The 50% rule says half of gross rent will be eaten by operating costs (not counting the mortgage). It is a napkin from a calmer insurance market. On a lot of 1990s ranches in 2026, the napkin is optimistic by a few hundred dollars a month — which is the entire cash-flow thesis.
This page replaces the slogan with a stack you can argue with. Pair it with the underwriting guide and the Deal Analyzer (remember: the on-site tools still use a 1.5% plug until you override the story in your own sheet).
A property the 50% rule likes and operations do not
Purchase $219,000. Market rent $1,750. 50% rule operating costs: $875. P&I at 20% down, 6.9%, 30-year: about $1,153. Implied cash flow: $1,750 − $875 − $1,153 = −$278 already, so this one never should have been a “yes” on the napkin. Flip the rent to $2,100 on a “value-add” story and the napkin suddenly prints +$72. That is how people buy a paint job.
Rebuild it:
| Line | 50% rule | Document-based |
|---|---|---|
| Taxes (post-sale estimate) | buried in the 50% | $240 |
| Insurance (quoted) | buried | $195 |
| HOA / city landlord fee | ignored | $45 |
| Owner-paid water/trash | buried | $70 |
| Vacancy (8%, this street) | buried | $168 |
| Turnover (1 month every 3 years, smoothed) | buried | $58 |
| Repairs / small capex reserve | buried | $140 |
| Property management (if you will not self-manage forever) | often forgotten | $168 (8%) |
| Operating total | $1,050 at 50% of $2,100 | $1,084 |
The totals look similar until you notice the 50% version assumed $2,100 rent you have not leased, omitted management because “I’ll do it myself,” and still has no roof. Self-management is a job. Price the job or admit you are buying employment.
Cash flow on quoted insurance and honest vacancy, $1,750 actual rent, no management: $1,750 − ($240+$195+$45+$70+$140+$140 vacancy) − $1,153 ≈ −$233. The value-add rent does not exist yet. The expense stack does.
Four lines that moved more than cap rate since 2022
- Insurance: non-renewals and wind/hail deductibles. A $5,000 deductible is a capex line pretending to be a policy.
- Taxes after a sale: the seller’s escrow statement is a historical document.
- Utilities you thought tenants paid: master meters, well equipment, lawn in HOA-mandated yards.
- Turnover: a $1,750 unit that needs $2,400 in paint and a week of vacancy every other year is $100+/month you never put in the 50%.
Monthly operating worksheet (copy this)
- Gross rent you can lease in 30 days: ______
- Vacancy % from that street’s days vacant, not a national 5%: ______ → dollar ______
- Taxes (post-sale): ______
- Insurance (quoted, including wind/flood if required): ______
- HOA / licenses: ______
- Owner utilities: ______
- Landscaping / snow if you promised it: ______
- Repairs (not capex): ______
- Capex reserve (roof, HVAC, plumbing, flooring, aged in): ______
- Management or the hours you will not invoice yourself for: ______
- Debt service (P&I + any impounds not already above): ______
NOI is 1 − (3 through 10), annualized. Cash flow is NOI − annual debt service. If you cannot fill lines 3–6 from documents, you are still in listing-photo phase.
What the on-site calculators are doing
The mortgage and deal tools subtract about 1.5% of purchase price per year as a blended stand-in, plus the mortgage. That will look “close” on a boring Midwest ranch with cheap taxes and a five-year-old roof. It will lie in Florida, coastal Texas, older Northeast cities with high millage, and anywhere an HOA can levy a special assessment. Treat the tool output as a first pass, then replace the plug with this worksheet. The mistake detector is most useful when you feed it the unflattering vacancy and capex, not the defaults that make the grade look like a B.
If you want one habit: never present a cash-flow number to yourself without listing the insurance quote date and the tax-assumption method next to it. Numbers without those two footnotes are marketing.