthly rate and the loan term to months. 

r monthly = 5%/12 = 0.004167

n months = 3 × 12 = 36

Using these values, the monthly payment (M) can be calculated as: 

M = £10,000 × 0.004167 × (1+0.004167)36 / (1+0.004167)36-1 = £299.71

Over 36 months, the total amount paid will be 36 × £299.71 = £10,789.56

In this amortised example, the borrower pays £789.56 in interest over the 3 years, with monthly payments gradually reducing the principal and interest.

Simple Interest Example

  • Principal (P): £10,000
  • Annual Interest Rate (r): 5%
  • Loan Term (t): 3 years

Simple Interest = P × r × t

Simple Interest = £10,000 × 0.05 × 3 = £1,500

Over the 3 years, the borrower would pay £1,500 in interest, resulting in a total repayment of £11,500.

Is simple interest the same as APR?

No, simple interest and Annual Percentage Rate (APR) are two different concepts in financing. APR is a percentage that measures the interest rate plus any additional fees associated with a loan or credit card. It's a broader measure of the cost of borrowing money than the interest rate, which only applies to the principal loan amount. APR can include fees like origination fees, which are associated with processing a new loan application. 

What types of loans or financial products use simple interest? 

??Simple interest loans calculate interest based only on the principal amount of the loan, making the interest costs easy to anticipate. This type of interest calculation is commonly found in specific financial products and short-term borrowing options, from bridging loans to car loans. 

Simple interest on loans

Simple interest loans calculate interest based only on the principal amount of the loan, which does not change over the loan term as long as payments are made on time. This type of interest calculation is straightforward and transparent, making it easier for borrowers to understand the total cost of their loans. Simple-interest loans are often preferred for short-term borrowing and specific financial products due to their ease of calculation.

Are bridge loans simple interest? 

Bridge loans are offered as simple interest when the loan is serviced (when the interest is paid monthly) or compound interest when the loan's interest is rolled up. Bridge loans provide temporary financing, typically for property transactions, until longer-term funding can be secured. With simple interest, the borrower pays interest only on the principal amount for the duration of the loan. Simple interest in bridging loans helps borrowers avoid the higher cost of compound interest.

What is serviced interest?

Serviced interest is used to bridge loans and other short-term financial products. It refers to interest payments that are regularly made by the borrower during the loan term rather than allowing interest to accumulate and compound. By servicing the interest, borrowers can ensure that their loan balance remains predictable.

Are student loans simple interest? 

The short answer is no; in the US, student loans are simple interest, but in the UK, they are compound interest (where the government calculates the daily interest and applies it to the monthly balance). 

Are car loans simple interest?    

Car loans are typically structured over a few years, with fixed monthly payments covering principal and interest (also known as amortised debt). The interest portion of each payment is calculated based on the remaining principal balance, meaning borrowers pay interest only on the amount they still owe. This helps borrowers to manage their finances more effectively. The transparency of simple interest car loans makes them popular for financing purchases. 

Final thoughts

Simple interest remains a cornerstone of many financial products, offering transparency and predictability in an often complex financial landscape.

Its straightforward calculation method makes it particularly appealing for short-term borrowing options like bridging loans and car loans. While it may not always represent the most cost-effective option for long-term financing, simple interest provides borrowers with a clear understanding of their financial obligations.

The comparison between simple and compound interest highlights the importance of understanding different interest calculation methods when considering loans or investments. Each has its place in the financial ecosystem, with simple interest favouring borrowers in short-term scenarios.

As we've seen, the application of simple interest extends beyond just loans. It plays a crucial role in various financial products, each tailored to specific needs and circumstances. From bridging finance to car loans, simple interest helps borrowers manage their finances more effectively by providing predictable interest costs.

However, consumers must remember that while simple interest is, well, simple to understand, the overall cost of borrowing involves more factors than just the interest rate. The Annual Percentage Rate (APR), which includes additional fees, often provides a more comprehensive view of borrowing costs.

In an era of increasingly complex financial products, the enduring relevance of simple interest serves as a reminder of the value of clarity and simplicity in financial transactions. As always, informed decision-making is key when navigating the world of loans and investments, and understanding simple interest is an essential part of that financial literacy toolkit.


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