Why It Matters
Here is how to read your number: add up all rent collected from units you don't occupy. Subtract your full PITI — principal, interest, taxes, and insurance — plus a maintenance reserve. Then add back the fair market rent your own unit would cost you if you rented it elsewhere. A positive result means your tenants are subsidizing your housing. A negative result means you still pay less out of pocket than you would renting solo — and you are building equity at the same time. Most house hackers land somewhere between break-even and a few hundred dollars positive per month. That gap is the real prize.
At a Glance
- What it is: Net monthly income after tenants pay rent and you subtract all ownership costs
- Formula: House Hack Cash Flow = Rental Income from Other Units − (PITI + Maintenance − Owner's Unit Value)
- Why it matters: Determines whether your tenants fully cover your housing costs — or by how much
- Typical range: Break-even to +$600/month for a well-underwritten duplex or triplex
- Key inputs: Gross rents, PITI, maintenance reserve, and the implicit rent value of your unit
House Hack Cash Flow = Rental Income from Other Units − (PITI + Maintenance − Owner's Unit Value)
How It Works
Rental income from other units. Count only the units you don't occupy. On a duplex, that is one unit. On a triplex, that is two. On a fourplex, three. Use actual signed lease amounts, not optimistic pro formas. If a unit is vacant, model it at 90% of market rate to reflect realistic stabilization, not best-case occupancy.
PITI as your cost baseline. The house-hack-expenses that matter most are your full housing payment — principal, interest, property taxes, and insurance. This is what your tenants are offsetting. A $2,800 PITI on a duplex where the other unit rents for $1,650 leaves you carrying $1,150 — which is likely cheaper than renting a comparable apartment in the same market.
Maintenance reserve. Budget 5–10% of gross rental income for repairs, appliance replacements, and turnover costs. Skipping this number creates fake positive cash flow that gets wiped out the first time a water heater fails. On a $1,650/month rental, that is $83–$165 per month set aside before you count cash flow.
Owner's unit value. This is the key adjustment that separates house hack math from standard rental math. Your unit has a fair market rental value — what you would pay to rent it elsewhere. Add that back into the equation. If your unit would rent for $1,400 per month and your PITI is $2,800, with $1,650 coming in from the tenant, your effective housing cost drops to $2,800 − $1,650 = $1,150. Compared to renting your unit at $1,400, you are $250 ahead monthly — and building equity.
Why the formula matters for underwriting. When evaluating whether to buy, run this formula before submitting an offer. If you need a conversion-permit to create a rentable unit or if the property requires a separate-entrance to qualify for certain loan programs, those costs affect your PITI and therefore your projected cash flow. Properties in areas with multifamily-zoning often command higher rents but also higher purchase prices — the formula helps you test whether the numbers pencil.
Real-World Example
Simone bought a duplex in Columbus, Ohio in early 2024 for $342,000 with 5% down under an FHA owner-occupant loan. Her numbers broke down like this.
Monthly PITI: $2,410. The other unit rented for $1,575. Her maintenance reserve at 8%: $126. Her unit's fair market rent in that neighborhood: $1,350.
House Hack Cash Flow = $1,575 − ($2,410 + $126 − $1,350) = $1,575 − $1,186 = $389 per month.
Simone's tenants effectively cut her housing cost from $2,410 to $389 — an 84% reduction. Before buying, she paid $1,340 per month renting a one-bedroom nearby. Now she pays $389 and is building equity on a $342,000 asset. Over 12 months that gap — $1,340 versus $389 — saves her $11,412, which is more than her entire down payment contribution of $17,100.
She also has a shared-wall unit configuration, which required verifying the property met code before closing. Once she confirmed it did, the deal was straightforward.
Pros & Cons
- Significantly reduces your largest monthly expense — housing — often by 50% or more
- Allows owner-occupant loan programs (FHA, conventional 5% down) unavailable to pure investors
- Builds equity in an asset while paying less than equivalent market rent
- Creates hands-on property management experience before scaling to larger portfolios
- Income from tenants is taxable, but deductible expenses offset a meaningful portion
- Proximity to tenants creates personal friction that arm's-length landlords never face
- Vacancy hits harder — losing one tenant on a duplex means losing 100% of rental income
- FHA loans carry mortgage insurance premiums that persist until you refinance, eroding cash flow
- Your unit's fair market rent is an implicit savings number, not cash in hand
- Selling triggers recapture questions on any depreciation claimed on the rental portion
Watch Out
Underestimating PITI on owner-occupant loans. FHA loans add an upfront MIP of 1.75% of the loan amount (often rolled into the loan) plus a monthly MIP of 0.55–0.85% annually. On a $325,000 loan, that is roughly $149–$228 per month on top of principal, interest, taxes, and insurance. Run the full payment, not just principal and interest, before committing to a purchase price.
Rent-ready assumptions. A unit that needs $8,000 in work before it leases is not producing income during your first two months of ownership. Factor pre-lease renovation costs into your year-one cash flow projection, not just your steady-state monthly formula.
Ignoring vacancy between your tenants. A duplex with annual turnover averages about 3–4 weeks of vacancy per unit per year. At $1,575/month, that is $1,181–$1,575 lost annually, or roughly $99–$131 shaved from monthly cash flow when amortized. Model it.
Confusing cash flow with wealth creation. Even if your monthly cash flow is break-even or slightly negative, you may still be winning — if your effective housing cost is below local market rent and you are accruing equity. The formula tells you cash position; it does not capture the full economic picture.
Ask an Investor
The Takeaway
House hack cash flow is the number that tells you whether your investment lives up to the pitch. Run it with real rents, real PITI, and a real maintenance reserve before buying. Add back your unit's fair market rent value to see your true effective housing cost. A duplex that produces $300/month positive cash flow while cutting your housing expense by $1,000 is a better deal than it looks on a standard cap rate analysis. That is the house hack math in action.