Add up what you cannot skip, choose the months of it to hold, count the saving that takes, and see what the cash earns and costs.
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Target = monthly essentials × months of coverage. Gap = target − already saved, floored at zero. Months to fund compounds the balance at APY ÷ 12 and adds the set-aside at month end, stopping at the target or beyond 1,000 periods. Yearly interest is target × APY ÷ 100, an approximation: the balance only reaches the target partway through the year.
$1,200 housing, $400 food, $300 transport, $300 insurance and $300 of minimum debt payments is $2,500 a month, so six months is a $15,000 target. With $8,000 saved the gap is $7,000; at $500 a month on 4% APY the fund is there in 14 months, reaching $15,535.24 with $535.24 of interest on the way. That $15,000 at 4% earns about $600 a year.
On an 18% APR card the same $15,000 would save $2,700 a year, so holding the full fund there costs about $2,100 a year net — arithmetic, not a recommendation.
It assumes a constant APY, a steady set-aside and no withdrawals; it cannot see whether your income holds or how long a gap lasts. Minimum payments are essential: missing them turns one emergency into a second.
Informational only — no advice on the order of your saving and debt.