A statement that books rent on the day the invoice arrives is telling you that the first of the month was a catastrophe and that the other thirty days were unusually profitable, which is true of the bank account and false about the business. Fixed costs are consumed continuously: the warehouse is occupied every day, the software runs every day, the salary buys a day of work every day. Proration is the arithmetic that puts them where they were consumed rather than where they were billed, and it is the single change that makes a per-day figure worth reading at all. It is also the reason a spreadsheet P&L stops being an argument about invoice timing and starts being a comparison between days.
- A monthly cost is charged at twelve times the monthly amount divided by 365 for each day it covers.
- A yearly cost is the yearly amount divided by 365; a one-off lands on the day it was booked.
- Without it, a daily figure is dominated by invoice timing rather than by trading.
- With it, two months with different billing dates carry the same daily burden and can be compared.
Why 365ths rather than the month's own length
Dividing a monthly cost by the number of days in each month is the obvious approach and it produces a subtly annoying result: February days carry more overhead than March days, so a short month looks structurally worse per day than a long one for reasons that have nothing to do with the business.
Multiplying the monthly amount by twelve and dividing by 365 gives every day of the year the same burden, which is what makes days comparable across a year. It costs a tiny discrepancy against a calendar month's exact total and buys the property that actually matters, which is that a Tuesday in February can be compared with a Tuesday in July.
Where each kind of cost lands
Recurring monthly and yearly costs spread across the days they cover. A one-off lands on the day it was booked, because that is genuinely the day it belongs to: a brand refresh or a piece of equipment was consumed at a point in time rather than continuously.
The one deliberate exception is category-based. Fixed costs categorised as marketing, a monthly influencer retainer for instance, leave the overhead line and join marketing spend, because that is what they are. They still spread daily, they simply spread onto a different line, and each cost is counted exactly once either way.
| Billed as it arrives | Prorated | |
|---|---|---|
| Day 1 | €3.000 of cost | €98,63 |
| Days 2 to 30 | nothing | €98,63 each |
| Day 1 profit | deeply negative | ordinary |
| Comparable with last month | only if billing dates match | always |
What it does to break-even
Break-even is the point where contribution covers the fixed block, and that block only means something if it is a rate rather than a series of shocks. Prorated, the question becomes how many orders a day it takes to cover a day's share, which is a number an operator can actually hold in their head and a break-even calculator can compute.
It also stops a genuine improvement from being invisible. Cancelling a subscription lowers the daily burden from the day it stops, and a statement that books lumps will show nothing until the next invoice fails to arrive, by which time nobody connects the two.
The two mistakes worth avoiding
The first is spreading a cost across days it did not cover. An annual insurance premium paid in March covers the twelve months from March, not the calendar year, and spreading it from January means eleven months carry a cost that had not started. The rule is to spread across the period the payment buys rather than across whatever period is convenient.
The second is spreading a one-off. A brand refresh, a piece of equipment or a legal fee was consumed at a point in time, and smearing it across a year turns a decision into a background hum. If somebody chose to spend it, the day they chose is where it belongs, and the recurring commitments are what the spreading is for.
Doing it by hand
In a spreadsheet the whole technique is one column. Total everything you pay regardless of sales, divide by the days it covers, and put the result on every row, which is why the daily P&L tracker has a column for exactly that and a note telling you what to put in it.
The discipline it demands is completeness rather than arithmetic. A prorated figure built from half your commitments is a precise number about a fictional cost base, and the most common omissions are the annual ones: insurance, the accountant, the domain renewals, the tool that bills once in January and is forgotten by March. Total everything first, then divide, and the resulting daily figure is one you can defend to anybody who asks where it came from.