Research & data

Where to Find the Numbers Your Calculator Needs

SFRCalc Editorial

Every calculator on this site — or anywhere else — is only as good as what you put into it. A model with a clean formula and a guessed rent number is still a guess. The hard part of underwriting a deal usually isn't the math, it's tracking down inputs you can actually trust: what a unit will really rent for, what it's really worth, what the repairs will really cost, and what a lender will really charge. Here's where each of those numbers actually comes from, and how much to trust any single source.

Rental income

Rent is the input that moves every other number downstream, so it's worth checking against more than one source rather than taking the first estimate you see.

Automated rent estimators (Zillow's Rent Zestimate, Rentometer, RentCast, and similar tools) give a fast starting range based on nearby comparable listings. They're a reasonable first pass, but they can drift 10–20% off in either direction, especially in thinner rental markets with fewer recent comps.
Live listings for comparable units on Zillow, Apartments.com, or local classifieds are more reliable than an automated estimate, because they show what landlords are actually asking right now — though asking rent and the rent a unit eventually signs for aren't always the same.
HUD Fair Market Rents, published annually by county and unit size, are a useful floor reference, particularly relevant if the unit could ever be leased through a housing-assistance program.
A local leasing agent or property manager can usually give you the most current, market-specific number, since they're seeing what actually gets signed on comparable units in that immediate area, not just what's listed.

Pull at least two of these before you plug a rent number into a calculator, and lean toward the more conservative one — overestimating rent is the single easiest way to make a bad deal look good on paper.

Purchase price and after-repair value (ARV)

Automated valuation estimates (Zillow's Zestimate, Redfin Estimate) are a reasonable starting range for a fairly typical property in an active market, and much less reliable for unique properties, rural areas, or anywhere with few recent sales to draw on.
County assessor and recorder records are public and show actual recorded sale prices for nearby properties — slower to update than the market, but a hard data point rather than a model's guess.
A buyer's agent pulling comparable sales from the MLS is generally the most accurate source available, since it reflects closed sales rather than active asking prices, and an agent can adjust for condition and features a spreadsheet can't see.
Paid comp and skip-trace platforms (PropStream and similar) aggregate public records and off-market data for investors who want to run comps in bulk without going through an agent for every deal.

Weight sold comps over active listings and automated estimates — an asking price says what a seller wants, and a Zestimate says what a model guesses; a closed sale says what actually happened.

Repair and rehab costs

This is the input people most often underestimate, and it's also the one automated tools can't give you at all. There's no substitute for someone looking at the actual property.

A licensed contractor walk-through is the standard first step — get at least two or three bids on any significant scope of work, since estimates for the same job can vary widely between contractors.
General cost-per-square-foot guides from sites like Angi or HomeAdvisor are useful for a rough, directional gut-check before you have real bids, but they're regional averages, not a quote for your specific property.
A written scope of work, item by item, keeps bids comparable to each other and makes it obvious later if something was missed.

Whatever the contractor bids add up to, build in a contingency on top — unexpected issues (electrical, plumbing, structural) are the norm on older properties, not the exception.

Financing rates and terms

Freddie Mac's Primary Mortgage Market Survey publishes a weekly national average for conventional 30-year rates — a useful baseline for what "the market rate" roughly is at any given time.
Rate-comparison sites (Bankrate, NerdWallet, and similar) let you see a spread of advertised rates across lenders, though advertised rates assume a strong borrower profile and can move once your actual numbers are underwritten.
Investor-specific rates — DSCR loans, portfolio lenders, hard money for a rehab — run higher than an owner-occupant conventional rate and aren't well captured by consumer rate sites. A mortgage broker who works with investors, or a direct quote from a DSCR or portfolio lender, is the more reliable source here.

Treat any advertised rate as a starting point, not a number to underwrite with. Before you rely on a rate for a real deal, get an actual quote or rate lock tied to your specific loan scenario.

Property taxes

The tax figure on a listing sheet is often the seller's current bill, not what you'll actually owe. In many counties, a sale triggers a reassessment to the new purchase price — which can mean a meaningfully higher tax bill than the one advertised. Check the county assessor or tax collector's website directly, and ask whether that jurisdiction reassesses at time of sale before assuming the listed tax figure will carry over.

Insurance

Insurance cost varies enormously by location, property age, and coverage type, and it's one of the few inputs that genuinely needs a live quote rather than a rule of thumb. Get quotes from at least two or three independent agents, or run the property through a landlord-insurance marketplace, rather than assuming a flat percentage of rent or value — that shortcut can be badly wrong in higher-risk areas (coastal, wildfire, flood zones) where premiums have moved sharply in recent years.

Vacancy and turnover assumptions

The Census Bureau's rental vacancy data, broken out by region, gives a broad benchmark for typical vacancy rates.
Local market reports, often published by commercial brokerages or property managers in a given metro, go deeper on submarket-level trends than a national number can.
Days-on-market for comparable rental listings in the immediate area is often the most direct, hands-on signal — if similar units are sitting for weeks before renting, that tells you more about real local turnover risk than any published average.

HOA dues, utilities, and other fixed costs

HOA or condo association documents (or a resale certificate, where required) show current dues and flag any pending special assessments — check for the latter specifically, since they can land as a large, one-time surprise cost.
The seller's trailing 12 months of utility bills, for any utilities the owner pays, are worth requesting directly rather than estimating.
A home inspection report won't give you a dollar figure, but it will tell you the approximate remaining life of the roof, HVAC, water heater, and other major systems — which is exactly what a CapEx reserve assumption should be based on.

No single source here is reliable enough to underwrite a deal on its own. Cross-check at least two sources for any number that materially changes your return, and when they disagree, use the more conservative figure rather than the one that makes the deal look better.

Once you've got real numbers, run them through the SFR, BRRRR, Flip, or any of the other calculators — every input mentioned here maps directly to a field in one of them.

This article is for general education and isn't financial or investment advice. Sources named here are examples for illustration, not endorsements or guarantees of accuracy — always verify current availability, terms, and data quality for your specific market before relying on them.