An emergency fund is money that exists so that a bad month does not become a bad year. It sits in a high-yield savings account, untouched, ready to cover a job loss, a medical bill, a car repair, or a broken furnace without borrowing or selling investments at a bad time.
The usual target is three to six months of essential expenses, not income. Essentials are rent or mortgage, utilities, groceries, insurance, minimum debt payments, and transportation. If those add up to $4,500 a month, the target is $13,500 to $27,000. People with variable income, a single earner supporting a household, or a job in a shaky industry lean toward six months or more. Two earners in stable fields can lean toward three.
For someone whose pay includes bonuses and stock, the fund does double duty: it also smooths the months between lumps, so a delayed vest or a small bonus year does not force a scramble. Some people hold their bills-only fund in savings and count a Roth IRA's contributions as a second-tier backstop.
The fund should be boring. It is not an investment, so it does not belong in stocks; a 30% market drop is often the same moment a layoff arrives. A savings account paying around 4% APY keeps it roughly level with inflation and available in a day.
The common mistakes are skipping it to invest faster, then selling investments at a loss when something breaks, and letting it drift into everyday spending because it sits in the same account as checking.