The mortgage constant explained

If you’re evaluating a mortgage for a house or a real estate investment loan, one metric can give you a quick way to compare the true borrowing costs: the mortgage constant. This percentage shows what you’ll pay annually in principal and interest relative to your original loan amount. It gives you an apples-to-apples comparison across different loan products and deals.

The mortgage constant focuses on principal and interest payments only and excludes property taxes, homeowners insurance, and other costs. The percentage gives insight into how a loan will impact your cash flow and overall financial picture over time. This tool ties directly into mortgage amortization.

Let’s dive into what a mortgage constant is, how to calculate it, and why it’s so important.

What is the mortgage constant, and how does it work?

The mortgage constant converts annual principal and interest (P&I) into a percentage of the original loan amount. It gives you a way to look at the annual cost of repaying the loan expressed as a percentage of the original loan amount.

Because fixed-rate loans have predictable amortization schedules, that percentage remains stable over time. That makes the mortgage constant a useful metric with these types of loans.

To understand why, let’s look at how a fixed-rate mortgage works. A fixed-rate, amortizing loan has scheduled principal and interest (P&I) payments that stay the same. Because payments are predictable, the annual debt service — the total amount paid each year toward the loan — is also predictable. And because of that, the mortgage constant is a reliable, stable metric.

The mortgage constant does not work for adjustable-rate or interest-only loans. With an adjustable-rate mortgage (ARM), the rate changes, which means the monthly payment changes, which in turn means the annual debt service changes. And if the annual debt service changes, the mortgage constant changes.

Interest-only loans have a similar issue. They don’t include regular principal payments during the interest-only period, so they don’t have a standard amortization pattern. Again, that means the mortgage constant is an unreliable metric.

So, the mortgage constant is most useful for traditional, fixed-rate, fully amortizing loans. Investors and lenders use it to model cash flow, compare loans, and assess risk.

Real estate investors rely on the mortgage constant to compare debt service against net operating income (NOI) and evaluate potential cash flow.

Lenders use it to evaluate annual debt service relative to the loan amount and determine whether a borrower can handle the monthly payments of a loan.

Financial analysts and underwriters use it to compare loans with similar terms.

How to determine your home loan constant: The mortgage constant formula

To calculate your home loan constant, you’ll need to know your total loan amount and monthly payment.

You can use the Rocket Mortgage mortgage calculator to help you figure out your monthly principal and interest payment, if you don’t know it already.

The mortgage constant formula

There are several ways to calculate the mortgage constant, including using an online calculator or a tool like Excel. However, if you prefer a hands-on approach, here's a simple formula you can use:

(Principal and Interest) × 12 ÷ Loan Amount = Mortgage Constant

The first part of the equation is referred to as your annual debt service. This is the total amount it would take to completely repay your loan in 1 year.

One thing to keep in mind is that the mortgage constant works a little differently than your amortization schedule. The loan constant is a fixed percentage that doesn’t change. However, if you look at your amortization schedule, you’ll see that at the beginning of the loan, you pay more toward interest and, later, more toward the principal. The total, however, remains constant each month and every year.

Calculating the mortgage constant: An example

Here’s a simple example of how to calculate the mortgage constant.

Let's say you borrow $415,000 with a 30-year fixed mortgage at a 5.5% interest rate in Los Angeles. Based on a mortgage calculator, your estimated monthly principal and interest payment would be about $2,356.

To figure out the mortgage constant, plug the numbers into the formula:

($2,356 × 12) ÷ $415,000 = (0.0681 × 100) = 6.81%

Quick note: Make sure you only use your principal and interest amount, since this formula doesn’t include other housing costs like taxes or insurance.

You can then use this formula to compare different loan options or investment opportunities to determine which one is more profitable. Typically, a lower mortgage constant indicates a more profitable investment when comparing the two.

Mortgage constants vs. other real estate metrics

The mortgage constant is just one component of a real estate investor's toolbox. Often referred to as real estate metrics, these tools work best when used in conjunction so that you can gain a more comprehensive insight into a property's financial potential.

Net operating income (NOI) vs. mortgage constant

In real estate investing, your goal is to make sure any property you buy turns a profit. Net operating income, or NOI, helps you estimate how much money an investment property can make. To calculate the NOI, take the total income and subtract all property management costs such as maintenance and taxes. Also, make sure to include all forms of income, such as parking fees or laundry machines.

Because NOI doesn't account for your mortgage payment, investors can combine both metrics to gain more insight into the profitability of the deal. For example, let's say the net operating income from the property is sufficient to comfortably cover the annual loan cost (the mortgage constant). In that case, the investment is more likely to be profitable, sustainable, and worthwhile.

In short, NOI is about the property’s income after operating expenses, while mortgage constant is about the loan’s annual debt service as a percent of the loan amount.

Cap rates vs. mortgage constants

The capitalization rate, also known as the cap rate, is another real estate metric that helps investors evaluate investment opportunities. This formula helps estimate cash flow compared to the investment property's market value. Unlike the mortgage constant calculation, the cap rate is typically used when financing isn't involved.

Here's the cap rate formula:

NOI ÷ Current Market Value = Cap Rate (as a percentage)

If you need a mortgage to buy the investment property, you can combine both metrics to gain more insight into how your loan payments affect profitability. If the cap rate is higher than the mortgage constant, it shows that the income from the property should be able to cover the loan payments while still generating a stream of income.

How to use your mortgage constant

To evaluate whether financing supports a property’s cash flow, investors often compare the mortgage constant to the cap rate.

Using the example above:

Cap rate calculation:
$51,000 ÷ $518,750 = 9.83%

Mortgage constant calculation:
($2,356 × 12) ÷ $415,000 = 6.81%

Compare the two:

Cap rate: 9.83%

Mortgage constant: 6.81%

Investors often look for a positive spread between cap rate and mortgage constant to support cash flow. When the cap rate is higher than its mortgage constant – as with the above example – the property is more likely to be profitable.

Mortgage constants and residential vs. commercial mortgages

Whether you're just getting started in real estate or already managing a few properties, it's helpful to know that you can have up to 10 residential mortgages. This includes homes with up to four units each, which still count as single-family properties under federal guidelines. That means you could potentially finance up to 40 units without needing a commercial loan.

When you're juggling more than one loan, the mortgage constant can help you compare loan options, spot which properties might cost you more in loan payments and make smarter choices for steady cash flow.

FAQ

Here are some common questions people have about the mortgage constant.

Why is the mortgage constant higher than the interest rate?

Since the mortgage constant takes both the loan’s principal and interest into consideration, it will always be higher than the interest rate on any amortized loan. The interest rate only factors in interest and not the loan’s principal.

Why is the mortgage constant important?

The mortgage constant is important because it helps determine the profitability and return on investment of your real estate purchase if you're financing it. A lower mortgage constant usually means higher profitability.

Should the cap rate or mortgage constant be higher?

In general, the mortgage constant for a loan should be lower than the cap rate. When the cap rate is lower than the mortgage constant, the investment is likely to be unprofitable.

The bottom line: Mortgage constants help you understand your debt service costs

Real estate investing can be an exciting endeavor, but it also comes with risks and complexities. That’s why it’s important to evaluate each opportunity using real estate metrics like the mortgage constant. This simple formula helps you compare loan offers, understand the cost of financing, and figure out whether a property is truly profitable. If you have more questions, you can discuss these metrics with your mortgage lender.

If you’re ready to buy a home or investment property, you can start an online application for a loan with Rocket Mortgage and discover a mortgage solution suited to your goals.