Every personal-finance article says the same thing: keep three to six months in an emergency fund. Almost nobody can say what the sentence means for them in pounds — three months of what? Salary? Spending? Spending after you’ve cancelled what you’d cancel in a crisis?
Pilots have a cleaner word for this: runway. How long the engine can be off before things get serious. It’s a division sum, it’s honest, and it fits in a spreadsheet smaller than this paragraph.
runway (months) = accessible cash ÷ essential monthly spend
The rest of this guide is making those two numbers true — which is where the “three to six months of salary” folklore quietly falls apart.
Number one: essential monthly spend
Not your income, and not your normal spending — your crisis floor: what one month costs when the income stops and you behave accordingly. Rent or mortgage, utilities, food, transport, insurance, minimum debt payments, the subscriptions you’d genuinely keep.
If you built the four-number budget, this is your Bills number plus a slice of Life — one honest afternoon’s work with SUMIFS over your real transactions. Measured beats estimated: most people’s guess is off by 20% or more, and the direction of the error is never the flattering one.
The salary version of the folklore fails here. Two people on the same £2,800 take-home can have crisis floors of £1,400 and £2,600 — the same fund gives one of them nearly twice the runway. Runway is about your costs, not your worth.
Number two: accessible cash
Money counts toward runway only if it’s reachable in days without penalty or loss: instant-access savings, cash ISAs, the current account surplus. Not the pension (locked), not the stocks & shares ISA (a crash and a job loss like to travel together — ask 2008), not the credit card (that’s borrowing with a countdown, not runway).
The model
Six named cells and you’re done — cash, floor, and:
=cash / floor → runway_months
=target_months * floor → target_fund
=MAX(0, target_fund - cash) → gap
=IF(gap=0, "Funded ✓",
"£" & TEXT(monthly_saving,"#,##0") & "/mo → " &
TEXT(gap/monthly_saving,"0.0") & " months to funded")
Set target_months to taste: three is a floor for steady salaried
work, six if income is variable or one income covers a household,
more if you’re contracting or your industry wobbles. The point of
the model is that changing your mind is a one-cell edit.
Why months beat pounds
A fund measured in pounds is a number with no feelings attached — £6,300 is just… an amount. The same fund expressed as 3.75 months of my actual life is information your gut can use. It changes both directions, too:
- Building: every £250 saved visibly buys you days. The bar moves. Progress you can see is progress that continues — the same psychology as the compounding gap chart.
- Spending cuts: trimming the crisis floor by £100 doesn’t just save £100 — it makes every pound already saved cover more time. The denominator is the quiet lever, and it’s the same lever that shortens the pension countdown. A cheaper life needs less rescue and less retirement. One fact, two payoffs.
When (not) to spend it
Last discipline, worth a cell comment in the sheet: the fund is for involuntary, income-shaped events — job loss, the boiler, the clutch, the emergency flight home. It is not for sales, weddings, or “it’s technically an emergency that the holiday is this cheap”. Spend it on a real one without guilt (that’s the job); refill it before resuming any other saving.
Runway first, then investing. An engine-off glide you’ve already measured is what makes every riskier, longer-term number — the ISA, the pension model, all of it — survivable when real life shows up.
Two numbers, one division, months not pounds. Folklore retired.