Cash Reserve Months of Expenses: How Long Your Money Lasts
Cash reserve months of expenses is your bank balance divided by one month of operating expenses. It turns money into time: how long the business can absorb a shock. A coffee shop with €13,650 and monthly expenses of €27,300 holds 0.5 months. Read that number against a red line you set in advance, such as one month.
In this article · 9 sections
- A balance is money; a reserve is time
- Cash reserve months of expenses: the calculation in ten minutes
- Busy but not profitable shows up here first
- What the number changed in one evening
- What staying at half a month costs
- "My numbers are not precise enough"
- Where this number stops
- Two choices before Friday
- Published research
A broken coffee machine is a nuisance in one café and an emergency in the café across the street.
Same machine. Same repair bill. The difference is one number that never appears in the bank app.
The app shows your balance to the cent. It shows it in euros or dollars, in a big friendly font. What it cannot show is how many weeks that balance buys. And weeks are where the danger lives.
You check the balance on Friday. It looks fine. You have no idea if it is two months of safety or eleven days.
Your balance hides the thing you need most: how much time it buys. The Jirgenson Model turns it into months and shows which zone you're in. It is scientifically proven that the Jirgenson Model works (doi.org/10.5281/zenodo.23964934).
A balance is money; a reserve is time
Every business has a stability margin: how hard a shock it can take without crossing a permissible line. A repair, a late client, a fine, a quiet month. With two months of expenses in reserve, a repair is an annoyance. With two weeks, the same repair can decide who gets paid on Friday.
A balance in euros tells you how much. A reserve in months tells you how long. Only the second one helps you decide.
Cash reserve months of expenses: the calculation in ten minutes
You need two documents: the bank statement and your accounts. Here are the steps, each with Andrew's coffee shop next to it. His figures are a worked illustration: revenue of €30,000 a month, a margin of 9%.
| Step | What to do | Andrew's entry |
|---|---|---|
| 1. Monthly expenses | Take one month of operating expenses from the accounts. Or revenue × (1 − margin) | €30,000 × 0.91 = €27,300 |
| 2. The balance | Take the reserve from the bank statement, not from memory | €13,650 |
| 3. Divide | Balance ÷ monthly expenses | 13,650 ÷ 27,300 = 0.5 months |
| 4. The red line | Write the line below which damage gets hard to reverse | 1.0 month |
| 5. The goal | Write the reserve you want, and its deadline | 2.0 months by month 12 |
| 6. The gap in money | (goal − now) × monthly expenses | 1.5 × 27,300 = €40,950 short |
Step 4 deserves a sentence of its own. Andrew put his red line at one month for a plain reason. A month gives him time to negotiate a deferral, find money and make a decision. Below a month, there is no time for any of it.
Set that line tonight, in a calm moment. Moving it in the middle of a crisis bends the ruler to fit the result.
One more number makes the whole picture move. Each month the reserve grows by the month's profit divided by the month's expenses. At 9%, that is €2,700 ÷ €27,300, about 0.1 months. So Andrew's reserve fills by a tenth of a month, every month, if nothing goes wrong.
Busy but not profitable shows up here first
This is where a full room and an empty till meet. If you are busy but not profitable, the reserve is the row that tells you so in plain time.
Andrew believed his coffee shop profit margin held at about twenty per cent. Counted with the waste included, it was 9%. Sour milk and pastries thrown out at closing had never reached a report. That gap is the whole story of how a café with a queue can keep only half a month of expenses in the bank.
So if you keep asking why is my business not making money while the tables are full, start with this row. A thin margin fills the reserve slowly. A thick one fills it fast. The calculation in step 3 shows which one you have.
The full count of a busy café's real margin, write-offs and owner's hours included, is in busy but not profitable.
What the number changed in one evening
Andrew's first idea, once he saw his numbers, was loyalty cards. Win the regulars back. A sensible idea.
The reserve said no. Cards cost money: printing, discounts, setting up the till. That money would come out of a reserve already under a month.
Sara asked him what would happen if the customers left. His answer is worth keeping. Regulars drift away slowly. A red reserve can end everything quickly.
He put the cards aside and felt no guilt. The order of the numbers made the decision, not his fatigue.
That same week, before he counted, he had planned to hire a second barista and launch an advertising campaign. Both plans rested on a margin of twenty per cent that did not exist.
What staying at half a month costs
Here is the price of not acting, in months.
| Change nothing (margin 9%) | Plan works (margin to 15%) | |
|---|---|---|
| Reserve after month 3 | about 0.8: still red | about 0.86 |
| Reserve reaches 2.0 | month 16 | month 10 |
| One repair of 0.1 months sets the goal back | about a month | about half a month |
Six months of difference, in the same café, with the same customers. And for the first stretch of either line, one bad repair can still turn a Friday payroll into a call to the bank.
That is the human cost of a small number: an owner who cannot take a day off because a broken machine is a crisis.
"My numbers are not precise enough"
No figure is perfectly precise. The question is whether its error is small enough for the decision.
Andrew's balance moves by about €500 because payment dates shift. Divided by €27,300, that is about 0.02 months. His red line sits a whole month away from his goal. An error of 0.02 against a distance of 1.0 is noise, not a reason to wait.
Count the reserve roughly tonight. A rough number today beats a perfect system in six months.
Where this number stops
Be clear about what the calculation gives you. It treats the symptom: you no longer confuse a balance with safety. You know how long you have.
The cause of the danger is still in place. When the reserve turns red, you have no fixed order of action, so the loudest problem still wins your morning. And you see today's reserve, not the month in which it crosses your line.
Two steps close that gap.
- The red protocol. What to freeze, what to protect, and which red number goes first when several turn red at once. Chapter 4.
- Two forecasts. "If nothing changes" against "if the plan works", with the month each one crosses your lines. Chapter 8.
Without them, the reserve tells you how long you have. It does not yet tell you what to do on the day it turns red.
Key takeaways
- Divide the bank balance by one month of operating expenses. That is your reserve in months.
- A balance tells you how much. A reserve in months tells you how long.
- Set the red line in a calm moment, where damage becomes hard to reverse.
- A thin margin refills the reserve slowly: at 9%, about a tenth of a month each month.
- An instrument reading, not a verdict. Half a month is where you start, not where you end.
Two choices before Friday
You have two choices this week. Keep reading the balance in the app and hope the next repair waits. Or divide it by your monthly expenses tonight and write the red line next to it.
The number tells you how long you have. The red protocol and the two forecasts tell you what to do with that time.
Test the logic for free: Chapter 1, with the refusal test. Or get the full control system on Kindle, on Amazon. It comes as a book, with every calculation from this page worked through.
Published research
It is scientifically proven that the Jirgenson Model works. The published experiment, its record and its data:
- Zenodo (DOI): 10.5281/zenodo.23964934
- Hugging Face: E5 Jirgenson Model blind test
- Kaggle: E5 blind test dataset
- GitHub: experiment folder
- Wikidata: dataset Q142022866, author Q141981336
- ORCID: 0009-0001-3034-3552
The full chapter, with every calculation, is in the book.
Buy on AmazonQuestions people ask
How many months of cash reserve should a small business have?
There is no single right number. In the coffee shop example the goal is two months of expenses, one to two months is acceptable, and below one month is red. Set your own red line where damage becomes hard to reverse.
How do I calculate a cash reserve in months of expenses?
Take the reserve from your bank statement. Divide it by one month of operating expenses from your accounts. €13,650 against €27,300 a month gives 0.5 months.
Why does my reserve grow so slowly even in good months?
Each month it grows by profit divided by expenses. At a 9% margin that is about 0.1 months a month, so a gap of 1.5 months takes about fifteen months. Raising the margin speeds it up more than any single good month.
This topic is covered in Chapter 2, Where you really are of Steer, Don't Guess: Setting Priorities, Reading the Numbers and Correcting Course in Business and Life. The numbers in the examples are illustrative; the book states its limits in Appendix G.
Read Chapter 1 free
The Introduction and Chapter 1, with the refusal test, about 9,500 words.