House Hacking Calculator: Run Your Numbers Before You Buy

Introduction

A house hack is not judged the way a rental is judged. A rental has to pay you. A house hack only has to cost you less than the rent you are already paying — a much lower bar, which is exactly why the maths gets wrong in both directions.

TL;DR: The number that decides a house hack is net monthly housing cost: PITI plus maintenance, minus what the other units bring in. Compare that against your current rent, not against a cap rate. Then check the two things most calculators skip — how far rent can fall before the deal stops working, and what the property cash flows after you move out and rent your own unit.

Why the usual comparison is wrong

The instinct is to judge a house hack against a rental property, using cap rate and cash-on-cash. That comparison does not hold, because you were going to pay for housing either way. A rental has to generate a return on capital. A house hack only has to beat your lease.

That is why a house hack that "loses money" as a rental can still be a good decision. If your net housing cost is $900 and you were paying $1,900 in rent, the property is $1,000 a month better than your alternative — even though no underwriter would call it cash-flowing.

The reverse trap is more common and more expensive: a duplex where rent nearly covers the mortgage looks like free housing, right up until taxes, insurance, vacancy and maintenance are added back.

Net housing cost = PITI + maintenance − rent collected after vacancy

What belongs in the payment line

Most beginner house-hack maths understates this line. The monthly payment should carry principal, interest, taxes, insurance and any HOA dues — plus any utilities that stay in the owner's name, which in older small multifamily is often water and trash.

A cheap duplex stops being cheap the moment those are layered back in. The calculator above keeps taxes and insurance as annual figures because that is how they are quoted, and divides them into the monthly view for you.

A worked example

A $425,000 duplex, FHA at 3.5% down, 6.5% over 30 years. You live in one side and rent the other for $1,650.

Line	Amount
Loan amount	$410,125
Principal & interest	$2,592
Taxes & insurance	$633
Total PITI	$3,226
Rent collected, after 5% vacancy	$1,568
Maintenance & capex, 10% of rent	$165
Net housing cost	$1,823

Against $1,900 of current rent that is a saving of roughly $77 a month — far less than the "rent nearly covers the mortgage" framing suggests, because rent covers about 49% of PITI, not all of it.

Worth sitting with that number. A $77 monthly edge is not compensation for becoming a landlord, and this particular deal does not improve when you leave: rent both sides at $1,650 and the property still runs about $421 a month negative, because two units at this rent cannot carry a 3.5%-down payment at 6.5%.

So the example is not a marginal deal that needs a nudge. It is a property whose rent-to-price ratio is too low to work at these terms, and the calculator's job is to surface that before you write an offer rather than after.

The two numbers that actually decide it

Break-even rent. Not "does this work at today's rent" but "how far can rent fall before it stops working". That is the question you can still act on while negotiating, and it tells you whether you are buying a deal or a coin flip. In the example above the rented side only covers PITI at about $3,795 a month — well over double market rent — which says plainly that this property never carries itself on one unit.

Cash flow after you move out. Owner-occupancy requirements are usually a year. After that the property is a pure rental carrying financing an investor could not have obtained. That is the asset you actually keep, and it is frequently the entire case for the deal — though as the example shows, not always. When this number is still negative with every unit rented, the house hack is buying you a discount on housing and nothing more, and it should be judged on that alone.

Where this calculator is deliberately simple

It does not model principal paydown, appreciation, or the tax treatment of the rented portion. All three favour the house hack, so the number above is conservative by construction.

It also assumes one rent across every unit. If you are renting three bedrooms at different rates, use the average — the break-even output still holds, because it moves with the total rather than the mix.

Finally, it says nothing about whether you want to live there. That is not a modelling gap; it is the honest limit of a spreadsheet. The occupancy requirement is a year of your life, and a deal you leave early is a deal that did not work.

How this fits the wider financing picture

FHA is why house-hack maths is appealing in the first place — the CFPB notes FHA loans still allow down payments as low as 3.5%, which changes the entry cost dramatically against a 20–25% investor loan. But a low down payment on weak monthly economics is still a weak deal, and this calculator is built to make that visible rather than hide it.

For the full financing framework, see House Hacking with an FHA Loan. If you are treating this as the first step toward a portfolio, What Is a DSCR Loan? covers how the second purchase gets financed once owner-occupancy is gone, and LLC for Rental Property covers the structure question that follows.

To underwrite the same property as a straight rental once you have moved out, run it through the rental property ROI calculator.

FAQ

What is a good net housing cost for a house hack?

There is no absolute threshold, because the benchmark is your own alternative. A useful result is one where net housing cost sits meaningfully below what you pay in rent today and still leaves room to fund reserves. A $1,400 net cost is excellent against $2,200 of rent and poor against $1,100.

Should a house hacking calculator include vacancy?

Yes. A calculator that omits vacancy and maintenance is comparing gross rent against a mortgage payment, which is the single most common reason first-time house hacks underperform their spreadsheet. Five percent vacancy and ten percent maintenance are conservative starting points, not padding.

Is the lowest owner cost always the best house hack?

No. Livability decides whether you complete the occupancy period, and a property you leave in month seven forfeits the financing advantage that justified buying it. Property quality and exit options matter alongside the monthly number.

How much rent do I need for the deal to break even?

That is the break-even rent output above. It solves for the rent at which the rented units exactly cover PITI, after vacancy and maintenance. If it sits far above market rent, the property will never carry itself on the rented side alone — common, and not automatically disqualifying, as long as you know it going in.

Does house hacking still work with only one room rented?

It can, but the margin is thin, because one room rarely covers much of a whole-house payment. Set units to 1 and rent to the room rate: if net housing cost still beats your current rent, the arrangement works. The break-even output will usually show that a single room cannot cover PITI on its own.

What happens to the numbers when I move out?

The unit you occupied becomes rentable, so gross rent rises by roughly one unit while the payment stays fixed. The "cash flow once you move out" output models exactly that. For many house hacks this is where the deal turns positive, and it is the reason to weigh one year of occupancy against the years that follow.

Conclusion

Run the property at today's rent, then run it again 10% lower. If both results still beat your lease and you can fund reserves without draining your liquidity, the deal is doing what a house hack is supposed to do — buying an asset at owner-occupant terms while your housing cost goes down rather than up.

Sources

• CFPB FHA Loans Page
• HUD FHA INFO 2025 Forward Mortgage Limits