08/19/2026
Buying a Home: Best to Start with Cash Flow
When planning a real estate purchase, I’ve found that the best place to start is by defining your maximum monthly payment, NOT the maximum amount that the bank is willing to lend you. Borrowing the maximum translates into the maximum payment the bank thinks you can make consistently without defaulting on the loan.
The rule of thumb that I use is 15% of your gross income. This can be difficult in markets like DC where housing is expensive. Ultimately, your family needs cash flow to live AND to save enough to one day retire and stay retired.
In that way, the 15% of gross income as a maximum monthly payment towards housing is a vitally important component of enabling your retirement savings plan.
Here is the math. Let’s say your household income is $500,000. 15% of the $500,000 is $75,000 per year, which represents the maximum you intend to put towards principal and interest on your mortgage. Divide the $75,000 by 12 and that translates to a monthly payment of $6,250. This is your maximum monthly payment.
Using Google, ask what the loan size is for a 30-year fixed loan with a $6,250/month payment and a 7% interest rate. I’m going to round up here, so the answer is $950,000 that you can borrow. The bank will likely want 20% down, so $950,000 divided by 80% will give you the target purchase price of the home, which using these assumptions is roughly $1,200,000. To arrive at the required down payment, subtract $950,000 from the $1,200,000 and you get a down payment of $250,000.
Note: the only time that I will say to go beyond the 15% is that you can see a clear path to a promotion and salary increase within a year or two that will get you to that 15% (ex. maybe you start off with 20% of gross income, but you know that your incomes will increase such that the 20% will soon become 15%)
Choosing the right mortgage size will not only put you in a position to save towards a funded retirement, but will also help keep your money stress in-check.