Duplex Mortgage Calculator
A duplex is two investments in one building: a mortgage you pay and rents you collect. Whether the deal works comes down to one subtraction, effective rental income minus every monthly cost, and most calculators only do half of it. This page does both halves. The investment tab runs the full deal analysis: mortgage payment, vacancy-adjusted rents, taxes, insurance, maintenance, management fees, cash flow, cash-on-cash return, and the 1% rule. The affordability tab answers the question buyers ask before any of that: given my income and debts, how much duplex can I actually qualify for?
Both tabs share nothing but the page. Run the investment analysis on any property you are considering, then run the affordability check to see whether a lender would agree with you.
Investors typically need 20-25% down; owner-occupants often qualify with less.
5-8% is typical; lenders often assume 25% vacancy on the income they count.
Charged on collected rent; enter 0 if self-managing.
Monthly cash flow:
Car loans, student loans, minimum card payments.
Lenders typically count 75% of gross rent toward your income.
Most lenders cap DTI at 43%; FHA allows up to 50-56.9% with strong files.
Max duplex price:
Investment math uses the standard amortization formula. Affordability uses your debt-to-income ratio with 75% of gross rent counted as qualifying income, the common lender practice.
Quick Answer
For the pre-filled duplex, $400,000 with $80,000 down and both units renting at $1,400, the investment tab returns a -$325.42 monthly cash flow: a $2,022.62 mortgage payment and $2,985.42 of total monthly expenses against $2,660 of vacancy-adjusted rent, for a -4.88% cash-on-cash return. On the affordability tab, $90,000 of income, $2,000 of monthly debts and a 43% DTI cap support a maximum duplex price of $445,938.42, because the lender counts 75% of the second unit's rent as qualifying income.
- Quick Answer
- How to use the Duplex Mortgage Calculator
- The duplex formulas
- Worked example: the $400,000 duplex, investment view
- Worked example: what $90,000 of income qualifies for
- Owner-occupied vs full rental: the same building, two deals
- House hacking a duplex: honest pros and cons
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use the Duplex Mortgage Calculator
Start on the investment tab with a real property, then check the affordability tab with your real income. The two tabs answer different questions, and a deal is only worth chasing when both answers work.
On the investment tab, enter the purchase price, down payment, rate, and term, then both units' rents and your expense assumptions. Vacancy matters: 5% is typical for the analysis, while lenders qualifying you for the loan often cut rental income by 25%. If you will live in one unit, select it, and the tool shows your personal monthly housing cost alongside the investor numbers. On the affordability tab, enter your income, monthly debts, available down payment, and the rent you expect from one unit; the tool applies your DTI limit and works backwards to the maximum loan and purchase price. If you want the lender's view of that ratio on its own, the 45 debt-to-income ratio calculator benchmarks your DTI against the bands lenders use.
The duplex formulas
Five formulas drive the investment tab, and one reversed formula drives affordability.
Monthly mortgage payment = Loan x (r x (1 + r)^n) / ((1 + r)^n - 1), with r the monthly rate and n the number of payments.
Effective rent = Gross rent x (1 - vacancy rate).
Cash flow = Effective rent - (Mortgage + Tax + Insurance + Maintenance + Management).
Cash-on-cash return = Annual cash flow / Down payment x 100.
1% rule = Monthly gross rent / Purchase price x 100.
Affordability: Max housing payment = (Annual income / 12) x DTI limit - Monthly debts + 75% of expected rent, then the loan formula is solved backwards for the maximum loan.
Two definitions deserve care. Net rental yield here is the year's rent after vacancy, tax, insurance, maintenance and management, but before the mortgage, divided by the purchase price. It is close to, but not identical to, a cap rate, which uses full net operating income; the cap rate calculator runs that metric on its own if you want to compare this duplex against other properties on a like-for-like basis. Cash-on-cash return divides by the down payment only. It does not include closing costs, so it reads slightly higher than tools that divide by total cash invested.
The 75% rental-income convention is not a quirk of this page. Lender guidelines, including the Fannie Mae Selling Guide section on rental income, build vacancy and collection loss into how much rent a lender will count, and the Consumer Financial Protection Bureau explainer on debt-to-income ratio and the 43% limit sets out why that cap shows up in underwriting.
Worked example: the $400,000 duplex, investment view
A $400,000 duplex with $80,000 down leaves a $320,000 loan at 6.5% for 30 years. Both units rent for $1,400. These are the values pre-filled on the investment tab, and every figure below is what the tool returns for them, verified line by line.
| Line | Calculation | Result |
|---|---|---|
| Monthly mortgage (P and I) | $320,000 loan, 6.5%, 30 years | $2,022.62 |
| Gross monthly rent | $1,400 + $1,400 | $2,800.00 |
| Effective rent | $2,800 x (1 - 5% vacancy) | $2,660.00 |
| Property tax | $4,800 / 12 | $400.00 |
| Insurance | $1,800 / 12 | $150.00 |
| Maintenance | $2,400 / 12 | $200.00 |
| Management fee | 8% of $2,660 effective rent | $212.80 |
| Total monthly expenses | Mortgage + tax + insurance + maintenance + management | $2,985.42 |
| Monthly cash flow | $2,660 - $2,985.42 | -$325.42 |
| Annual cash flow | -$325.42 x 12 | -$3,905.01 |
| Cash-on-cash return | -$3,905.01 / $80,000 | -4.88% |
| Net rental yield | Net rent before mortgage / $400,000 | 5.09% |
| 1% rule | $2,800 / $400,000 | 0.70% |
Negative cash flow does not automatically kill the deal: appreciation and principal paydown still build wealth, and each monthly payment retires part of the loan. But you would be feeding the property $325 every month, and the honest reading of the verdict badge is that this price and these rents do not carry this mortgage. Either the price comes down, the rents are genuinely higher than entered, or the deal is an appreciation bet and should be judged as one.
Worked example: what $90,000 of income qualifies for
With $90,000 of annual income, $2,000 in monthly debts, $50,000 for a down payment, a 6.2% rate over 30 years, a 43% DTI cap, and $1,600 of expected rent from one unit: qualifying rent = 1,600 x 0.75 = $1,200; max monthly housing payment = (90,000 / 12) x 0.43 - 2,000 + 1,200 = $2,425.00. Working the amortization formula backwards, that payment supports a $395,938.42 loan, so the maximum duplex price is $445,938.42. Verified against the calculator above.
Notice what the rent does here. Without the $1,200 of qualifying rent, the same income and debts would support a far smaller payment. That is the duplex advantage in underwriting: the second unit helps you qualify for the building that houses you. It is also why lenders verify market rent from an appraisal rather than taking your figure: the qualifying income is only as real as the rent. Before you rely on any approval estimate, the approved mortgage calculator cross-checks what a lender is likely to approve from income and debts alone.
Owner-occupied vs full rental: the same building, two deals
Living in one unit changes both the math and the mortgage. You collect only the other unit's rent, but your personal housing cost becomes total expenses minus that rent, often far below what a single-family home would cost you.
Full rental (both units let)
Gross rent $2,800, effective rent $2,660 at 5% vacancy. Cash flow -$325.42 a month on the worked example.
You need investor financing: typically 20 to 25% down, and a slightly higher rate.
The property has to stand on its own numbers, because no tenant is subsidising your housing.
Owner-occupied (you live in Unit 1)
Only Unit 2's rent counts: effective rent $1,330. Your monthly housing cost is $1,549.02 on the same building.
You can often qualify with 3.5 to 5% down through FHA or conventional two-unit programs.
The comparison is not against zero rent: it is against what a home of your own, plus a separate rental investment, would cost you.
Lenders reward the owner-occupied route because you are on site and the rent still counts at 75% toward qualifying income. The trade-off is landlording from next door, which is either convenient or miserable depending on your tenants. Tax treatment also splits by unit: the rental half is a rental activity with its own deductions, covered in the IRS guide to residential rental property in Publication 527, while your own half is treated as your home.
House hacking a duplex: honest pros and cons
Pros
- The second unit's rent counts toward qualifying income at 75%, so you can buy more building than your salary alone supports.
- Low owner-occupant down payments (3.5 to 5%) get you into an income-producing asset with far less cash than an investor loan.
- You are on site: vacancies get filled faster, and small repairs do not need a management company.
- One mortgage, one roof, one tax bill covers both your housing and your first rental.
Cons
- Negative cash flow on paper is common at today's prices and rates, as the worked example shows: the deal can still cost you monthly.
- Your tenant is your neighbour. Late rent and noise complaints arrive at your own front door.
- This tool's cash-on-cash return divides by the down payment only, so it flatters the return versus tools that count closing costs too.
- Selling or moving out can change your loan terms and insurance; an owner-occupied rate assumes you actually live there.
If the duplex numbers stay negative at every realistic rent, widen the search rather than forcing the deal: the buying rental property calculator runs the same style of analysis across property types, including closing costs and repairs in the cash invested.
What competing calculators miss
The ranking tools split into two incomplete halves. One group computes the mortgage payment and subtracts rent, ignoring vacancy, management, taxes, and insurance, which is how a deal that looks positive on screen loses money in reality. The other group runs a full rental analysis with cap rates and long projections, but never answers the affordability question: given my income and debts, what will a lender actually approve? General rental property calculators also bury the duplex-specific facts that decide these deals: the 75% rental-income rule, the owner-occupied housing cost, and the very different down payments for occupants versus investors.
This page keeps the full investment analysis with every expense line visible, adds the DTI-based affordability tab with the 75% rule lenders actually use, and shows the owner-occupied housing cost in the same result box. Its limits are stated rather than hidden: no closing costs in the cash-on-cash denominator, no PMI line, and no amortization schedule. A tool that names what it leaves out is more useful than one that quietly leaves out vacancy.
Frequently asked questions
It depends on occupancy. Owner-occupants can use FHA loans with 3.5% down or conventional loans with 5% down on two-unit properties. Investors buying a non-owner-occupied duplex typically need 20-25% down, and rates run slightly higher.
Monthly gross rent should be at least 1% of the purchase price. A $400,000 duplex should gross $4,000 a month to pass. It is a screening shortcut, not a verdict: many solid deals in expensive markets fail it while cash-flowing fine after realistic expenses.
Many investors target 8-12% cash-on-cash, meaning the down payment earns that return yearly from cash flow alone. Anything positive beats a property that needs monthly feeding, but compare against your other options for the same capital.
Most lenders count 75% of gross market rent as qualifying income, holding back 25% for vacancy and maintenance. They use appraiser or lease-based rent figures, not your optimistic projection, and some require landlord experience or cash reserves.
The math generalizes: add the extra units' rents into the two rent fields (split them however is convenient). FHA financing extends to four-unit properties for owner-occupants, which is why house-hacking scales to fourplexes.
Not necessarily, but it means the investment costs you money every month. Positive leverage cases exist: strong appreciation markets, below-market purchase prices, or value-add renovations that raise rents. Run the numbers with conservative vacancy and maintenance before counting on appreciation to bail out negative cash flow.
Start with 5% for a stable area with strong rental demand, and 8-10% where turnover is higher or one unit sitting empty for a month is a realistic year. Remember the scale: with only two units, one empty month in one unit is already about 4.2% vacancy for the year, so small numbers stop being conservative fast.
No. The cash-on-cash return divides annual cash flow by the down payment only, and there is no PMI or mortgage insurance premium line in the expenses. That makes the return read slightly higher than tools that divide by total cash invested. If you are putting less than 20% down, add an insurance estimate to the annual insurance field as a rough allowance.
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References
- Fannie Mae, Selling Guide, "Rental Income" (topic B3-3.1-08) - how lenders document and calculate rental income, including the vacancy allowance behind the 75% convention. Fannie Mae Selling Guide: Rental Income. Accessed October 2026.
- Consumer Financial Protection Bureau, "What is a debt-to-income ratio? Why is the 43% debt-to-income ratio important?" - the DTI definition and cap used in mortgage underwriting. CFPB: Debt-to-Income Ratio and the 43% Limit. Accessed October 2026.
- Internal Revenue Service, Publication 527, "Residential Rental Property" - rental income, expenses and depreciation for the rented units of a small multifamily property. IRS Publication 527: Residential Rental Property. Accessed October 2026.
- Consumer Financial Protection Bureau, "What is private mortgage insurance?" - when PMI applies on low-down-payment loans and how it adds to monthly cost. CFPB: What Is Private Mortgage Insurance?. Accessed October 2026.
- U.S. Department of Housing and Urban Development, "FHA Loans" - FHA eligibility for owner-occupied properties of up to four units, including low down payments. HUD: FHA Loans. Accessed October 2026.
- Freddie Mac, Single-Family Seller/Servicer Guide Section 5306.1, "Rental income" - rental income underwriting requirements for investment and owner-occupied two-unit properties. Freddie Mac Guide: Rental Income (5306.1). Accessed October 2026.