Reserves are the cash buffer between a bad month and a forced decision. The furnace that dies in January doesn't care about your cash flow — and the investors who lose properties are rarely the ones with bad deals, but the ones with good deals and no liquidity when two problems arrived together.
How much to hold
A common floor: 3–6 months of full operating costs (mortgage included) per property, PLUS a capital reserve that accrues $250–$400 per unit per year toward big-ticket replacements. Newer buildings and diversified multi-unit cash flow sit at the low end; an older single-family with one tenant and one furnace deserves the high end — one vacancy there is a 100% income outage. Lenders often require 6 months of payments per financed property; treat that as a minimum, not a target.
Fund it like a bill
Reserves fail when they're an intention instead of a transfer. Automate a monthly move — for example 10% of collected rent — into a separate high-yield savings account per portfolio (or per property, if that keeps you honest) until the target is met, then let the capital reserve keep accruing. Separate account matters: money that sits in checking gets spent.
A component view for capital reserves
Rough US replacement costs and lives: roof $8,000–$15,000 every 20–30 years; furnace/AC $4,000–$8,000 every 15–20; water heater $1,200–$2,000 every 10–12; exterior paint/siding and flooring on their own cycles. Divide each component's cost by its remaining life, sum across components, and you have YOUR building's real annual capital accrual — usually more informative than any per-unit rule.
Worked example
A duplex runs $2,350/month all-in costs. Operating reserve target at 4 months: $9,400. Component math: roof 10 years out ($12,000 → $1,200/yr), HVAC 8 years ($6,000 → $750/yr), water heaters ($300/yr), other ($550/yr) ≈ $2,800/year capital accrual, $233/month. When the sewer line fails at $5,200, it's a withdrawal and an invoice — not a credit-card balance or a fire-sale conversation.
When this rule of thumb breaks
Reserves have an opportunity cost, and past a point they're idle capital — a 10-property portfolio doesn't need 10 × 6 months, because vacancies and failures don't correlate perfectly; portfolio-level pooling with a floor per property is more capital-efficient. A large unused HELOC can substitute for part (not all) of a cash reserve — credit lines get frozen in exactly the downturns you need them. And underfunding reserves to "make the deal work" means the deal never worked.