Recently, more people have been asking about how to use online real estate return calculators. It seems simple since you just input a few numbers and get results automatically, but when you actually try it, it can be confusing to know which numbers to enter where. Many people mistakenly think that the number generated from just entering the purchase price and monthly rent is the cap rate.

Let's take Frederick, Maryland as an example. According to Zillow, the average home value at the end of June 2026 is about $469,700, and the average rent is around $2,200 per month.

The calculation is straightforward. If you calculate $2,200 per month over 12 months, the annual rental income is $26,400. Dividing this by the home price of $469,700 gives you about 5.6%.

However, caution is needed here. This 5.6% is technically closer to the gross return. Not all of the $2,200 rent goes directly into your pocket when you own the house.

You also have to pay property taxes and insurance. There may be repair costs, and if a tenant moves out, there will be vacancy periods. If the house has an HOA, there will be additional management fees. If you use a professional management company, you need to factor in their fees as well.

Let's apply the 50% rule to estimate operating costs as half of the rental income. If we take half of the annual rental income of $26,400 as expenses, the net operating income would be about $13,200. Dividing this by the home price gives a cap rate of about 2.8%.

Comparing this to the initial calculation of 5.6%, the difference is significant.

Of course, the 50% rule is just a quick way to review. If you're actually investing in a property, it's much more accurate to calculate each factor like property taxes, insurance, HOA fees, expected repair costs, and vacancy rates individually. Since property taxes in Frederick can vary based on the address, it's also important to check based on the actual property address.

Cash-on-cash return is different. This measures how much money the actual cash you invested in the house is generating.

For example, if you put down 20% to buy the house, you would consider the down payment and closing costs as your actual investment. Then, you would subtract operating expenses as well as mortgage principal and interest from the rental income, and divide the cash left over after one year by your investment.

So, even for the same house, the cash-on-cash return can vary significantly depending on the interest rate and how much you put down. In contrast, the cap rate is a metric used to compare the profitability of the property itself, regardless of whether you took out a loan.

You also shouldn't just trust the average rent values. In Frederick, the rental levels for 2-bedroom and 3-bedroom single-family homes differ, and there can be significant variations based on the neighborhood and condition of the house.

The home price should also reflect the actual price of the property you want to buy, rather than just the regional average, to yield meaningful results.

Ultimately, online calculators are better thought of as tools that calculate the assumptions you input rather than machines that provide answers.

If I were looking at investment properties in Frederick, I would first check the purchase price and expected rent to see the gross return. Then, I would subtract the actual operating costs to calculate the cap rate, and finally, I would include the down payment and mortgage to check the cash-on-cash return.

It's important not to conclude that "this house has a return of 5.6%" just because the calculator shows that number. What matters more than the calculator is understanding what is included in that number and what is missing.