Property calculators divide by who you are. A buyer needs affordability and payment tools. An owner needs equity, refinancing and sale-proceeds tools. An investor needs return metrics, and those are the ones most often misused.

The recurring theme across the investor tools is that a rental pays you in four ways — cash flow, principal paydown, appreciation and tax treatment — and most analyses count only the first. A property that looks like it barely breaks even can be returning well into double digits once the others are included.

The ideas these tools share

A rental pays you four ways: Cash flow, principal paydown, appreciation and tax treatment. Most analyses count only the first, which is why a property that looks like it barely breaks even can be returning well into double digits. It is also why a strong cash-flow number alone does not make a deal good.
Leverage cuts both ways: Borrow below the cap rate and debt raises your return on invested cash. Borrow above it and debt destroys return. The test is whether your loan constant exceeds the cap rate, and no assumption about future appreciation changes the first year of arithmetic.
Value is NOI divided by cap rate: That single relationship explains most of what happens to a property price without anything changing at the property. If market cap rates rise, your building is worth less even though it produces exactly the same income. Underwriting an exit cap at or above your entry cap is the conservative convention for this reason.
NOI excludes financing on purpose: Net operating income is income after operating expenses but before mortgage payments, so two buyers with different loans can compare the same building. The moment you add debt service you are measuring your deal rather than the property.
The costs sellers omit are always the same ones: Property management, capital reserves and realistic vacancy. Strip those from an offering memorandum and a 7.5% cap becomes something closer to 5.5%. Rebuild the expense side yourself before trusting any headline yield.

Which one do you need?

You are buying a home to live in: Mortgage affordability first, then the payment calculator for a specific property.
You are deciding whether to refinance: Refinance break-even — and check whether the new term is longer than what you have left.
You need cash from your equity: HELOC vs cash-out refinance, which compares the cost of the money rather than the monthly payment.
You are evaluating a rental: Investment property ROI for the full picture, cap rate to compare properties, cash-on-cash to compare deals.
You are flipping: ARV and the 70% rule — and make sure the selling commission is in your cost base.
You are selling: Home sale proceeds for the full net, agent commission for the fee alone.

Where these go wrong

Using the seller expense figures
Offering memoranda routinely omit management, reserves and realistic vacancy. Those three lines are usually the difference between a deal that works and one that does not.
Refinancing an entire mortgage to access part of it
If your current rate is below the refinance rate, a cash-out refinance reprices the whole balance to reach a fraction of it. A second-position loan leaves the first mortgage alone.
Assuming cap rate compression on exit
Underwriting a sale at a lower cap rate than you bought at is assuming the market does your work. Model the exit at or above your entry cap and let compression be upside rather than the plan.
Judging a rental on cash flow alone
Cash flow is one of four returns and usually the smallest early on. Ignoring principal paydown makes a sound deal look marginal.

Common questions

What is the difference between cap rate and cash-on-cash return?

Cap rate is net operating income over purchase price and ignores financing entirely, so it describes the property. Cash-on-cash divides your actual cash flow by the cash you invested, so it describes your deal. One building has a single cap rate and as many cash-on-cash returns as there are ways to finance it.

How much should I budget for rental expenses?

About 50% of gross rent over a full ownership cycle, excluding the mortgage — covering tax, insurance, management, maintenance, capital reserves and vacancy. Your first year will almost certainly be cheaper, which is exactly why the figure is expressed over a cycle rather than a year.

Do these include property taxes and insurance?

Where they matter, yes, and as inputs rather than assumptions — both vary enormously by state. Property tax ranges from roughly 0.3% of value in Hawaii to over 2% in New Jersey, so any calculator that hardcodes a national average is wrong for most of the country.

What cap rate is a good cap rate?

There is no universal answer, because cap rates price risk and location. A 5% cap in a stable metro and an 9% cap in a thin rural market can carry the same real risk-adjusted return. Compare against what similar properties in the same submarket actually trade at, never against a national figure.

Should I use gross rent or net operating income?

Net operating income, always, for anything comparing properties. Gross rent multiples ignore the expense side entirely, which is where two superficially identical buildings most often differ.

How much should I set aside for capital expenditure?

Roofs, heating systems and appliances fail on a schedule even when nothing goes wrong. Reserving a percentage of rent monthly turns an unpredictable disaster into a predictable line item, and its absence is the most common reason a projected return never materialises.