How to Calculate Cap Rate on a Rental Property (2026 Guide for Small Landlords)

You found a duplex. The seller says it pencils out. Your gut says one thing and the listing sheet says another. We have been there. Cap rate is the one number that settles most of those arguments, and below we show you exactly how to calculate it on a real duplex, what a good cap rate looks like in 2026, and the five mistakes that trick landlords into buying bad deals.

A cap rate (capitalization rate) measures how much money a property earns each year compared to what it costs to buy — before any loan. Think of it as the building's return if you paid all cash.

It matters for three reasons:

• It compares deals apples to apples. A $200,000 fourplex and a $450,000 duplex both boil down to one percentage.
• It shows how the market prices risk. A higher cap rate means more income for the price, but usually more risk or slower growth.
• It protects you from overpaying. If similar buildings near you sell at a 6% cap rate, a listing that only supports 4% is overpriced or has problems.

The Cap Rate Formula (Memorize This One Line)

Cap Rate = Net Operating Income (NOI) ÷ Purchase Price × 100

And NOI is:

NOI = All rental income − Vacancy − All operating expenses

Operating expenses include taxes, insurance, repairs, reserves, management fees, and any utilities you pay. They do not include the mortgage, income taxes, or big one-time renovations.

What Cap Rate Does NOT Tell You

• It ignores your loan. Two identical buildings can cash flow very differently depending on financing (see our cash flow guide).
• It ignores appreciation. Cheap 8% cap markets often grow slowly; 4% cap coastal cities often grow fast.
• It ignores your labor. Self-managing adds income the formula never sees.

Here is the example we will use for the whole article: a 1970s duplex in a Midwest college town, listed at $300,000, with both units renting at $1,400 per month.

Step 1: Start With Gross Scheduled Rent

$1,400 × 2 units × 12 months = $33,600 per year

That assumes every unit stays full and every tenant pays on time. That never happens, so:

Step 2: Subtract Vacancy and Add Other Income

• Vacancy at 5%: −$1,680
• Laundry income: +$600
• Effective gross income: $32,520

Step 3: List Every Real Operating Expense

Notice no mortgage payment appears in this list. That is on purpose. Track every recurring cost you can so nothing hides from the math.

Step 4: Divide NOI by the Purchase Price

• NOI = $32,520 − $13,600 = $18,920
• Cap rate = $18,920 ÷ $300,000 = 6.3%

Step 5: Sanity-Check the Answer

At 6.3%, the property's income pays back its purchase price in about 16 years, before appreciation and loan paydown. Run the same math on three nearby sales. If your number is far above or below theirs, something is wrong — either your expenses or the seller's price.

There is no universal good number. Cap rates move with interest rates, and in 2026 they vary hugely by market:

Compare the Cap Rate to Your Borrowing Cost

This is the move most beginners skip in 2026: subtract your mortgage rate from the cap rate.

• Cap rate 6.3% with a 7% loan = negative leverage — every borrowed dollar drags your return down.
• Cap rate 6.3% with a 6% loan = positive leverage — the loan amplifies your return.

If rates sit above local cap rates, buy with a bigger down payment, wait, or shop markets where cap rates run higher.

Three Actions to Take Before You Offer

1. Pull sold comps nearby and estimate the neighborhood cap rate.
2. Get a real rate quote from your lender before you fall in love with a deal.
3. Only offer where cap rate minus mortgage rate is near zero or better.

These three get mixed up constantly. They answer different questions:

Our Simple Screening Order

1. 1% rule filter — kill the obvious duds in seconds.
2. Cap rate check — price the survivors against the local market.
3. Cash-on-cash return — run real loan terms on the finalists.

Only step 3 tells you what your bank account will actually feel. Steps 1 and 2 get you there without wasting your weekends.

You can do all of this by hand, but one mistyped formula quietly wrecks an offer. So we built a free cap rate calculator into Property Aura, and it is embedded below for you to use right now:

What to Enter (and What to Leave Out)

• Include: taxes, insurance, repairs, CapEx reserve, management fee, utilities you pay, and honest vacancy.
• Leave out: the mortgage payment, income taxes, and one-time renovation budgets (add those to your total cost instead).

What the Output Should Tell You

1. A cap rate at or above your local market norm.
2. Positive or near-zero leverage versus your quoted loan rate.
3. A cash-on-cash return you would actually accept — many landlords want 8% or better.

Why We Put the Calculator Inside the Tool You Manage With

Spreadsheets cannot pull in what you actually spent last quarter. Property Aura can — and that is exactly what NOI is supposed to reflect.

Mistake 1: Subtracting the Mortgage Payment in the Cap Rate Math

The Mistake: Plugging your loan payment into operating expenses and calling the result a cap rate. That is a financed return, not a cap rate. The Cost: You can no longer compare properties to each other or to the market, and financing-dependent math hides overpaying.

Mistake 2: Skipping Vacancy, Maintenance, and CapEx Reserves

The Mistake: Using the seller's numbers with 0% vacancy and zero reserves. The Cost: The cap rate reads 8% when the real number is 5%. That gap can wipe out years of cash flow the first time the roof fails.

Mistake 3: Trusting the Pro Forma Instead of Actual Numbers

The Mistake: Using projected market rents instead of the in-place leases. The Cost: If real rents are $100 per unit lower than the pro forma, that is $2,400 a year on our duplex — roughly $38,000 of value at a 6.3% cap rate. Always ask for the rent roll and actual expense history.

Mistake 4: Comparing Cap Rates Across Markets or Property Types

The Mistake: Rejecting a solid 4.5% deal in a growing city because you read that 8% exists somewhere else — or comparing a long-term duplex to a short-term rental. The Cost: You either freeze and buy nothing, or you chase high-cap deals in shrinking towns where vacancy eats the extra yield.

Mistake 5: Using List Price Instead of True Total Cost

The Mistake: Dividing NOI by the sticker price while ignoring closing costs (2–5%) and day-one repairs. The Cost: Our $300,000 duplex is really about $318,000 with closing costs. The cap rate silently drops from 6.3% to 5.9%. Good calculators let you add those costs in.

1. Do I include my mortgage payment when I calculate cap rate?

No. Cap rate measures the building, not your loan. Mortgage payments belong in cash-on-cash return and cash flow math.

2. Is a higher cap rate always better?

No. Higher cap rates usually mean more risk — weaker tenant demand, slower growth, or bigger repair bills. Match the number to your risk tolerance.

3. What cap rate should I look for on a small duplex in 2026?

In most Midwest and Southern markets, 6% or better is a fair target. In large coastal metros, 4–5% can still be a fine deal if rents and values are stable.

4. Does cap rate change after I buy the property?

Yes. Cap rate is NOI divided by value, so every rent raise, expense change, and market swing moves it. Re-check it yearly — Property Aura updates NOI automatically from your real books.

5. How is cap rate different from ROI?

ROI is broader — it can include appreciation, loan paydown, and your time. Cap rate is deliberately narrow: one year of building income against one purchase price. Use cap rate to shop, and ROI to judge the full picture.

Cap rate will not tell you everything, but it will stop you from buying a pretty building that loses money. Run every deal through our free cap rate calculator, then let Property Aura keep tracking real income and expenses after closing — rent collection, maintenance, and tax-ready accounting in one simple dashboard. Try Property Aura free and make your next offer with numbers, not nerves.