Its usually recommended to have 3-6 months of expenses in the event of income loss, but its only a rough guideline. In some industries or economic scenarios, you may go almost 12-24 months with a job or significant income (I leave out part time jobs or gig for keeping this simple).
In short, we don't save enough in America (for a variety of reasons), and we don't have a proper social safety net for when that comes back to bite us.
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Have 6-12 months of expenses saved in your emergency fund. This is critical. When possible max out your retirement accounts (a Roth IRA can be used as an emergency fund in a pinch; contributions can be removed at any time penalty fee, but this should be a last resort). Save more. Live below your means.
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https://www.reddit.com/r/personalfinance/wiki/commontopics
Infographic: https://i.imgur.com/fb7Dtmh.png
This implies setting your mortgage repayment rate at a level which leaves you with sufficient income to overpay it. Having set things up this way, you can then size your emergency fund on the assumption that you won't have to pay your mortgage out of it.
Why do this? Simply put, the savings on interest are massive and this improves your financial stability a great deal over time.
Why not do this? It requires careful and diligent financial planning, and that sort of person probably doesn't need telling how to take advantage of the subtleties of mortgages.
An investment property isn't liquid, and it's also leveraged.
If you're overleveraged, you'll make extra profits during the inflationary boom, but you'll get wiped out during the next recession.
It also depends on rent and your lifestyle.
I'm able to save 25%-50%+, but I live cheaply.
I'm luckier than the author since I have folks to crash with (relatives) and can borrow some of the items needed to get a new job (computers, car, etc). I have also been on the other end by letting a friend have a room while he got back on his feet.
From what I'm reading, you need to be making about 400k (household income) to make it to the 1%.