Most investors treat their buy box (price range, markets, property types) as a fixed identity: "I buy sub-$200K duplexes in the Midwest." The math disagrees. Your buy box is a function of the mortgage rate, and when the rate moves, the box moves, whether you redraw it or not.
The mechanics: what one point costs
On a $120,000 investor loan (a $160,000 property with 25 percent down, 30-year amortization), the monthly principal and interest looks like this:
| Rate | Monthly P&I | vs. 6.5% |
|---|---|---|
| 5.0% | about $644 | +$114/mo cash flow |
| 6.5% | about $758 | baseline |
| 8.0% | about $881 | -$123/mo cash flow |
A three-point swing moves monthly cash flow by roughly $237 on this one modest loan. For context, a solid cash-flow deal at this price point nets $250 to $400 per month. The rate environment is not background scenery; it is a co-author of every deal you underwrite.
Nothing about the house changed. The tenant pays the same rent, the county charges the same taxes. Yet the identical property is a good deal at 5 percent, marginal at 6.5, and dead at 8.
Three things to re-derive when rates move
1. Your maximum purchase price. Work backward from cash flow instead of forward from price. Decide your minimum acceptable monthly cash flow, then solve for the price where the deal still clears it at today's rate. When rates rise one point, that break-even price drops meaningfully; offers that were rational last quarter are overpays now. The reverse is also true, which is why buyers who re-derive quickly in falling-rate windows beat the crowd to repricing.
2. Your markets. Rising rates thin the herd unevenly. A market whose typical deal cleared $300 per month has cushion; a market that averaged $120 goes underwater entirely. Higher rates concentrate the viable map into the strongest cash-flow markets, and falling rates expand it again. The comparison of your borrowing rate against market cap rates (explained here) tells you which side of the line each market sits on.
3. Your property types. Because two rents carry one loan, duplexes and small multifamily absorb a rate increase better than single-family homes at the same price. Investors who were single-family-only at 5 percent often find that at 7 percent only the multifamily math still works.
Underwrite with the live rate, not a vibe
Two practical rules:
- Use the investor rate, not the headline. Investment-property mortgages price above the owner-occupant averages you see in the news, commonly by around half a point. Underwriting at the headline rate flatters every deal in your pipeline by the same silent margin.
- Refresh the rate every time you underwrite. A spreadsheet with a hardcoded rate from three months ago is quietly mispricing everything. PadSweep pulls the current 30-year average with an investor premium on every run (and lets you override it with your actual quoted rate), so every deal is measured against the money you would really borrow.
The upside of doing this well
Rate discipline is not just defense. Most of the market underwrites with stale assumptions, in both directions. When rates rise, they overpay; when rates fall, they are slow to see that yesterday's rejects now cash flow. An investor who re-runs the full math (the checklist is here) on the whole market at the current rate sees the repriced deals first.
Re-running an entire market's math after every rate move is exactly the kind of chore software should do. PadSweep re-underwrites every listing with the live rate on every search, so your buy box redraws itself. Try it on your market.