Two calculations in one: the emergency fund you should hold based on your monthly expenses, and your FIRE number - the pot needed to live off your investments using the 4% rule.
Emergency fund
£
months
FIRE number
£
Emergency fund target
£12,000
6 months of expenses
Emergency fund£12,000
FIRE number (4% rule, × 25)£750,000
More cautious (3.5% withdrawal)£857,143
The 4% rule (pot = annual spending × 25) is a starting guideline from historic data. Many UK planners use a more cautious 3–3.5% withdrawal rate for a long retirement. Keep your emergency fund separate, in easy-access cash.
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Emergency fund: your financial shock absorber
An emergency fund is easy-access cash that covers essential spending if your income stops or a big bill lands. A common UK guideline is three to six months of essential expenses - nearer six (or more) if your income is variable, you are self-employed, or you are the sole earner. It is the foundation that stops a setback turning into debt.
Your FIRE number
FIRE (Financial Independence, Retire Early) uses the 4% rule as a rough guide: if you can live on 4% of your invested pot a year, your money has historically had a good chance of lasting. So your FIRE number is roughly your annual spending x 25. It is a guideline, not a guarantee - sequence of returns, fees and a long retirement all matter.
Worked example
Monthly essential expenses 2,000.
Six-month emergency fund = 12,000.
Annual spending 30,000 gives FIRE number about 750,000 (30,000 x 25).
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Frequently asked questions
How many months should my emergency fund cover?
Three to six months of essential spending is the common range. Choose nearer six (or more) if your income is irregular, you are self-employed, or others depend on you.
Is the 4% rule safe?
It is a useful starting guideline based on historic US data; many UK planners use a more cautious 3-3.5% withdrawal rate for a long retirement. Treat your FIRE number as a target to refine, not a precise finish line.
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Important: BrickCrunch provides general information and estimates only — this is not financial, tax, mortgage or legal advice. Calculations are simplified and your circumstances may differ. UK tax and rate figures are checked against gov.uk (21 June 2026 (2026/27 tax year)) but rates change and errors are possible. Always confirm figures with gov.uk or a qualified professional before making decisions.
