People put real effort into deciding how big their emergency fund should be, then park it in the first account they think of. Which is a shame, because where the money lives matters nearly as much as how much of it there is.
Start with the job description. An emergency fund has exactly one job: to exist, in full, on the day something goes wrong. The boiler dies, the car fails, the job disappears. Whatever the emergency is, the fund has to be there, all of it, available now. Everything about where you keep it follows from that.
The three wrong places
Not in shares. I know, the market usually goes up, and watching savings earn less than an index fund stings a bit. But markets have a talent for dropping at the worst possible moment. Job losses cluster in downturns, which is exactly when your invested fund would be worth less than you put in. An emergency fund that shrinks during emergencies isn't doing its one job.
Not locked in fixed terms. A fixed-term account pays a little more in exchange for your money being unreachable, or reachable only with a penalty. Emergencies don't book ahead. If getting your own money out involves a waiting period or a sheepish phone call, it's not emergency money.
Not mixed into your current account. This one fails quietly. The money is technically there, but it sits in the same pot you spend from, and balances that look healthy get spent. Not on anything dramatic. Just a slightly nicer weekend here, a "we can afford it" there, and a year later the buffer has evaporated without a single emergency happening.
The right place is boring
What's left after those three is the answer: a separate, instant-access savings account paying a decent rate. Boring, and correct. You want three things. Instant access, because emergencies are. A real interest rate, because the fund shouldn't rot while it waits. And a separate account, because a small bit of friction between you and the money keeps it intact. It should take a deliberate transfer to spend it, not a contactless tap.
What the numbers look like
Say your essentials come to $3,700 a month: housing, food, bills, transport, the things that don't stop when your income does. A fund covering 6 months of that is $22,200. Which sounds enormous, and stated as one lump sum, it is.
So don't stare at the lump sum. Say you've got $3,000 saved already. Put it in that instant-access account, set up a $400 automatic transfer each month, and at 4% APY you'll reach $22,200 in about 44 months. The interest is doing some quiet lifting there, and it does more as the balance grows. Under four years to go from "one bad month from trouble" to a fund that covers half a year. On autopilot, once you've set up the transfer.
Your numbers will differ, so run your own. The emergency fund calculator works out your target from your actual essential spending, and the savings goal calculator tells you how long your monthly amount will take to get there, interest included.
Boring is the feature
Nobody brags about their instant-access savings account, and that's rather the point. This is the one part of your finances where dull is the goal. The clever, exciting stuff can happen elsewhere, with money you can afford to strap a rocket to. The emergency fund just sits there, earning its modest interest, waiting for a bad day. And when the bad day comes, it'll be the best account you own.