The balance your bank shows is an honest and useless figure. It says what is there now, not what is left once the rent, the insurance and the gym subscription have gone.
Money left to live on answers the other question, the one you actually ask on the 15th: how much can I still spend without it going wrong?
What it counts
What comes in, minus everything that must go out: fixed charges, your share of the household, loan repayments. What remains is yours, for the rest of the month.
The formula fits on one line, but each of its terms has to be set correctly, or the figure lies.
What comes in is the net figure after tax if you are on prélèvement à la source, France’s pay-as-you-earn system. Not the gross, not the pre-tax net: what actually lands in the account. If your income varies, take the lowest of the last three months rather than the average. A budget built on a good average breaks in the first lean month.
What must go out falls into three families. Fixed charges that leave on their own: rent, insurance, subscriptions, mobile. Your share of shared charges if you live with others. Loan repayments, including the ones forgotten because they are small: paying for a phone in four instalments is still a repayment.
What remains is not savings. It is the money for groceries, transport, going out, haircuts and the chemist. If you want to save, savings belong in the outgoing column, not in what remains.
An example that does not cheat
Take someone on 2,100 € net a month.
Rent and charges 620 €, home insurance 18 €, mobile 15 €, shared internet 12 €, transport 65 €, gym 29 €, car loan 180 €, automatic savings 150 €. Total fixed outgoings: 1,089 €.
Money left to live on: 1,011 € for the month, around 33 € a day.
That figure changes everything. It turns “can I afford this restaurant” into a question you can answer, on the 18th, without opening a calculator.
Why it barely moves
A figure that changes every day is not a landmark. It moves only when something real moves: a charge written down, a spend recorded, income arriving.
That is precisely what separates it from the bank balance. The balance drops on the 1st when the rent goes, climbs on the 28th when the salary lands, and makes the end of the month feel better than it is. Money left to live on moves only with your decisions.
I used to check my balance every morning and get nothing from it. Now I look at one figure a week.
Camille, 29
The three traps
Forgetting annual charges. Property tax, insurance paid in one go, the car service, local taxes: real outgoings that appear in no particular month. Divide them by twelve and put them with the fixed charges. A figure that ignores 1,200 € of annual spending overstates by 100 € a month.
Counting savings as available. If your savings transfer leaves on the 5th and you still count it as spendable, you will spend the same money twice. Savings that leave automatically are a fixed charge.
Taking your best month’s income. A bonus, a thirteenth month, a refund: those are exceptional. They must not raise your reference figure, or the month without a bonus becomes a hard month for no visible reason.
Living with others, same maths
Living with one person or three does not change the nature of the figure, it changes what goes in the outgoing column. Your share of the shared charges belongs there, worked out with whichever split you agreed: equal, by income, or set by hand.
What does not belong there is what the others spend on themselves. Your figure is yours. Theirs is theirs, and it does not have to be visible for shared charges to be split correctly.
The only figure worth watching
A budget broken into forty lines feels like control. In practice nobody keeps it past two months.
One figure, updated when something moves, lasts for years. It does not say everything, but it says the only thing that changes a decision: can I, or do I wait until next week.