Everyone divides spending into fixed and variable, and almost everyone divides it by the wrong test. The question people ask is "is the amount always the same?", and the question that matters is a different one: can I decide not to have this cost this month?
That sounds like an academic distinction and it is not. A budget exists to answer where there is room to move, and that is exactly what the right question reveals and the wrong one hides. Electricity changes every month and you cannot decide not to pay it: it is fixed. A subscription is the same amount to the cent, and you can cancel it this afternoon: it is not fixed at all, it is the most variable cost you have.
Sorting by amount leaves the cancellable and the uncancellable sitting in the same place, and then you look at the budget for somewhere to cut and see nothing.
What is the difference between fixed and variable expenses?
The ability to decide, this month, without changing your life.
A cost is fixed when the decision about it has already been made and is not open now: a contract, a loan, an instalment, a service the house depends on. You can change it, but that is a project, not a Tuesday choice.
A cost is variable when the decision is open every day: what you buy, where you buy it, how much you buy. That is not the same as being frivolous. Eating is compulsory; this week's groceries are a decision.
Put in a table, the four combinations show up at once:
| Cost | Does the amount change? | Can you decide not to have it? | Where it lives |
|---|---|---|---|
| Rent or mortgage | No | No | Fixed |
| Electricity, water, gas | Yes | No | Fixed, at the worst month's figure |
| Subscriptions | No | Yes | Looks fixed, is variable |
| Groceries | Yes | Yes | Variable, with a floor |
The row that matters is the third, and it is the one nearly every budget files in the wrong place.
Why are subscriptions the hard case of fixed expenses?
Because they have the shape of a fixed bill and the nature of variable spending, and the shape is what you see.
They leave on the same day, for the same amount, without anyone doing anything. That is exactly how rent behaves, so the brain files them next to rent and stops looking. The difference is that rent has to be paid and streaming does not, and the only thing keeping streaming paid is that nobody has decided again.
The test is short and works on any of them: if you cancel today, does something stop working tomorrow? If the answer is no, or "I'm not sure", it is not fixed. It is variable spending in disguise, and it is the only part of a budget where cutting costs no effort at all, only a decision that was never made.
This connects to what we wrote about money that disappears without anyone deciding anything: subscriptions are the version of that story that leaves by direct debit.
What about fixed expenses whose amount changes every month?
Budget them at the worst month, not at the average.
Electricity in winter and electricity in summer are two different bills with the same name. Put the average in the budget and you get a plan that is right in April and October and wrong in the other ten months, with the added problem of being wrong on the low side precisely when the bill is biggest.
Use the highest figure from the last year. In the months when the bill comes in below that, the difference is not money left over: it is the difference that will pay for winter. A budget that survives the worst month survives all the others, and the feeling of slack in July is false when January is two months away.
How do you estimate a variable expense without inventing it?
From the typical month, which is neither the average nor the best month.
The average lies because a single abnormal month drags the whole thing. The best month lies because it never happens again. What works is the typical month: look at the last six months of that category, drop the highest and the lowest, and take one of the ones left in the middle.
And there is a second part, which is the one most often missing: variable spending has a floor. Groceries do not go to zero, however well the discipline goes. Before putting a limit on a category, ask what it costs in a month when everything goes well, and do not set the limit below that. An impossible limit is not ambition, it is a guarantee the budget gets abandoned in month three.
Between fixed and variable expenses, where is the room?
In the two places the right division has just made visible.
- In the costs that look fixed and are not. Subscriptions, insurance that has never been compared, old tariffs, gyms nobody goes to. Cutting here is a decision rather than a daily effort, which is why it sticks.
- In the floor of the variables, not the ceiling. Cutting groceries by ten per cent every week is a project that wears you down. Changing where you shop once is a decision that lasts months.
What almost never works is going after the fixed costs seriously, because moving house or renegotiating a loan is weeks of work, and while the budget is not yet running it is far too early for that.
And occasional costs that are neither fixed nor variable expenses?
Those are the third drawer, and they are usually missing entirely.
Car insurance, road tax, the service, the dentist, school supplies. They are not variable, because you do not decide the amount; they are not monthly fixed costs, because they do not come out every month. File them as fixed and the budget gets a line that is nearly always empty. File them as variable and they blow the category in the month they turn up.
They deserve a place of their own, and the way to give them one is to give them a monthly share even in the months when nothing goes out. It is written up in full in how to build a budget that survives month two, because it is the main reason budgets die.
Where to start sorting fixed and variable expenses, today
Three things, and the second is the one that produces the result.
- Run your list of costs through the right test. Not "does the amount change?", but "can I decide not to have this this month?". Some will move box.
- Look at the ones that moved from fixed to variable. That is where the room you could not see has been sitting, and it costs no discipline at all.
- Put the worst month's figure on the fixed bills that vary. Electricity, water, gas. Summer's slack belongs to winter.
Sorting a cost properly does not make a budget stretch further. What it does is make the budget answer when you ask it where there is room to move, which is the only question anybody really asks it.
We keep a page of frequently asked questions about budgeting and everyday money, with a short answer to each one and the full piece one click away. If you want to see it, it is here: frequently asked questions.
This is educational writing. It is not financial advice or an investment recommendation; for advice on products, speak to a professional registered with the CMVM.