I always had a general "just in case" cushion for my rentals, but it was never really tied to anything specific, just a round number that felt safe.

Then I actually mapped out every major system across my properties by age and expected remaining lifespan, and added up what replacing everything on a realistic timeline would actually cost, spread out year by year. The number that came out was different from my old cushion in an uncomfortable way, some years it was way more than I'd been keeping aside, other years way less.

What changed for me wasn't the total amount saved, it was realizing my old cushion was smoothing over years that are naturally lumpy. A year where three systems age out at once isn't bad luck, it's just math you can see coming if you're tracking it.

How are others sizing their reserve, a flat number that feels safe, or something tied to what's actually aging in the property?