Real Estate Investing Basics

How to Accurately Calculate Cash Flow on Your Next Rental Property

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When I was a kid, “Duck Tales” was one of my favorite TV shows.

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Each day after school I’d watch the adventures of Huey, Dewey, and Louie as they fought off the Beagle Boys who wanted good ‘ol Scrooge McDuck’s hard-earned money. They always managed to thwart the efforts of the villains and save Scrooge’s money. And, as a reward to himself, Scrooge McDuck and the three nephews would take a swim in his vault of money. Yes, you remember. They would jump off the diving board head first into mountains of gold coins. As a kid, nothing seemed more exciting than that.

So how did Scrooge get so much money? To use Scrooge’s own words, by being “smarter than the smarties, and tougher than the toughies.” In other words, Scrooge was good at business. He was smarter than the rest.

Yes, this is only a cartoon, but I think there is a valuable lesson to be learned here. If you want to succeed and swim in your own river of cash, you need to be smarter than the rest. I believe the best way to do this is through a solid grasp on the numbers.

Math was not most people's favorite subject in school, but it might be the most important for a real estate investor. Understanding how your business makes money is imperative in helping it make more. Therefore, I want to focus on one of the most important aspects of real estate math: cash flow.

In layman's terms, cash flow is the amount of income left in your business after all the bills have been paid. This amount is often expressed as a monthly dollar amount. In the real estate rental business, cash flow is the income left after paying out expenses such as the mortgage, taxes, insurance, vacancies, repairs, capital expenditures, utilities, and any other expenses that affect the property.

How to Calculate Cash Flow


Cash flow might seem really easy to calculate, but trust me, a lot of people get it wrong. At its core, it is very simple. To calculate cash flow ,you simply subtract the expenses from in the income:

Cash Flow = Total Income – Total Expenses

Easy enough, right? So…how do so many people screw this up? The fact is that while the equation is simple enough, the items that make up the equation are loaded with complexity. Let’s take a look at both to see what I mean:

Total Income

While the total income might be the same as the total rent, many times it won’t be. There may be other sources of income you need to account for, such as application fees, late fees, and laundry income.

When analyzing a property for cash flow, it’s wise to list out all possible sources of income, but be conservative. It’s best to err on the side of caution and assume you’ll be getting less than you actually hope to.

Total Expenses

Ah…this is when things get complicated. You see, most people can figure out the income, but when it comes to expenses, one mistake can be the difference between incredible success and catastrophic failure. When dealing with rental properties, there are A LOT of expenses you’ll encounter. For example, just off the top of my head, you might have:

  • The mortgage
  • Mortgage insurance (PMI or MIP)
  • Property taxes
  • Property hazard insurance
  • Flood insurance
  • Earthquake insurance
  • Water
  • Sewer
  • Garbage
  • Electricity
  • Natural gas
  • Propane
  • General maintenance upkeep
  • Landscaping
  • Repairs
  • New appliances
  • Capital expenditures
  • Office Supplies (stamps, envelopes, paper, etc.)
  • Software
  • Gas/mileage
  • HOA (Homeowners association) dues, fees, and assessments
  • City taxes
  • Advertising
  • Payroll
  • Property management
  • Vacancy rate
  • Probably a lot more I’m not thinking of…

To further complicate things, not all expenses are going to occur each month, so it is often best to calculate a certain percentage for those expenses when planning for the future. For example, you may not have any vacancies right now, but you might assume your property will be empty one month out of the year. Therefore, you will want to include 1/12th, or 8.3 percent, for your monthly vacancy expense.

Some expenses, like office supplies and gas, you may choose to ignore on individual houses. But if you are buying a larger multifamily, I’d recommend including it because it does add up quickly.

Related: Visualizing Cash Flow—How to Accurately Budget Expenses


Example: Let’s Calculate Carl’s Cash Flow

Carl is trying to calculate his monthly cash flow for a duplex he might buy.

According to the real estate agent, the property will rent for $600 per unit. Carl would be responsible for paying $125 per month for water/sewer and $50 for garbage. He also learns that properties like this duplex in this area sit vacant for 5 percent of the year. Carl assumes he'll spend 8 percent per month on repairs. He also will plan on setting aside 5 percent each month for capital expenditures, such as a new roof, heating system, or other big ticket items down the road.

Finally, Carl calculates that his mortgage payment will be $470 per month (taxes and insurance not included). The property taxes are $960 per year and the insurance would be $600 per year. Finally, Carl will be using a local property management company that charges 10 percent per month to manage the property.

How much cash flow can Carl plan to receive?


$600 x 2 = $1,200 per month in income


  • Mortgage: $470
  • Taxes: $80
  • Insurance: $50
  • Vacancy: $60
  • Repairs: $96
  • CapEx: $60
  • Water/Sewer:$125
  • Garbage:$50
  • Management: $120

Add all these up and it comes to $1,111 per month in expenses. 

Income – Expenses = Cash Flow

$1,200 – $1,111 = $89 per month


How to Calculate Cash-on-Cash Return on Investment

Bonus: If Carl spent $20,000 to acquire this property (down payment and closing costs), what kind of cash-on-cash return on Investment would this be for Carl?

The cash-on-cash return on investment is the annual cash flow a real estate investor receives based on the cash they invested. It is one method to compare the returns an investor might get through rental real estate compared with other investment methods. To determine your cash-on-cash return, use the following formula:

Cash-on-Cash Return = Annual Cash Flow / Total Investment 

Therefore, since Carl would be expecting $1,068 per year (with a $20,000 investment) the formula would be:

$1,068 / $20,000 = 5.34%

Carl's estimated cash-on-cash return on investment would be 5.34 percent. Keep in mind, this return does not take into account potential appreciation, loan paydown, tax benefits (or payments), or other forms of increase or decrease in his profit. However, I like to use the cash-on-cash return because, at minimum, this is what I probably will expect. If the property appreciates or the loan pays down over 30 years…awesome! However, I don't necessarily base a deal on this.

How to Calculate Cash Flow Using the 50 Percent Rule

Another, much quicker way, to estimate cash flow is using a technique known as the 50 percent rule. This rule of thumb states that a rental property's expenses tend to be about 50 percent of the income, not including the mortgage principal and interest (P&I) payment. The formula would look like this:

Cash Flow = (Total Income x .5) – Mortgage P&I

Let’s use the same information we used earlier with Carl and the duplex he was interested in. In that example, the property was expecting to bring in $1,200 per month in income, with a mortgage P&I of $470 per month. Using the 50 percent rule, we can see:

  • Cash Flow = ($1,200 x .5) – $470
  • Cash Flow = ($600) – $470
  • Cash Flow = $130

Keep in mind that the 50 percent rule is only a rule of thumb and generally not as accurate as using the actual expected numbers. In this example, Carl determined using the 50 percent rule that he could expect $130 per month in expenses, but the actual numbers we did earlier showed only $89 per month.

Therefore, the 50 percent rule is a good tool to use to quickly analyze a rental property, but should never take the place of a thorough analysis of a property. Think of the 50 percent rule as a “quick filter” that allows you to estimate cash flow in under a minute, enabling you to analyze dozens of properties while looking for a deal with potential that you can run a more thorough analysis later on.

Finally, it would be a shame not to mention that all these numbers you can do online using the BiggerPockets Rental Property Calculator. While you can easily do these numbers on a spreadsheet or on the back of a napkin, I find it helpful to use the online calculators to ensure my math is correct and that I’ve considered all the potential expenses. Furthermore, you can save your analysis reports and even print out your report to give to potential lenders, partners, or others.

Check out the video below to learn more about the rental property calculator:

Did you find this helpful, and is there anything you’d like to add about calculating cash flow? 

Share with a comment below!

Brandon Turner is an active real estate investor, entrepreneur, writer, and co-host of the BiggerPockets Podcast. He is a nationally recognized leader in the real estate education space and has tau...
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    Taylor Jennings
    Replied about 6 years ago
    Nice explanation. Definitely set the standard for simplicity and detail.
    Rajib Miah
    Replied about 6 years ago
    Brandon, thanks for such an awesome article with real life examples. This one is going in my bookmarks bar for future reference.
    Replied about 6 years ago
    I’m looking at a Homepath duplex and currently trying to calculate the cash flow. I’ll be living in one of the units so do you think I should include my “rent” in the analysis to get a better picture? (Otherwise, it’s negative cash flow). Also, how should I go about getting an idea of what the expenses are in that area? To get an idea of rent costs, I’ll be knocking on doors this week to talk and ask. Should I do the same with their expenses? There are not many duplexes in this area so the comps are outdated.
    Replied about 6 years ago
    I’d think for a duplex you’d want to run the calculations as if you were renting both halves, but make sure to look at your personal budgeting to make sure the “rent” you would be paying is in line with what you can afford. examples; a) rent on each half of duplex is $700, you are currently paying $750/mos rent in an apartment — Mortgage, taxes, etc are $1k/mos total = go for it! b) You are currently renting for $1k. TOTAL costs Mortgage+ insurance, taxes, etc. = $2k. Duplex rents at $1k per unit= use caution. In effect you’ll be having to pay for repairs, vacancies, etc for BOTH units out of your own pocket. Fine if you can afford it not so good if you live paycheck to paycheck. A duplex is often an excellent idea, but it isn’t “free” housing.
    Ben Staples Investor from Chicago IL
    Replied over 5 years ago
    Great article! Really helpful
    Josh Lyons from Charlotte, NC
    Replied over 3 years ago
    Thanks for this article Brandon… very helpful! Quick question… in calculating cash flow, should legal and accounting fees be considered as well? Is there a certain percentage of the rent that could be used to calculate these costs per month? Thanks!
    Brice Naylor from Loveland, Ohio
    Replied over 3 years ago
    Got the cash on cash calculation right 🙂
    Ashley Cote Software Developer from Mobile, Alabama
    Replied almost 3 years ago
    I know this is an old post, but I’m just starting out. I’m a huge numbers fan so this post was exactly what I was looking for. I enjoyed the test example as well (I did get it right 😀 ) Thanks!
    Ben Lemer from Canton, South Dakota
    Replied almost 3 years ago
    nailed it!! great post!!
    Eric Carr Real Estate Broker from Los Angeles, CA
    Replied about 2 years ago
    “When I was a kid, Duck Tales was one of my favorite TV shows.” Now I’ve got the intro song stuck in my head!!!