Why flip math is different from buy-and-hold math
Flipping and long-term rental investing both start the same way — buying a property — but the financial engine underneath is completely different. A buy-and-hold investor cares about monthly cash flow and appreciation over years, tracked with tools like a Cap Rate Calculator. A flipper cares about a much tighter window: how much does this specific deal cost from purchase to sale, and how much is left over once every cost — including the ones that only exist because you're moving fast — gets subtracted.
That tighter window is exactly why flip underwriting rewards precision. A rental investor with a slightly-off maintenance estimate has years to absorb the error. A flipper with an off-by-10% rehab budget can watch an entire deal's margin disappear in a single project.
The full formula, beyond the 70% rule
The 70% rule is a fast screening tool, not a full underwriting model. Here's the complete math.
Total Project Cost = Purchase Price + Purchase Closing Costs + Rehab Budget + Holding Costs + Selling Costs
Holding costs
Holding Cost = (Loan Interest + Taxes + Insurance + Utilities) × Hold Months
Loan interest on hard money or renovation loans is typically interest-only for the hold period, unlike a standard amortizing mortgage.
Net profit and ROI
Net Profit = ARV − Selling Costs − Total Project Cost ROI = Net Profit ÷ Total Cash Invested × 100
Selling costs — agent commissions plus closing costs on the sale — commonly run 6-8% of ARV, and are easy to underweight when a deal looks good on paper.
What actually moves your margin
- ARV accuracy. The single most consequential number in the entire deal — and the easiest to get wrong by using aspirational rather than genuinely comparable sales.
- Rehab budget precision. Contractor bids beat per-square-foot rules of thumb, especially on older homes with unknown conditions behind walls.
- Hold period. Every extra month of permitting delays, contractor scheduling, or slow resale adds real, compounding holding cost.
- Financing cost. Hard money rates run meaningfully higher than conventional financing — the tradeoff is speed and flexibility.
- Selling costs. Commonly the largest single expense category after rehab, and the one most often underweighted in a quick mental estimate.
Two flips, one delayed by three months
Both examples share the same $220,000 purchase price and $45,000 rehab budget, targeting a $340,000 ARV.
Example A — on schedule, 6-month hold
Financing and carrying costs stay contained to the planned timeline. Net profit comes to roughly $33,100 after selling costs — about a 39.8% return on cash invested. Worth flagging: the 70% rule would cap the offer at $340,000 × 70% − $45,000 = $193,000, so this $220,000 purchase price is actually about $27,000 above that threshold. The deal still works because the margin and financing costs land favorably — a reminder that the 70% rule is a conservative screen, not a pass/fail test.
Example B — delayed to a 9-month hold
Same purchase, same rehab, same ARV — but permitting delays and a slower resale stretch the hold period by three months. Net profit drops to roughly $26,250 (about 29.2% ROI), even though nothing about the property itself changed — the additional three months of loan interest and carrying costs alone account for the difference.
Model your own timeline, including a delayed scenario, with the House Flip Profit Calculator.
Benefits of underwriting before you bid
✓ What you gain
- A maximum offer price grounded in math, not gut feel
- A stress-tested view of what a delay would cost before it happens
- Clarity on whether hard money terms still leave room for profit
- A number to walk away from if a seller won't come down enough
- A repeatable process for evaluating the next deal faster
✗ What the math can't guarantee
- That comps used for ARV will still be valid by the time you sell
- Hidden structural or systems issues discovered mid-renovation
- How quickly the finished property will actually sell
- Local permitting timelines, which vary widely by jurisdiction
- Shifts in buyer demand between purchase and resale
Practical tips
Treat the 70% rule as a first filter, not a final answer
It's fast and useful for screening dozens of potential deals quickly, but always run the full holding-cost and selling-cost math before making an actual offer.
Get contractor bids before you're under contract, if possible
A rehab budget based on real bids, even preliminary walkthrough estimates, is far more reliable than a generic per-square-foot assumption.
Build in a contingency, then respect it
Add 10-15% to your rehab budget for the unexpected — and don't quietly absorb overruns into a shrinking margin without acknowledging the deal has changed.
Model a slower sale scenario before you commit
If the deal still works with an extra month or two of hold time added, you have real margin for error. If it only works on the fastest possible timeline, that's a fragile deal.
Understand the tax treatment before you count your profit
Flips held under a year are typically taxed at short-term capital gains rates, which can meaningfully reduce what you actually keep — factor this in before comparing a flip's return to a buy-and-hold alternative.
Underwrite your next flip
See net profit, ROI, and a full cost breakdown before you make an offer.
Bottom line
The 70% rule gets a deal onto your shortlist. Full underwriting — accurate ARV, real contractor bids, honest holding-cost assumptions, and a stress-tested timeline — decides whether you actually make an offer. Skipping straight from the rule of thumb to a signed contract is how profitable-looking flips turn into break-even ones. Run the complete numbers with the House Flip Profit Calculator before you bid.
Frequently Asked Questions
What is the 70% rule in house flipping?
The 70% rule suggests paying no more than 70 percent of a property's after-repair value minus estimated repair costs, leaving room for holding costs, selling costs, and profit margin.
What is ARV in real estate?
ARV, or after-repair value, is the estimated market value of a property once planned renovations are complete, based on recent comparable sales of similarly renovated homes in the same area.
What costs are included in flip profit calculations?
Purchase price, closing costs on purchase, rehab budget, holding costs like loan interest, insurance, taxes and utilities during the project, and selling costs including agent commissions and closing costs on sale.
What is a good ROI for a house flip?
Many experienced flippers target a 10 to 15 percent net profit margin on the after-repair value before considering a deal worthwhile, since that margin needs to absorb both planned costs and unplanned ones.
How are holding costs calculated on a flip?
Holding costs are typically estimated as a monthly rate covering loan interest, property taxes, insurance, and utilities, multiplied by the expected number of months the project takes from purchase to sale.
What is hard money and why do flippers use it?
Hard money is short-term, asset-based financing from private lenders, typically carrying higher interest rates than a conventional mortgage but funding much faster and based more on the deal than the borrower's income history, which suits the short timelines of flipping.
How is a flip taxed differently from a long-term rental sale?
Flips held for less than a year are typically taxed at short-term capital gains rates, usually higher than the long-term rates on properties held over a year. Tax treatment depends on individual circumstances and should be confirmed with a tax professional.
How long does a typical house flip take?
Most flips take four to nine months from purchase to sale, depending on the scope of renovation, permitting requirements, and how quickly the finished property sells in the local market.
Should I include my own labor as a cost in the rehab budget?
Many flippers include it even when doing some work themselves, since it makes the deal's true profitability comparable to a scenario where all labor is contracted out, and avoids overstating the margin.
What is a realistic rehab contingency to budget?
Adding 10 to 15 percent on top of contractor bids is a common practice, since older homes in particular tend to reveal hidden issues, such as plumbing or structural problems, once work begins.
How do I estimate ARV accurately?
Use recent sales of genuinely comparable, similarly renovated properties within the same neighborhood, ideally sold within the last three to six months, rather than aspirational comps from a different area or condition level.
What happens if a flip takes longer than planned?
Every extra month adds real holding costs, primarily loan interest, taxes, insurance, and utilities, which directly reduces net profit. This is why realistic timelines matter as much as accurate repair estimates.
Can I flip a house without using my own cash?
Some investors use hard money combined with private capital partners to minimize personal cash invested, but most financing structures still require some cash for the down payment, closing costs, and contingency reserves.
What is the biggest mistake new flippers make?
Overestimating ARV and underestimating both rehab costs and timeline are the most common mistakes, and any one of the three can turn a profitable-looking deal into a break-even or losing one.
How do I calculate my own flip profit before making an offer?
Use the free House Flip Profit Calculator on CalcPro, which totals purchase price, rehab budget, financing costs, holding costs, and selling costs against your target ARV to show net profit and ROI.