Three to six months of expenses. That's the standard answer for how big an emergency fund should be, and as standard answers go, it's a decent one. But it treats every household the same, and households aren't the same. The right size for yours depends on how steady your income is and what would actually go wrong.
Start with the question the fund exists to answer: if the paycheck stopped or the transmission died, how long until real trouble? Two steady earners with separate employers can lean toward the smaller end. Three months, maybe less. One income, commission-based pay, or self-employment pushes you the other way. Six months, sometimes more. The fund isn't a trophy. It's a buffer sized to your specific downside.
Where you keep it matters almost as much as the size. This money has one job, and that job is sitting still until something breaks. A plain savings account you can reach in a day or two is fine. Don't chase yield with it, and don't invest it. An emergency fund that's down 20% the month you need it wasn't an emergency fund.
Now the fair pushback: idle cash is expensive. Every dollar parked in savings is a dollar that isn't paying down debt or growing somewhere else. That's true, and it's why some people eventually restructure how their cash sits so the same dollars can do two jobs at once. The line-of-credit approach at Dynamic Banking is one version of that. But that's a graduate move. Get the boring cash buffer in place first, because the fancier setups all assume your basic footing is solid.
Something to be aware of: don't let the perfect number stall you. Whether your target is three months or six, a fund holding $1,000 beats a plan holding zero. Start where you are, automate a monthly transfer, and let it build. If you want help figuring out your own number, the personal banking basics page is a good next stop.