The formula
target = monthly expenses × months you want covered
Where it comes from
An emergency fund is measured in months of expenses, not in a round figure, because what it has to cover is time without income. Three months is the usual floor and six is the common target; anyone self-employed or on unstable income needs more.
How to work it out by hand
- Add up what you actually spend in a month, not what you earn
- Multiply by the number of months you want covered
- Subtract what you already have set aside
- Divide the gap by what you can save each month
What is worth knowing
The number to use is expenses, not income: what the fund has to replace is your outgoings while there is nothing coming in. Two things people get wrong. They size it on their salary, which makes the target far larger than it needs to be, and they keep it somewhere it cannot be reached in a day. An emergency fund is not an investment: its job is to be there instantly and intact, so it belongs in an instant-access account, not in the market. Losing 20% of it in a crash at the exact moment you lose your job defeats the point.