Fixed vs variable costs, and why the mix decides your risk

Two businesses can pull in the exact same revenue this month and face wildly different odds of surviving the next one. The difference usually isn't sales or smarts — it's the shape of their costs. How much of what you spend stays the same no matter what, versus how much rises and falls with the work you actually do, quietly sets how much risk you're carrying. Most founders never look at this until a slow month forces them to.

Every dollar you spend behaves in one of two ways. A fixed cost doesn't move when sales move — your office rent is the same whether you book one client or fifty. A variable cost rises and falls with volume — sell more units, you spend more on the materials to make them. Knowing which is which isn't accounting trivia. It tells you what happens to your profit when business is great, and what happens when it isn't.

Let's get precise about the two buckets, then look at why the mix between them is the real lever on your risk.

The two buckets, defined

Fixed costs are the bills that show up at roughly the same size every month regardless of how much you sell. Think rent, full-time salaries, insurance, and software subscriptions — your accounting tool charges the same whether you process ten invoices or ten thousand. You commit to these in advance, and they don't care about your sales chart.

Variable costs scale with volume. They go up when you do more and down when you do less. The clearest examples:

• Raw materials and the cost of the products you resell
• Payment processing fees — roughly 2.9% plus a fixed cents charge on each card transaction
• Shipping and fulfillment on every order that goes out the door
• Hourly contractors and freelancers you only pay when there's work

A few costs live in between. A salesperson on base salary plus commission, or a phone plan with a flat fee plus per-usage charges, is "semi-variable" — part fixed, part moving. Others jump in steps: hire one more manager or rent a second unit, and a "fixed" cost lurches up to a new level. These step costs feel fixed until growth forces the next jump.

The mix sets your operating leverage

Here's the idea that ties it together. Operating leverage means how heavily you lean on fixed costs. The more of your spending that's fixed, the more leverage you have — and leverage cuts both ways.

If your costs are mostly fixed, every extra sale is almost pure profit, because the big bills are already paid. A busy month is spectacular. But a slow month is brutal: the rent and salaries don't shrink, so you can fall below break-even fast. Break-even is simply the sales level where you cover all your costs and make zero — no profit, no loss.

If your costs are mostly variable, you're more resilient. When sales drop, your costs drop right alongside them, so it's hard to bleed badly. The trade-off is that your upside is capped — a great month is good, not life-changing, because each sale always drags its costs along with it.

The same revenue, two very different months

Picture two consulting shops, each booking $50,000 of revenue in a normal month. Studio A is fixed-heavy: $35,000 in salaried staff and rent, plus $5,000 in variable costs. Studio B is variable-heavy: $10,000 in fixed costs, plus $30,000 in contractors and pass-through costs that scale with project work.

In the normal month, A keeps $10,000 and B keeps $10,000 — identical. Now a bad month hits and revenue falls to $30,000. Studio A's fixed bills don't budge, so it spends roughly $35,000 fixed plus about $3,000 variable and loses around $8,000. Studio B's variable costs shrink with the work, so it spends $10,000 fixed plus about $18,000 variable and still scrapes out roughly $2,000 in profit. Same business size, same downturn — one is hemorrhaging, the other is fine.

Flip it to a $70,000 boom month and the picture reverses: Studio A's mostly fixed base means most of that extra revenue falls to the bottom line, while Studio B hands much of it back out in scaling costs. Neither structure is right or wrong. The fixed-heavy one is a high-stakes bet on staying busy; the variable-heavy one is a steadier ride with a lower ceiling.

Know your monthly nut

The single number to pull out of all this is your "monthly nut" — the total fixed cost you have to cover before you've earned a dollar of profit. Add up rent, salaries, insurance, software, and every other bill that arrives no matter what. That figure is your line in the sand: the minimum revenue your business must produce just to stand still.

Once you know your nut, decisions get sharper. You can see how many slow weeks of cash you can absorb, judge whether a new salaried hire is worth raising your floor, and decide when to keep a cost variable — a contractor instead of an employee — to stay flexible while you're still proving demand.

This week, list every recurring cost and tag each one fixed or variable, then total the fixed column. That single number is your monthly nut — and it's the first thing to know before you take on any new commitment.