Find out what you actually pay to live in a property when rental income from the other units offsets your mortgage and expenses.
Picture paying $300 a month to live in a place your neighbors pay $1,800 to rent. That's house hacking when it works: you buy a small multi-unit property, live in one unit, and let the tenants in the others cover most of your mortgage. The catch is that the math only works on the right property at the right price — and a fourplex that looks like a slam dunk in the listing can quietly cost you money once vacancy and maintenance are real. This calculator tells you which it is. Enter purchase price, down payment percentage, mortgage rate, total units including yours, and average rent per rental unit, and it models your full monthly costs — mortgage, taxes, insurance, utilities, maintenance, and vacancy — then subtracts rental income to show your actual monthly housing cost.
That bottom-line figure is the number that matters. A buyer paying $2,400 a month on a mortgage who collects $1,600 in rent from tenants has a net housing cost of $800 — versus $2,400 for a comparable single-family home. The calculator also shows Monthly Cash Flow, which turns positive when rental income exceeds all costs. Some buyers land at zero or slightly positive from day one; others target near-zero housing costs and treat cash-flow neutrality as the win.
How rental income actually offsets your mortgage
The calculator models a property with Total Units (including yours) and uses Rent Per Unit (average for rental units) to compute Total Rental Income. If you buy a fourplex, live in one unit, and rent the other three at $1,000 each, your monthly rental income is $3,000. That figure runs through the vacancy reserve — if you set vacancy at 6%, the effective rental income drops to $2,820 to reflect the realistic expectation that units will occasionally sit empty.
That effective rental income is then compared to Total Costs — your mortgage payment plus property taxes, insurance, the utilities you pay for common areas, and a maintenance reserve. The difference is Monthly Cash Flow. Positive means the property pays you; negative means you are subsidizing it; zero is often the target for a first house hack where the goal is free or near-free housing rather than investment return.
Your real housing cost versus what you see on paper
The output labeled Your Housing Cost is what house hacking is really about. It is your share of the total monthly cost after rental income is credited. For many buyers targeting a duplex or triplex, this number lands somewhere between $200 and $600 — far below what renting a comparable unit in the same market would cost, and often below what a single-family mortgage payment would be.
What makes this figure powerful is that it captures the full cost picture. Owners who run this estimate by comparing the mortgage payment to rental income without factoring in vacancy, maintenance, property management, and their own utilities consistently underestimate carrying costs by $400–700 a month. The calculator builds those line items in explicitly so Your Housing Cost reflects the real operating experience, not just the loan payment minus collected rent.
Maintenance reserve: the input most first-timers skip
Maintenance Reserve % represents the percentage of the property's value set aside annually for repairs, appliance replacement, and capital improvements. A standard rule of thumb is 1–2% of value per year, which on a $400,000 property means $4,000–$8,000 annually — $333–$667 per month. New construction runs lower; older properties run higher. The reserve is set as a percentage of monthly mortgage and costs, so you can scale it to your property's condition.
Skipping this input or setting it to zero produces an artificially optimistic Monthly Cash Flow. Deferred maintenance in a rental unit does not disappear — it compounds. Running with a realistic reserve from the start reflects the actual return you will experience over three to five years rather than an optimistic first-year projection.
Vacancy rate: planning for real-world tenant turnover
Vacancy Rate % is the expected percentage of time rental units sit vacant across the year. For a duplex in a tight rental market, 4–5% may be realistic; in a slower market or with tenant turnover challenges, 8–10% is more conservative. The calculator applies this rate to gross rental income to produce effective rental income — the figure that actually flows to your bottom line.
A 6% vacancy rate on $1,000-per-unit rent in a three-unit property means about $60 per unit per month is reserved against vacancy — $180 total reduction in the rental income line. Over a year, that is $2,160 in vacant time baked into the model. Buyers who have never managed rental property often find that running at 8–10% vacancy for the first year is wise until they have demonstrated their ability to retain and replace tenants efficiently.
Using the calculator to compare properties before touring
The most practical use of this tool is running candidate properties before you visit them. Pull the listing price, estimate rents from comparable units in the same building or neighborhood, and enter rough numbers for taxes, insurance, and vacancy. In two minutes you get a directional Monthly Cash Flow and Your Housing Cost for each property.
This pre-screening step eliminates properties where the numbers simply do not work at asking price, even with generous rent assumptions. It also identifies properties where a small price reduction or rent increase makes the math viable. Going into showings with a screen-filtered shortlist — rather than touring emotionally and calculating later — is the discipline that separates buyers who execute house hacks successfully from those who end up with an expensive lesson.
How to use it
- Enter Purchase Price and set Down Payment % — remember that owner-occupant house hack loans often qualify for 3.5–5% down as a primary residence.
- Set Mortgage Rate % and enter Total Units (including yours) — the number of all units in the property.
- Enter Rent Per Unit (average for rental units) — the rent you realistically expect to charge, not the optimistic ceiling.
- Fill in Property Tax (monthly), Insurance (monthly), and Utilities You Pay for common areas.
- Set Maintenance Reserve % and Vacancy Rate %, then read Your Housing Cost and Monthly Cash Flow.
Who it's for
- First-time buyer evaluating a duplex — A buyer considering a $385,000 duplex models the purchase at 5% down, renting the other unit at $1,200. The calculator shows a $510 monthly housing cost versus $1,800 in rent for a comparable single-family home.
- Buyer comparing a triplex versus a duplex — The same buyer runs both properties — the triplex is $480,000 but has two rental units at $1,100 each. The calculator shows the triplex produces better cash flow despite the higher price.
- Investor checking whether a fourplex qualifies as primary residence — A buyer entering a $620,000 fourplex with three $1,050 rental units sees whether the Your Housing Cost is low enough to justify the larger purchase relative to a simpler duplex.
- Owner modeling a rent increase scenario — An existing house hacker with a $900/unit current rent wonders what a $100 increase across two units does to their monthly housing cost — runs it in 30 seconds and sees the full impact.
Key terms
- House hacking
- Buying a multi-unit property, occupying one unit as a primary residence, and renting the remaining units to offset or eliminate housing costs.
- Monthly Cash Flow
- Total Rental Income (after vacancy adjustment) minus Total Costs. Positive cash flow means the property generates income above expenses; negative means the owner subsidizes the gap.
- Vacancy Reserve
- A percentage of gross rent set aside to account for periods when units sit empty between tenants. Applied against gross rental income to produce a realistic effective income figure.
- Maintenance Reserve
- A monthly dollar amount set aside for repairs, maintenance, and capital improvements. Typically modeled as 1–2% of property value annually to reflect long-term ownership costs.
Frequently asked questions
Can I use a conventional loan for a house hack?
Yes, provided you occupy one of the units as your primary residence. Duplexes, triplexes, and fourplexes all qualify for conventional and FHA financing when owner-occupied. Beyond four units, you generally need commercial financing. FHA allows down payments as low as 3.5% on owner-occupied multi-unit properties, which is why house hacking is accessible even for buyers without large down payments.
What Utilities You Pay should I include?
Include only utilities billed to you as owner — common area electric, water/sewer if your building is on a shared meter, and lawn or trash services. If tenants pay their own utilities directly, set this to zero or a small common-area amount. In buildings with shared utilities, this can be $150–400 a month depending on the property size.
How should I estimate Rent Per Unit if the units are vacant?
Pull active rental listings in the same building type and neighborhood from Zillow, Apartments.com, or Facebook Marketplace. Use median rent for comparable units, not the listing high. Discounting by 5–10% from the market high gives a conservative projection that holds up better under real-world conditions.
Does this tool work for short-term rental house hacking?
The calculator is built for traditional long-term rentals. Short-term rental income varies by season, platform fees, and management cost in ways that require a different model. If you are planning to Airbnb the non-owner units, the STR vs LTR Comparison tool gives you a more appropriate framework for that analysis. Run your target property's numbers here before you make an offer — the two-minute model is free and no account is needed.