Fixed Costs Are Promises, Variable Costs Are Choices

The lease was for five years, and I signed it in an afternoon.

I remember the afternoon better than almost anything else from that period — the specific pen, the way the agent said "standard terms" about a clause I now think about at three in the morning. Twenty minutes of actual decision-making. And then sixty consecutive months during which that twenty minutes reached forward and removed £3,400 from my account, every single time, regardless of what I thought about it that month.

Here is the thing I did not understand at the time and would like to have understood: I did not have a £3,400 monthly cost. I had a decision I had made once, that was still executing.

Nobody teaches the difference, because accounting teaches you to sort costs by behaviour — does it move with volume? — which is correct and tells you nothing about what to do on a Tuesday. The useful sort is by authorship. Who is making this decision, and when? It is the same move as treating double-entry as a theory of the self: the categories are only worth keeping if they change what you do.

Operating leverage, worked¶

The business version of this is a real and well-defined mechanism, and it's worth doing the arithmetic explicitly rather than asserting it, because the size of the effect surprises people.

Two businesses. Same product, same price of $10 a unit. Different cost structures.

• Alpha carries $10,000/month in fixed costs and $2 of variable cost per unit. Heavy machinery, long lease, salaried staff.
• Beta carries $2,000/month fixed and $8 variable per unit. Contractors, rented space by the hour, materials bought per job.

At 1,500 units a month they look similar and Alpha looks better:

Monthly volume	Alpha (high fixed)	Beta (low fixed)
1,000 units	−$2,000	$0
1,500 units	+$2,000	+$1,000
2,000 units	+$6,000	+$2,000

Check it yourself — at 2,000 units Alpha books $20,000 revenue, less $10,000 fixed, less $4,000 variable, giving $6,000.

Now watch what a 33% swing does from that middle row.

Volume up a third: Alpha goes from $2,000 to $6,000 — it triples. Beta goes from $1,000 to $2,000, a doubling. Alpha wins the good year decisively.

Volume down a third: Alpha goes from $2,000 to −$2,000. Beta goes to exactly zero. Alpha is now losing money every month while Beta simply stops making any.

Same revenue drop. One business is bleeding and one is idling.

That's the part most people miss. Signing a five-year lease is not a cost decision. It's a forecast — an assertion about volume across sixty months, made by someone who had the information available on one particular afternoon.

The household version is starker¶

Now the personal ledger, where the same structure runs and the numbers are public.

The Bureau of Labor Statistics' Consumer Expenditure Survey for 2024, released in December 2025, reports that average annual expenditures for all consumer units were $78,535, against average pre-tax income of $104,207. Here is where it went:

Housing33.4%

Transportation17.0%

Food12.9%

Personal insurance & pensions12.5%

Healthcare7.9%

Entertainment4.6%

Cash contributions2.9%

Education2.0%

Housing and transportation together were 50.4% of everything. Both are dominated by fixed commitments: rent or mortgage, insurance, a car payment. Entertainment — the category that gets attacked first in every belt-tightening conversation ever held — was 4.6%, and housing was 7.3 times larger.

There's one more number worth flagging. Housing was the only major category with a statistically significant increase in 2024, up 3.3%. The largest fixed block is also the one growing.

So the household equivalent of Alpha's five-year lease is a mortgage, a car loan, and a set of subscriptions — and the household equivalent of eliminating that lease is a set of actions nobody describes as budgeting: moving, selling the car, downgrading the plan.

The turn¶

The obvious read is that people are bad at arithmetic. Cut the 33.4% instead of the 4.6%, obviously.

I don't think that's it, and the real reason is more interesting and much harder to fix.

Cutting a variable cost is making a decision. Cutting a fixed cost is revoking one.

Skipping the coffee is a choice you make about the future. Breaking a lease, selling a car, moving out of a place your family likes — those require you to go back and declare that a previous decision, made by you, was wrong. Not merely no longer optimal. Wrong, publicly and in front of people who watched you make it.

That's a different act, and it costs something arithmetic doesn't price. Fixed costs feel immovable not because they are, but because moving them means overturning a verdict you already delivered. Everyone will look at you and see someone downsizing, which reads as a failure signal even when it's the most competent thing available.

This is exactly the shape of technical and personal debt — a past commitment that keeps charging you interest in the present, where the payments stay individually below the threshold at which anyone acts. And it's why buffer days behave the way they do: a household with high fixed costs has a high daily burn, so its resilience is structurally low no matter how disciplined the discretionary spending is.

High fixed costs are high operating leverage on your own life. The good years are genuinely better — the nice flat, the reliable car, the standing arrangements that make everything smoother. And the bad year is catastrophic in a way it simply isn't for someone whose outflows are mostly decisions they're still making.

That's not an argument for minimising fixed costs. Alpha's structure wins decisively when volume holds, and a life with no commitments in it is not obviously a good life. It's an argument for knowing which bet you have placed — because most people placed theirs on an afternoon, in twenty minutes, with a pen, and have never once revisited it. Organisations do the identical thing at scale, which is why a founder's calendar quietly becomes the company's real cost structure: a commitment made once keeps allocating a resource long after anyone is deciding.

The practice¶

1. 1

   Sort your outflows by authorship, not by category

   Take last month's statement and mark each line: did I decide this last month, or did a past decision execute? Not essential-versus-luxury — that's a moral sort and it doesn't help. Authorship. You'll typically find that 60–70% of your money left without any decision being made in the period, which is the whole point.

2. 2

   Compute your own operating leverage

   Divide fixed monthly outflow by total monthly outflow. Above roughly two-thirds and you have Alpha's structure: excellent while income holds, brutal on a bad quarter. Below half and you're Beta — less upside, far more able to absorb a shock. Neither is wrong. Not knowing is.

3. 3

   Put an expiry date on every fixed commitment

   The dangerous property of a fixed cost is that it renews silently. Write down, for each one, the next date at which it could be changed without penalty — lease break, contract end, refinance window. Most people cannot name a single one of these dates, which means every commitment is functionally permanent by default.

4. 4

   Attack the 33.4%, once, properly

   One serious look at housing or transport is worth more than a year of discretionary discipline. Do the division: entertainment is $3,609 and housing is $26,266, so a 13.7% reduction in housing equals eliminating your entire entertainment budget — every subscription, every meal out, every ticket, gone, matched by rent about one-seventh lower. It is also enormously more effortful and socially costly, which is precisely why it stays untouched.

5. 5

   Before signing anything long, state the forecast out loud

   A five-year commitment is a claim about five years. Say it in words — "I am asserting my income holds at or above X until 2031" — and notice whether you'd defend that sentence. If you'd hesitate to write it down, you are about to write it down anyway, in the form of a signature. Price it the way you'd price any other multi-year obligation.

The pen¶

I got out of that lease eventually, two years early, at a cost I'd rather not put in print, and it was the single best financial decision of that period.

What I remember is not the negotiation. It's the feeling in the week beforehand — that ending it would be an admission, that the correct and dignified thing was to serve out the sixty months I had committed to, as though a lease were a sentence and I owed it to someone to complete it.

Nobody was owed anything. There was a break clause, a number, and a calculation that came out clearly in one direction. The only thing standing in front of it was that signing had felt like a decision and un-signing felt like a confession.

The money leaves either way. The question is only whether anyone is still choosing — and if a past version of you is doing the choosing, it's worth checking whether they'd make the same call knowing what you know now. They generally wouldn't. They only ever had the afternoon.

It's just business.

Sources

1. U.S. Bureau of Labor Statistics — Consumer Expenditures, 2024 (released 19 December 2025)
   +How this was checked

   Average annual expenditures for all consumer units were $78,535 against average pre-tax income of $104,207. Housing $26,266 (33.4%), transportation $13,318 (17.0%), food $10,169 (12.9%), personal insurance and pensions $9,797 (12.5%), healthcare $6,197 (7.9%), entertainment $3,609 (4.6%). Housing was the only major category with a statistically significant increase, up 3.3%.

2. Worked operating-leverage example — arithmetic shown in full above
   +How this was checked

   The Alpha/Beta comparison uses no external figures: at $10 per unit, Alpha ($10,000 fixed, $2 variable) breaks even at 1,250 units and Beta ($2,000 fixed, $8 variable) at 1,000. Every profit figure in the table is computed from those inputs and can be checked directly. BLS Consumer Expenditure Survey homepage linked for the underlying household data series.