Ask how much cash to hold per rental and you get three different answers, all stated as gospel. One camp says $5,000 a door. Another says three to six months of operating expenses. A third says 10% of rent set aside every month. They cannot all be right, because they are measuring different things, and none of them looks at the one property you actually own. A reserve is not a vibe. It is a number you can derive from the roof over the tenant's head.
The honest way to size a reserve is to build it from parts: a capital reserve for the big systems that wear out on a schedule, a vacancy reserve for the months a unit sits empty between tenants, and a cash cushion for your insurance deductible when something breaks all at once. Add those three, and the rules of thumb stop competing and start agreeing, because each one turns out to be a rough proxy for a piece of the same total. Below is how to derive your own per-door number, then reconcile it against the shortcuts so you know whether yours is high, low, or about right.
Reserves are three separate buckets, not one
Lumping all your set-aside cash into a single “reserve” hides what it is for, which is how landlords end up either over-saving or getting caught short. Split it into three buckets and each one has a clear job and a clear way to size it. The capital reserve replaces things that wear out. The vacancy reserve covers the gap between tenants. The deductible cushion covers the day a pipe bursts and your landlord insurance makes you pay the first few thousand dollars before it pays anything.
- Capital reserve. Money for the roof, the furnace, the water heater, the appliances, and the flooring, sized from how much useful life each one has left. This is the bucket the $5,000-a-door rule is really trying to approximate.
- Vacancy reserve. One to two months of rent per unit, so a turnover does not force you to cover the mortgage out of your own paycheck. This is what the “months of expenses” rule is reaching for.
- Deductible cushion. Enough to pay your insurance deductible outright, typically $1,000 to $5,000, plus a little for the small repairs that never rise to a claim.
Sizing the capital reserve from component life
The capital reserve is the one most people guess at, and it is the easiest to actually calculate. Every major system has a typical useful life and a replacement cost. Take the cost, divide by the years of life left, and you get the annual reserve that system needs. Sum those across the property and you have a defensible capital reserve, not a borrowed average. The component lifespans worth pricing are exactly the ones you should already be checking on a pre-purchase inspection, which is the moment you learn how much life each system has left.
Say you own a single-family rental. A roof runs roughly $9,000 and lasts about 25 years, so it needs about $360 a year. A furnace runs about $5,000 over a 20-year life, or $250 a year. A water heater is near $1,500 over 10 years, $150 a year. The kitchen appliances together might be $3,000 over 12 years, $250 a year. Flooring of $5,000 over 12 years adds another $417. Add those and the property needs about $1,427 a year flowing into a capital reserve, which is where the “10% of rent” rule comes from: on a $1,400 monthly rent, that is roughly 8.5% of gross rent, in the same neighborhood as the shortcut.
The starting balance matters as much as the annual contribution. If that roof is already 22 years into a 25-year life, you do not have 25 years to save for it; you have 3, which means you need most of its $9,000 on hand now, not later. A young house with a new roof and a recent furnace can run a thinner capital reserve because the clock has reset. This is why two houses at the same rent can need very different reserves, and why a flat per-door figure misleads. Age, not price, drives the number.
A reserve that only funds the annual contribution but ignores how old the systems already are is the most common way landlords get surprised. Reserve for the remaining life, not the full life. An appliance halfway through its run is already half a liability sitting on your books.
The vacancy reserve and the deductible cushion
Vacancy is the cost the capital reserve ignores. Even a well-run single-family rental turns over every few years, and each turnover means lost rent plus make-ready costs. Holding one to two months of rent per unit means a normal vacancy is an annoyance, not an emergency. If you want to size this precisely instead of by rule, your local vacancy rate and your own turnover history tell you how often the gap actually shows up and how long it lasts.
The deductible cushion is the smallest bucket and the one most people forget. A claim does not pay until you have covered your deductible, and on a landlord policy that deductible is often $1,000 to $5,000. When a tree comes through the roof, you write that check first. Hold it in cash so an insured loss never turns into a financing problem on top of a repair problem.
Reconciling the three rules of thumb
Put the buckets together and the competing rules of thumb stop fighting. Take that single-family example: a capital reserve building toward roughly $5,000 to $9,000 of imminent replacements, a vacancy reserve of two months at $1,400 (so $2,800), and a $2,000 deductible cushion. That totals somewhere between $9,800 and $13,800 once the capital bucket is funded, which is why “$5,000 per door” reads as a floor rather than a target. The $5,000 covers the deductible and a thin vacancy buffer, but not aging systems.
- $5,000 per door roughly matches the deductible cushion plus a partial vacancy reserve. It works for a newer property with young systems and breaks on an old one.
- Three to six months of expenses is a different cut at the vacancy and operating side. It is most useful as a survival test: could you carry the property through a long vacancy and a bad repair month with no rent coming in?
- 10% of rent set aside monthly is the capital reserve expressed as a flow instead of a balance. It funds the bucket over time but tells you nothing about whether you have enough on hand today for the roof that is already due.
Each rule captures one bucket and misses the other two. The reason to derive your own total is that you stop arguing about which shortcut is right and start holding a number that fits your actual roof, your actual rent, and your actual policy. Reserves are also one of the quiet defenses that keep a rental property portfolio from unraveling the first time two units turn over in the same quarter.
Reserves, lenders, and where to keep the cash
The reserve you hold to run the property is not the same as the reserve a lender wants to see at application. Underwriters run their own test, often expressed as a number of months of principal, interest, taxes, and insurance per financed property, and those are Fannie Mae and lender underwriting guidelines that shift over time, so do not treat any specific month count as permanent; confirm the current requirement with your loan officer when you apply. The same savings can satisfy both, but the operating reserve exists whether or not a lender ever asks. Keep the money liquid in a high-yield savings or money-market account, separate from personal spending, and never in anything you would have to sell at a loss to fix a furnace in winter.
Sizing all of this well depends on knowing your real repair spend and your real vacancy, which is exactly what a guess cannot give you. This is where rents.ai helps: its per-property finances and NOI analytics show your actual expense history and occupancy, so next year's reserve target comes from your own numbers instead of forum folklore. It will not pull component lifespans or price a roof for you, though; that estimating work, and the inspection that feeds it, is still yours. Before you ever record a transaction, run the deal first: the full underwriting walkthrough and the cash-on-cash calculator tell you whether a property can carry its own reserves before you sign. A reserve you derived is a reserve you can defend at 2 a.m. when the phone rings.