How to work out your essential monthly expenses

There's one number sitting behind almost every money decision: what you need per month to live. It's the base of your emergency fund, your budget, knowing what you need to earn, and deciding whether you can leave a job.

Almost everyone gets it wrong, and always in the same direction: too low, by 20% to 30%.

Why it's always underestimated

Three reasons that stack:

People picture a quiet month. A month without the vet, without the birthday, without the flat tyre. That month exists, but it's one in four.

Non-monthly expenses get forgotten. The half-yearly insurance, the vehicle registration, the service, the tuition. They aren't in your head when you do the maths, and they're exactly what breaks the month.

"Essential" gets confused with "cheap". Essential groceries aren't the absolute minimum possible: they're what you actually buy to eat every day.

The calculation, in three steps

1. Take three real months, not one

One month lies. With three, the odd events average out and the structural stuff shows up.

If you've never tracked your spending, it can be reconstructed — it's in how to rebuild a month of spending. It'll come out worse than if you'd logged it, but it works as a starting point.

2. Keep only what you'd still pay with no income

The test: if you lost your job tomorrow, would you still be paying this next month?

In:

• Housing: rent or mortgage, service charges
• Utilities: power, gas, water, internet, phone
• Everyday food
• Minimum necessary transport
• Health: insurance, ongoing medication
• The minimum payment on every debt
• Insurance you can't cancel
• Anything that depends on you: childcare, elder care, a pet

Out: going out, delivery, clothes, streaming, travel, gifts, the gym.

None of that is frivolous — it's what you'd pause in an emergency. It's a different category, not a bad one.

3. Add the non-monthly expenses, spread across the year

This is the step almost nobody takes, and the one that changes the number most.

List what you paid last year that happened once or twice: insurance, registration, taxes, car servicing, tuition, document renewals. Add it all up, divide by twelve, and add that to your essential month.

It's money you know is going out, even if you don't know exactly when.

What to do with the number

It's the floor of your budget. Everything you build above it is a decision; up to it, there's no negotiating this month.

Multiplied by 3 to 6, it's your emergency fund. How many months depends on your situation, and the calculation is in emergency fund.

It's your income floor. If you invoice, it's the number your weakest month gets compared against — see irregular income.

It's what makes your situation comparable over time. If your essential month rose three points as a share of your income over six months, that's real information, and you won't see it any other way.

Review it twice a year

It changes when your life changes: you move, rent goes up, someone starts depending on you. And where prices move fast, every three months.

In one line

Take three real months, keep what you'd still pay with no income, add the once-or-twice-a-year expenses spread across twelve, and review it twice a year. That number is the floor under almost every other money decision you make.

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