5 Numbers Every New Investor Must Know Before Their First Deal

A reader emailed last week with a question I get often enough that it's worth answering in public: "What numbers should I actually know before my first deal?"

Most first-deal underwriting fails not because the deal was bad. It fails because the underwriter modeled the wrong numbers. Any deal spreadsheet has fifty cells. Most of them are decoration. Five matter most — and right now, in May 2026, the data context shifts which five carry the most weight.

The 30-year mortgage just printed at 6.30% (Freddie Mac PMMS, week of April 30). The FHFA House Price Index is up 1.7% year-over-year — the slowest pace in over a decade. Builder incentives are running at more than double their normalized levels. Each of those facts changes a different number on your spreadsheet.

Here are the five you can't be wrong about.

#1 — The down payment is not 20%

The default mental model says "20% down, conventional financing, investment property." That's one rung on a ladder. There are others — and on a $250,000 duplex, the rungs spread wider than most beginners realize.

• FHA 3.5%, owner-occupied: $8,750 down
• Conventional 5%, owner-occupied: $12,500 down
• Conventional 25%, pure investment (2-4 unit): $62,500 down
• DSCR 25%, pure investment: $62,500 down (skips personal DTI)

Same brick. Same tenants. A 7.1× spread between the lowest and highest down payment. What changes is whether you live in one unit for the first 12 months. Note: Fannie Mae requires 25% down on non-owner-occupied 2-4 unit small multifamily — the familiar "20% down on investment property" applies only to single-family rentals, not duplexes.

What most beginners miss: the house hack on-ramp. If you'd buy this duplex anyway, and you can stomach a year of sharing a wall with a tenant, FHA owner-occupancy lands at $8,750 down vs the $62,500 conventional 25% — roughly $54,000 you keep in your reserves instead of writing it on the closing check. That's not a small detail. That's the difference between buying one duplex and being able to buy a second one in 18 months.

Run BOTH numbers before you decide. Then the down payment becomes a strategy choice, not a default.

#2 — Reserves are the money you keep, not the money you spend

Reserves are the money the lender wants to see in your account AFTER you close. Not used for the deal. Sitting there. Untouched.

The standard formula:

• 6 months PITI — the lender's minimum on most investment products
• 12 months total operating expenses — the industry best-practice floor

For our $250,000 duplex with a $1,640 monthly PITI: lenders want $9,840 minimum, and survival math says closer to $20,000 including operating buffer.

What most beginners get wrong: lumping closing costs and reserves into the same bucket. They're not the same. Closing is the one-time price of acquisition — title, transfer, inspection, lender fees, prepaid taxes and insurance, usually 2-5% of purchase price. Reserves are the ongoing safety net for the months when rent doesn't arrive on time and the water heater doesn't make it through February.

The discipline is simple: never let your reserves drop below 3 months PITI for any deal, no matter how good. Walking away from one deal is cheaper than the alternative on the deal that broke you.

#3 — The bank's math, not yours

The math here works backwards from how most people think about it. You decide what you can afford based on your budget. The bank decides based on something else entirely. Your debt-to-income ratio is what the lender uses to make that call — not your budget, not your gut, not your spreadsheet.

The ceilings:

• Conventional: 43-45% back-end DTI manual underwrite (up to 50% with Fannie Mae's automated underwriting / DU)
• FHA: up to 50%
• DSCR: skips personal DTI entirely — uses property cash flow instead

For an $85,000 W-2 income: $7,083 monthly gross × 45% = $3,187 maximum total monthly debt service. That includes the new mortgage you're about to take on AND every other recurring debt obligation already on your credit report — student loans, auto payment, credit card minimums, child support if applicable.

The most common first-deal mistake is doing this math AFTER finding the property. By then, the existing $480 car payment, $310 student loan, and $120 in credit card minimums have already eaten $910 of your monthly capacity. That leaves $2,277/month for the new mortgage's PITI. At 6.30% with 20% down — and accounting for typical property taxes and insurance — that caps you around a $350,000–$370,000 purchase. Without those existing debts, the same $85,000 income could carry roughly a $500,000 purchase. The car loan and the student loan are eating about $140,000 of buying power.

The fix: pull your credit, sum your existing payments, subtract from the ceiling, and shop deals that fit. Verify with a lender before you make a serious offer. Your DTI is the gate; everything else is downstream of that gate opening.

#4 — The cash-flow benchmark vs the S&P 500

Real estate competes with index funds. The question your spreadsheet has to answer: does this deal beat the alternative?

The B-class small multifamily target — the most accessible segment for a first-time investor — has two numbers most operators converge on:

• 6%+ cap rate on the as-purchased basis
• 8-10% cash-on-cash return on the actual money you put in (with $200/door per-month positive cash flow as a sanity check on the per-unit math)

Right now, the buyer leverage is unusual. Lennar's Q1 2026 earnings showed incentives running at 14.1% of sales price — more than double the normalized 4-6% range in healthy markets. Builders are sitting on heavy unsold completed inventory. The market is offering you negotiation room you couldn't dream of in 2022. That leverage matters most when your underwriting is disciplined.

What most beginners get wrong: trusting the seller's pro-forma expense numbers. Sellers want the deal to look good. Their pro-formas almost always understate expenses by 20-30%, especially on maintenance, capex, and vacancy.

The fix: apply the 50% expense rule as a sanity check (taxes + insurance + maintenance + capex + management + vacancy ≈ 50% of gross rent for B-class properties). Then add your own conservative 5-7% vacancy on top. If the deal still hits a 6%+ cap rate after that, it's a real deal. If it only works using the seller's numbers, the deal exists only on the seller's spreadsheet.

#5 — Honest appreciation is 1.7%, not 6%

This is the number most beginner spreadsheets get wrongest, by the widest margin.

FHFA's House Price Index for February 2026, released April 28, came in at 0.0% month-over-month and +1.7% year-over-year. That's the slowest national appreciation in over a decade. In real terms — adjusted for inflation — prices are roughly flat.

Most beginner spreadsheets model 4-6% appreciation. That number comes from the long-run historical average, which dates back to the 1970s and includes the housing boom. It's not what's happening right now.

What this means for your first deal: if your spreadsheet pencils only when you assume 4% appreciation, your deal doesn't actually pencil. The cash-flow numbers from #4 have to carry the deal on their own. Appreciation, if it shows up, is a bonus. It's not the thesis.

The fix: model 1.7% appreciation in your spreadsheet. If the deal still works with cash flow, principal paydown, and minimal appreciation — it works. If it only works at 4-6%, you're betting on regression to the mean. That's a real bet to take, but it's a bet, not a baseline. Beginners shouldn't bet the farm on a bet.

What these five numbers actually do

These five don't make the deal. They tell you whether the deal will keep you alive in month 14, when a tenant stops paying and a major capex hit lands in the same month.

Walking away from a deal because the numbers don't fit isn't failure. It's the discipline that lets you say yes — with confidence — to the right deal when it shows up. And it will show up. Builder incentives are real. Inventory is bifurcating. First-time buyers are 1-in-5 of all home buyers right now (the lowest share since NAR began tracking in 1981) — but that's a moment, and moments end.

If you're trying to underwrite your first deal this spring, run these five numbers BEFORE you fall in love with a listing. If you've been waiting for rates to drop before you start screening — these five tell you why the math doesn't actually require lower rates to work. And if you're tempted to tap your 401(k) to clear the down-payment hurdle — run these five against the deal first, then weigh the trade-off.

These five aren't the only numbers that matter. They're the five you can't be wrong about.