Real estate investing is a math problem with five canonical formulas — the 1% rule, 50% rule, cap rate, cash-on-cash return, and the 70% rule for flips. Every successful investor uses them; most retail investors misapply them. The difference between "this deal looks good on Zillow" and "this deal will produce 12% cash-on-cash with a defensible margin of safety" is whether you've run the numbers honestly.
What follows is what we call the Five-Formula Real Estate Screening Framework: how each formula works, what it actually tells you, where it breaks down, and how they combine into a decision that holds up after closing. We use a single worked example — a $300,000 single-family rental — throughout, so the same numbers flow through every formula and you can see how a deal evolves from screen to underwriting to purchase decision.
Formula 1: The 1% Rule (Initial Screen, Not Final Answer)
The 1% rule asks: does monthly rent equal at least 1% of purchase price? A $300K property should rent for $3,000/month minimum. The rule isn't a quality threshold — it's a noise filter. Properties that fail 1% almost never cash-flow at acceptable rates; properties that pass 1% deserve deeper analysis.
The rule has been declining in usefulness as property prices have outpaced rents in many markets. In high-cost markets (coastal cities, urban California), 1% properties no longer exist outside genuinely distressed neighborhoods. In Midwest and Southeast markets, 1% remains achievable. The framework now is: 1% in low-cost markets, 0.7-0.8% in mid-tier markets, 0.5-0.6% in high-cost markets — adjusted to the local rent-to-price ratio.
Our example: $2,400/month rent ÷ $300,000 = 0.8%. Below 1% strict, but in line with mid-tier market expectations. Proceed to next formula.
Formula 2: The 50% Rule (Operating Expense Reality Check)
The 50% rule asserts that operating expenses (everything except mortgage payment) consume roughly 50% of gross rental income across a long enough timeline. Property tax, insurance, vacancy, maintenance, repairs, capital expenditures, property management — they average out to half of rent.
Newer investors consistently underestimate operating expenses because they think in maintenance terms (repairs as they happen) rather than capital-expenditure terms (the roof replacement that will happen in year 18 and costs $15,000). The 50% rule forces honest accounting by combining everything into a single proxy.
| Expense category | % of gross rent | Example ($2,400 rent) |
|---|---|---|
| Property tax | 10-15% | $300/month |
| Insurance | 3-5% | $90/month |
| Vacancy reserve | 5-8% | $150/month |
| Repairs & maintenance | 5-10% | $180/month |
| Capital expenditures | 5-10% | $180/month |
| Property management | 8-10% (if used) | $240/month |
| Total | 40-58% | ~$1,140/month |
The 50% rule gives us: $2,400 - 50% = $1,200/month net operating income (NOI) before mortgage. That number drives the next formula.
Formula 3: Cap Rate (The Unleveraged Return)
Cap rate is the rate of return on a property if you bought it cash. It strips out financing and tells you how the property itself performs.
Our example: $1,200/month × 12 = $14,400 annual NOI. $14,400 ÷ $300,000 = 4.8% cap rate. That's typical for residential single-family in a stable market. Commercial properties target 6-10% caps; multifamily 5-8%; high-quality residential rentals 4-6%.
Cap rate is the right framework for comparing properties on a like-for-like basis. It's the wrong framework for comparing real estate to stocks or other investments because it omits leverage. A 5% cap rate on a leveraged real estate investment can produce 12-20% cash-on-cash returns — which is where the next formula matters.
Formula 4: Cash-on-Cash Return (The Leveraged Reality)
Cash-on-cash measures return on your actual cash invested — usually the down payment plus closing costs, not the full purchase price. With 25% down on our $300K property:
- Down payment: $75,000
- Closing costs: ~$6,000
- Initial rehab: ~$5,000
- Total cash invested: $86,000
Mortgage payment on $225,000 at 7% over 30 years: ~$1,497/month principal and interest. Adding tax and insurance escrow brings the total PITI to roughly $1,890/month.
Monthly cash flow: $2,400 rent - $1,140 operating expenses - $1,497 P&I (ex-PITI escrow already in operating) = -$237/month. The property loses $237/month.
That's the moment where most retail investors look up "house hacking" or "force appreciation." For a buy-and-hold investor in 2026, that's the moment to walk away. Negative cash flow is acceptable only when you have a specific catalyst (significant appreciation potential, rent growth coming, value-add play in progress) — none of which apply to a stabilized rental property in a normal market.
A property has to clear positive cash flow on day one to fit conservative buy-and-hold criteria. Our example doesn't, which means either purchase price has to drop or rent has to rise before this becomes a deal. That's the screening framework working correctly.
Formula 5: The 70% Rule (Fix-and-Flip Math)
Fix-and-flip investing uses different math. The 70% rule says: maximum purchase price = 70% of after-repair value (ARV) minus repair costs.
Example: ARV $300,000, repair estimate $40,000. Max offer = ($300,000 × 0.70) - $40,000 = $170,000. The 30% spread covers selling costs, holding costs, financing costs, and the flipper's profit margin (typically 8-15% of ARV).
The 70% rule is widely cited and widely misunderstood. Two failure modes drive most flip losses:
- Optimistic ARV estimates. "Comps support $320K" turns into actual sale at $295K when the market shifts during the rehab. The rule has to use conservative comps, not the top of the range.
- Optimistic repair estimates. New investors consistently underestimate by 25-50%. The rule has to use repair estimates that include a 20% contingency for the surprises every flip discovers.
Honest application: discount the ARV to the 25th percentile of comps, inflate repair estimates by 25%, and the 70% rule then produces deals that actually work.
The BRRRR Strategy (Putting It All Together)
BRRRR — Buy, Rehab, Rent, Refinance, Repeat — is the strategy that uses all five formulas in sequence. The goal: recycle invested capital so each new property uses minimal new cash.
Buy
Purchase below market value, typically through distressed sale, off-market deal, or value-add opportunity. Use the 70% rule for screening: buy at 70-75% of ARV minus rehab.
Rehab
Forced appreciation through renovation. Critical: rehab must increase appraised value by at least the rehab cost plus 15-20% to make BRRRR work mathematically. Cosmetic upgrades (paint, flooring, kitchen, bath) typically deliver this; structural work (roof, foundation, HVAC) typically doesn't.
Rent
Stabilize the property with quality tenants. The cap rate of the stabilized property determines refinance loan amount, so rent maximization is critical here. Document rent history thoroughly for the appraiser.
Refinance
After 6-12 months of seasoning (varies by lender), refinance based on the new appraised value. If ARV is $300K and you can refinance at 75% LTV, the new loan is $225K — which may exceed your original purchase plus rehab cost, returning your full initial capital.
Repeat
With original capital recovered, repeat the cycle on a new property. The strategy compounds when it works; it fails when one of the steps doesn't perform (rehab over-runs, appraisal lower than expected, refinance rate higher than projected).
Where These Rules Break Down
The five-formula framework assumes a relatively normal market. Three conditions break it:
- Rapid appreciation markets. When prices rise 15%/year, cap rates compress and cash-flow disappears. The math still works; it's just that buy-and-hold isn't the right strategy. Appreciation plays use different formulas (price-to-rent ratios, expected appreciation modeling).
- Distressed markets. When prices fall, even "good deals" lose money. The math doesn't fail; the assumption that prices stabilize fails. Risk buffers have to be larger.
- Specialty property classes. Short-term rentals, multifamily, mobile home parks, commercial — each has its own conventions that supersede or modify these residential rules. The framework still applies, but the % thresholds differ.
Common Mistakes That Kill Deals
- Ignoring property management. If you self-manage, you're an unpaid employee. Either pay yourself (and account for it in operating expenses) or factor 8-10% in costs even if you'll self-manage initially.
- Underestimating capital expenditures. Roofs, HVAC, water heaters, electrical, foundations all have predictable replacement cycles. Reserve for them in operating expenses; otherwise year-15 surprises eat all your profit.
- Counting appreciation as income. Appreciation is real but unrealized and unpredictable. Run the numbers as if the property never appreciates — if the deal works on cash flow alone, appreciation is bonus.
- Confusing cap rate with returns. Cap rate is the unleveraged metric for comparing properties. It's not what you earn. Cash-on-cash is.
- Skipping the inspection. Inspections cost $400-800. A skipped inspection costs $30,000+ when the foundation issue surfaces. Always inspect.
Run the Numbers
The five formulas above are the screening framework. Apply them honestly and the bad deals filter themselves out:
- Cap Rate Calculator — quick unleveraged-return analysis.
- Cash-on-Cash Return Calculator — leveraged-return reality check.
- BRRRR Calculator — full BRRRR cycle math including refinance amount.
- Rental Property ROI Calculator — comprehensive holding-period analysis.
- Mortgage Calculator — for the financing leg.
- Refinance Break-Even Calculator — when the rate-and-cost trade actually works.
Real estate investing rewards discipline. The math gives you the discipline; the harder skill is walking away from deals that don't pencil out. Most retail investor losses come from forcing a yes on a no deal because the screening framework was applied too generously. Apply it strictly and the framework protects you from yourself.