Size it on essentials, not lifestyle
The target is six months of what it costs to survive: housing, food, energy, transport, minimum debt payments. Not gym, streaming or eating out; in a real emergency those pause. For many people this cuts the target nearly in half, and a target you believe in is one you fund.
Compute your essentials once, honestly. That number times six is the goal. Times three is the first respectable base camp.
Make the deposit automatic and the amount honest
A standing order the day after payday, sized so you never notice it enough to cancel it. £50 a month you sustain for three years beats £300 a month you abandon in February. If money is genuinely tight, start with £10; the habit is the asset, and the amount can grow with every payrise (give the fund half of each raise before your lifestyle sees it).
Windfalls (tax refunds, bonuses, sold clutter) go straight in until base camp. This is the one account where boring lump sums are welcome.
Where to keep it (UK)
Easy-access savings account, paying a real rate, at a different bank from your current account: reachable in a day, invisible day to day. Most basic-rate taxpayers pay no tax on the first £1,000 of interest (£500 for higher-rate) thanks to the personal savings allowance, so a cash ISA is usually unnecessary for this job, especially with the cash ISA limit falling to £12,000 for under-65s from April 2027.
Never invest the emergency fund. Its job is to be there on the worst day, which is precisely the day markets are usually down.
The milestones that keep you going
£500: most small disasters die here, and the fund has already changed your relationship with a bad week. One month of essentials: breathing room. Three months: genuine resilience; a job loss is a project, not a catastrophe. Six months: freedom, the quiet kind that changes what you can say yes and no to.
When it is funded, stop. Redirect the standing order to investing, and only top the fund back up after it does its job.