How to calculate principal and interest payments

How to calculate principal and interest payments

How to calculate principal and interest payments is a question many are asking, especially with the current buzz around rising interest rate hikes. It’ s not uncommon to overhear someone lament, ‘I haven’t even made a dent in the principal loan amount. It’s all interest!’ 

This highlights the need to truly grasp the mechanics of borrowing money. When you take out a loan, you’re not just paying back the borrowed amount, or the principal; you’re also contending with the added cost of interest.

Understanding this calculation is key.

So, in this guide, we’ll break down the process of determining principal and interest payments. We’ll make the terms clear, explain the main ideas, and even provide tools to help you do the math on your own.

man using a calculator while holding a miniature house

What is the Principal and Interest Part of Your Monthly Payment?

Understanding the difference between principal and interest is the first step. The principal is the amount of money you initially borrow. Interest is essentially the cost of borrowing that money, usually expressed as a percentage of the principal, known as the interest rate.

Breaking Down Your Principal and Interest Loan 

When you make a monthly payment on your loan, it’s usually split into two parts:

  • Principal repayment: This is the part of your payment that goes directly towards paying off the original amount you borrowed.
  • Interest payment: This is the part of your payment that goes towards the cost of borrowing the money.

The total of these two amounts equals your monthly loan repayment.

man showing amount in the calculator

The Role of Interest Rate in Your Monthly Payments 

The interest rate plays a significant role in determining your monthly loan repayment. It is often expressed as an Annual Percentage Rate (APR) but is charged on a more frequent basis, usually monthly.

You can either have a fixed interest rate, meaning it stays the same over the home loan term, or variable, meaning it can fluctuate based on market trends. You would’ve determined the type of interest rate when you chose your specific loan product. 

The Impact of Loan Amount and Term on Interest

The amount of money you borrow (the principal) and the term of your loan (how long you take to pay it back) also significantly influence your total interest payment. Borrowing a larger amount or taking a longer time to repay the loan will generally result in more interest paid over the loan term.

How is Interest Calculated on a Loan?

Interest on a loan is calculated based on the unpaid balance of your loan. This is done by multiplying your loan balance by your interest rate and dividing this by the number of days in the year. This gives you your daily interest charge. 

The daily interest amount is then multiplied by the number of days in the month to give your monthly interest charge.

How to Calculate Principal and Interest Payments

To calculate your principal and interest payments, you need to know your loan amount, interest rate, and loan term. Here are the steps:

  • Step 1: Convert your annual interest rate to a monthly rate by dividing by 12.
  • Step 2: Multiply your loan amount by your monthly interest rate to get your monthly interest payment.
  • Step 3:To calculate your monthly principal payment, subtract your monthly interest payment from your total monthly payment.

To illustrate, let’s say you have a loan of $820,000 with an annual interest rate of 5% and a term of 30 years. Your monthly interest rate would be 0.004167 (5% divided by 12), your monthly interest payment would be $3,416.67 ($820,000 x 0.004167).

If this was an interest-only loan, you would be obligated to pay $3,416.67 per month. However, if it’s principal & interest loan, you’re total repayments would be the interest plus the amount set by your lender. In this instance it would be $3,416.67 + $985.33 = $4,402. 

What is an Interest-Only Loan?

An interest-only loan is a type of loan where, for a set period, the borrower pays only the interest on the principal balance. The principal amount remains unchanged during this period.After the interest-only period ends, the loan typically becomes a standard amortizing loan. This means you’ll start paying both the principal and the interest. 

Factors Influencing How Much Interest You Pay

Several factors can affect the amount of interest you pay on a loan. These include:

  • Interest rates: A higher interest rate means more interest paid over the life of the loan.
  • Loan amount: A larger loan leads to more interest paid because interest is calculated as a percentage of your loan balance.
  • Loan term: A longer loan term means more interest paid because you’re borrowing the money for a longer time.
  • Repayment frequency: More frequent payments can reduce your interest, as it’s calculated based on your outstanding balance.
  • Number of days in a month: Since interest is calculated daily, the number of days in a month can slightly affect your interest payment.
  • Additional repayments: Making extra repayments can reduce your loan balance faster, thus reducing the amount of interest you pay.

2 people discussing amount cost shown in calculator

The Effect of Making Extra Payments

Making extra payments on your loan can significantly reduce the amount of interest you pay. Extra payments reduce your loan balance faster, meaning there’s less balance for the interest to be calculated on – which can result in significant savings over the life of the loan.

Using Online Calculators to Simplify the Process

While it’s beneficial to understand how to calculate your monthly payments manually, using an online principal and interest calculator can simplify the process. There are many online calculators available that can calculate your mortgage repayments, interest payments, and even how much you can save by making extra repayments.

Key Takeaways

  • Monthly loan repayments are split into two components: principal repayment and interest payment. The principal is the original amount borrowed, while interest represents the cost of borrowing.
  • The total interest paid over a loan’s duration is influenced by the principal amount, loan term, and interest rate.
  • Making extra payments on a loan can lead to significant interest savings over the loan’s lifespan.

Owning a property, particularly an investment one, offers more savings avenues than just extra loan repayments. Properly structuring your portfolio paves the way for smart tax decisions. Ready to optimise your investments? Contact Property Tax Specialists today.


Please note that every effort has been made to ensure that the information provided in this guide is accurate. You should note, however, that the information is intended as a guide only, providing an overview of general information available to property buyers and investors. This guide is not intended to be an exhaustive source of information and should not be seen to constitute legal, tax or investment advice. You should, where necessary, seek your own advice for any legal, tax or investment issues raised in your affairs.

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