Ask a landlord how much a vacant unit costs them and most people say the rent. That's the obvious cost and it's real, but it's not the whole number. There's also the utilities you're still paying with nobody in the unit. The marketing and showings. The turnover cleaning and any repairs you do between tenants that you wouldn't have prioritized with someone living there. Days of your own time, which has a cost even if you don't invoice yourself for it.
Add it up honestly and a unit sitting empty for six weeks usually costs more than one and a half months of rent by the time you count everything. Most landlords never do that math because it's scattered across a bank statement, a receipt folder, and memory.
The number that actually matters isn't "is this unit vacant right now," it's "how much vacancy has cost me this year, and is that trending up or down." That's the number that tells you whether your pricing is off, whether your turnover process is too slow, or whether one particular property is quietly dragging on your returns every single year.
You can't see that trend from a spreadsheet you update when you remember to. You need vacancy tracked as a fact of the portfolio, the same way rent is, so the chart just exists rather than being a project you'd have to sit down and build.
Most landlords underestimate vacancy cost by a wide margin, not because they're bad at math, but because nobody ever showed them the actual total in one place.