A budget almost never fails because someone cannot do arithmetic. It fails because the first version is written in an optimistic mood, with numbers that describe the month the person wishes they had rather than the month they actually live. Rent and the car payment get copied in correctly, and then everything that is irregular gets a round number that feels responsible.
Look at the last three months of your bank statements before writing a single line. The pattern that matters is not the big bills, which you already remember, but the small, frequent ones you do not: the delivery fees, the top-up purchases, the second coffee, the app that renewed on a Tuesday. Those are the categories where the gap between planned and real usually lives.
Give irregular costs a monthly home even when they do not arrive monthly. Car maintenance, dentist visits, school supplies, the annual insurance premium — divide each one by twelve and treat it like a bill. A budget that ignores these is not a budget, it is a list of the predictable expenses, and the unpredictable ones will arrive anyway and look like a personal failure.
Finally, keep the first version loose. Leave a line called “unassigned” with real money in it, and spend the first two months measuring rather than restricting. A plan that survives contact with an ordinary month, with its birthdays and flat tires and bad days, is worth more than a precise plan you abandon on the fourteenth.
