A range wide enough to fit nobody
Three months of spending, or six. The standard advice spans a factor of two, and it is handed out identically to a tenured employee with a working partner and to a freelancer supporting three children alone. Those two households need very different buffers, and a range that wide barely constrains anything.
So this calculator refuses the flat rule. It starts from three months of essential spending as a floor, then adds cover for the circumstances that actually determine how long an income gap might last and how badly it would hurt. Freelance, commission-based and seasonal income can disappear faster and return more slowly than salaried work, so income insecurity stretches the buffer. Dependants stretch it again, because more people relying on the money means less flexibility to cut during a gap. And a household running on a single income has no second salary to absorb the shock. The output is a number sized to your situation, with the three- and six-month figures shown alongside for reference. Even then, treat it as a starting point rather than a prescription: job market conditions, health, visa status and industry all move the right answer, and no calculator sees any of them.
Essentials, not lifestyle
The most common sizing mistake is measuring against total spending. An emergency fund covers the months when you are not earning, and in those months you would not be spending normally; holidays, restaurants and subscriptions are the first things to go.
Count housing, food, utilities, transport, insurance, childcare, and the minimum payments on any debt. For most households that comes to 55–70% of normal spending, which makes the target substantially smaller and more achievable than the naive calculation. Pinning down your own essentials line is the fiddly part, and the sizing guide goes at it category by category.
Eight months of honest runway
Chloe is a freelance designer with one child and no second income. Her total spending is £2,900 a month, but her essentials are £1,900. A naive "six months of spending" would demand £17,400. This calculator takes three months as the base, then adds for insecurity, a dependant and single income: about 8.3 months of essentials, £15,700.
Similar headline number, arrived at honestly, and look at what happened along the way. Sizing against essentials rather than everything let her carry more months of cover for slightly less money. Eight months of genuine runway beats six months of pretending she would keep her normal lifestyle while unemployed.
Marcus makes the contrast. Salaried in a stable sector, dual income, no dependants, essentials of £1,600: his figure is 4.5 months = £7,200, less than half Chloe's, correctly.
Where the months should live
Instant-access savings, separate from your current account, ideally without a linked card. Accessible in a day, not tempting on a Friday night, and covered by FSCS deposit protection as long as the provider is UK-authorised. Accessibility is a real requirement, not a nicety: a fund locked in a 12-month fixed bond is not an emergency fund, whatever its interest rate says. A mortgage offset account passes the test, since the money stays reachable while reducing interest. A credit line fails it, because the lender can withdraw the facility exactly when conditions worsen, which is precisely when you would need it.
Should the fund be invested instead? No. Its job is to be there in exactly the situations where markets may also be falling, and that correlation is the whole reason to hold cash. Money you will not need for five-plus years belongs in investments; this money does not. The cost of holding cash is real, though, and it caps how big the fund should sensibly be: cash loses to inflation, so an oversized buffer quietly shrinks in real terms. The inflation calculator shows how much, and the Bank of England's inflation calculator makes the erosion concrete with actual CPI history.
High-interest debt reorders the queue as well. Carrying a large buffer while paying 22% on a card usually costs more than the security is worth; most planners suggest one month of essentials first, then attacking the debt, and the debt payoff planner handles that sequencing. Some emergencies should never touch the fund at all, because insurance handles them far more efficiently than cash ever will.
One bookkeeping note to finish. The fund counts towards your net worth, and towards your net worth tracker totals. It does not count towards your FIRE number, because it is not funding retirement; it is defending the plan that will.