Unit Economics: Can One Sale Pay For Itself?

Unit economics asks one question: when you sell one more of your thing to one more customer, do you make money? If yes, grow. If no, stop spending on marketing immediately — scaling a broken unit will bankrupt you faster. Most failed startups had bad unit economics and kept pouring fuel on the fire.

The formula, disarmed

Per-unit contribution margin = Price per unit − Variable cost per unit. That's it. 'Variable cost' means the costs that scale with the sale: the T-shirt itself, the payment processing fee, the shipping, the API call the customer triggered. If the math is positive, each new customer adds money to your business. If negative, each new customer removes money.

A tiny SaaS example

Every new paying customer is worth about $24/month before the fixed costs (your time, subscriptions, marketing). If you acquire them cheaply, you're printing. If they cost $200 to acquire, it takes 9 months to break even, assuming they stay. That's the whole game — unit margin vs. how long they stay vs. what they cost to get.

The five variable costs to always track

1. Payment processing fees (2.5-3% typically)
2. Cost of goods sold per unit (for physical or API-heavy products)
3. Per-customer infrastructure (hosting, database rows, storage)
4. Customer support cost per customer (time x hourly rate)
5. Refunds / chargebacks (usually 1-3% of revenue)

A spreadsheet formula for your unit model

In any sheet (Google Sheets / Excel / Notion), build these six cells: A1: Price per unit B1: =29 A2: Payment fee % B2: =0.029 A3: Fixed per-tx fee B3: =0.30 A4: Variable cost per unit B4: =4.00 (sum of API + hosting etc) A5: Contribution margin B5: =B1 - (B1*B2) - B3 - B4 A6: Contribution margin % B6: =B5/B1 Update B4 any time you learn real numbers from usage data. If B6 ever drops below 0.3 (30%), re-read your pricing lesson and consider raising prices.Live unit-economics calculator

Why investors obsess over unit economics

When investors ask 'do the unit economics work?' they're asking 'can you eventually become profitable just by growing?' If your contribution margin is positive and growing, growth will eventually compound into profit. If it's negative, growth just compounds losses. This is why some startups with huge revenue are still untouchable — the unit model is broken and no amount of growth will fix it.

What 'good' looks like

A good founder can, at any moment, tell you their per-unit contribution margin to the dollar, what the top three variable costs are, and which one they're working on reducing next. They update the number monthly. They treat it like a heart rate — a small drop is fine, a big drop is an emergency.