The 3 7 3 Mortgage Rule: A Quick Payment Estimator

You're scrolling through home listings, daydreaming about a backyard, when a price tag catches your eye: $400,000. Your first thought isn't about square footage or school districts. It's a much more basic, gut-level question: "Can I afford the monthly payment on that?"

This is where the so-called "3 7 3 rule" in mortgage lending pops up. It's a back-of-the-napkin trick that promises a quick answer. I've seen it mentioned in online forums and heard first-time buyers whisper it like a secret code. But after working in real estate finance for over a decade, I've also seen the confusion and misplaced confidence it can create. Let's break down what the 3 7 3 mortgage rule actually is, how to use it, and—more importantly—when you should absolutely ignore it.

What Is the 3 7 3 Rule? (The Simple Math)

The 3 7 3 rule is a mental shortcut for estimating your monthly principal and interest payment on a conventional 30-year fixed-rate mortgage. The numbers stand for:

• 3% Down Payment: It assumes you're putting down a minimum down payment of 3%. This is a real thing, often available through programs like conventional 97 loans or certain first-time buyer initiatives.
• 7% Interest Rate: It assumes your mortgage interest rate will be around 7%. This was historically a rough average, but as we all know, rates swing wildly.
• 30-Year Loan Term: It assumes the standard 30-year amortization schedule.

The "rule" part is this: For every $100,000 of the home's purchase price, your estimated monthly principal and interest payment will be about $700.

That's it. That's the whole rule. It's not magic, just a simplified calculation based on those three fixed assumptions. The problem is, your real-life mortgage application isn't based on fixed assumptions. It's based on you, the market, and the fine print.

How to Use the 3 7 3 Rule: A Step-by-Step Walkthrough

If you want to use it as a very preliminary filter, here's how.

1. Take the home's list price. Let's say $325,000.
2. Figure out how many $100,000 chunks it contains. $325,000 / $100,000 = 3.25.
3. Multiply that number by $700. 3.25 x $700 = $2,275.

This $2,275 is your rule-of-thumb estimate for the core mortgage payment (principal & interest). I keep a small table like this in my head for quick reference when I'm driving through neighborhoods with clients:

Notice I said "core" payment. This is the first big trap. The $700 per $100k does not include property taxes, homeowners insurance, or private mortgage insurance (PMI). Your actual monthly payment will be hundreds of dollars higher. Forgetting this is the single most common mistake I see rookies make with this rule.

Where the 3 7 3 Rule Falls Dangerously Short

This is where the 10-year perspective kicks in. The 3 7 3 rule isn't just simplistic; it can be dangerously misleading in today's market. Here's why you should be skeptical.

The 7% Interest Rate Assumption is a Roll of the Dice

In early 2023, rates hovered near 7%, making the rule seem prescient. By late 2023, they spiked above 8%. As I write this, they might be in the high 6s. If your actual rate is 6.5%, your payment is lower than the rule estimates. If it's 8.5%, it's significantly higher. Basing your affordability on a generic 7% is like planning a picnic based on last year's weather.

It Ignores the Full Monthly Payment (PITI + PMI)

Your lender doesn't care about your principal and interest estimate. They underwrite based on PITI: Principal, Interest, Taxes, and Insurance. With a 3% down payment, you'll also have PMI. Let's revisit our $400,000 example with real numbers:

• 3 7 3 Rule P&I Estimate: $2,800
• Likely Property Taxes (varies wildly): +$400 to $800/month
• Homeowners Insurance: +$100 to $150/month
• PMI (for 3% down): +$150 to $250/month

Suddenly, that $2,800 is actually $3,450 to $4,000 per month. That's a budget-breaking difference if you only planned for the rule's number.

It Locks You Into a Specific Down Payment Scenario

What if you're putting down 10%? Or 20% to avoid PMI? The rule's 3% down assumption becomes useless. The payment at 20% down is fundamentally different due to a lower loan amount and no PMI. Using the rule in this case gives you a wildly inflated and scary estimate that might discourage you unnecessarily.

Better, More Accurate Ways to Estimate Your Payment

Ditch the napkin. Use these methods instead. They take two more minutes and give you a real picture.

1. Use an Online Mortgage Calculator (The Right Way)

Go to a reputable site like the Consumer Financial Protection Bureau's (CFPB) Owning a Home tools or a major lender's site. Input these four things:

• Home Price
• Your Actual Down Payment Amount (in dollars or %)
• Your Estimated Interest Rate (get a quote or use a current market rate)
• Your ZIP Code (for estimating taxes & insurance)

This spits out a PITI estimate that's light-years ahead of the 3 7 3 rule.

2. The 25% Front-End DTI Rule of Thumb

Many financial advisors suggest your total monthly PITI payment should not exceed 25% of your gross monthly income. This is a better affordability check than the 3 7 3 rule because it's tied to your finances.

3. Get Pre-Approved (The Only Way to Be Sure)

All rules and calculators are just guessing. A mortgage pre-approval from a lender is the definitive answer. They pull your credit, verify your income and assets, and give you a conditional commitment for a specific loan amount and rate. This tells you exactly what you can afford and makes you a serious buyer. Resources from Freddie Mac explain the pre-approval process well.

Your Top Questions About Mortgage Rules, Answered