"How much can I still spend this month?" The left-to-spend-per-day method

A budget usually answers the question after the fact: where did last month's money go. Useful, but the question on the evening of the 14th, with a full online basket, is a different one: how much can I still spend before the month ends without going over?

The answer is one division. What is left of the month's targets, divided by the days that remain. The sum is easy. It is the three traps around it that make it wrong.

The basic sum

Take a 31-day month, on the morning of the 14th. There are 31 − 14 + 1 = 18 days left, today included: the day that is just starting can still cost something.

The month's targets were set category by category, for a single person who rents. Here is where things stand at the end of the 13th.

An example built for this article. Irregular expenses, covered further down, are not in it.

That leaves €330 for 18 days: 330 / 18 = €18.33 a day. That is the raw figure. It is correct, but it does not tell the whole story yet.

Trap one: the month in progress is not a full month

By the 14th, €1,390 of €1,720 has gone, 81% of the budget, while only 42% of the month has passed (13 days out of 31). Put that way, it looks alarming. Set against last month's full total it would look the opposite: €1,390 against, say, €1,700 last month, and you would feel comfortable.

Both readings are wrong for the same reason. Fixed costs land early in the month: rent on the 3rd, insurance on the 5th, most subscriptions before the 10th. They account for €935 of the €1,390 spent. Treating them at the same pace as groceries means comparing half a month with a whole one.

The better reading separates the two. Fixed costs are paid, or due on a known date: here, €15 of subscriptions still to come on the 20th. Variable spending is judged on its pace. Its total target is €770, or 770 / 31 = €24.84 a day on average. By the 13th, an even pace would have cost €323. The actual figure is €455.

So the useful figure is not €18.33 but €17.50 a day for variable spending: 770 − 455 = €315, divided by 18 days. More to the point, setting it against the €24.84 of the start of the month says what happened: the month ran ahead, and the remaining eighteen days have to cost 30% less per day than planned.

A daily figure is still an average. Nobody spends €17.50 every day; the shopping happens on Saturday and Tuesday costs nothing. Times seven, it becomes a more useful envelope: €122.50 a week for everything variable.

Trap two: targets built from wishes

A left-to-spend figure only means something if the target behind it can be met. Budgets written on a Sunday evening tend to start from what you would like to spend, not what you do spend. In the first month the figure turns red on the 10th, and you stop looking at it.

The approach that lasts starts from history: each category's average over the last three complete months, rounded up, then trimmed where you actually want to act. One or two categories at a time. Our view, with no study to prove it: a grocery target set 30% below the three-month average rarely survives more than a few weeks; 10% below is an effort you notice without breaking anything.

Trap three: irregular expenses

Car insurance comes once a year, presents in December, the car service in spring. If they go into the budget of the month they land in, that month looks like a disaster and the others look easy. In the example, a €420 car insurance renewal arriving on the 14th would turn €330 left into €90 over. A false alarm if the money had been put aside, a real one if it had not.

The fix is an old one: smooth them out. List what comes round every year, divide by twelve, and set that amount aside each month, in a separate savings account, on payday.

An example built for this article.

These expenses stay out of the daily figure: they do not have a monthly target, they have a reserve. When the bill arrives, it is checked against the reserve balance, not against this week's budget. This person's income then has to cover €1,720 of everyday spending, €150 of reserve and their long-term saving. If those three lines do not fit together, it is the target that needs rethinking, not the division.

What the figure does not do

Left to spend per day is a steering tool, not a verdict. It does not say whether €380 of groceries is too much or too little for you; it only says whether the month is following the path you chose. Nor does it replace the overall picture: a month held to the euro is not a healthy budget if the total target leaves nothing to save.

The link to the long term is direct all the same. In the example, the €132 the variable spending ran ahead would, if it became the norm every month, mean €1,584 a year less to invest. How small drifts like that move a financial independence date is worked out in our piece on no-buy years.

Where IgniFI fits

IgniFI's budget page shows this figure at the top of the current month, in a "left to spend" card: the sum of the month's targets, minus what has been spent in those categories, divided by the days left, today included. Only categories with a target count, a transfer filed as savings, to a savings account or a PEA, is not spending, and a refund reduces the category it lands in. To get started, the app suggests targets drawn from your last three complete months, rounded up to the next ten euros.

One honest caveat: the card does not split fixed costs from variable spending. On this article's example it would show €18.33 a day, the €15 subscription due on the 20th included. Leaving irregular expenses without a monthly target is, on the other hand, enough to keep them out of the sum.