variable cost
Also called: variable costs
A cost that grows with every unit sold or delivered, such as a tutor's payout per lesson or a payment fee per purchase.
Variable costs grow with every unit delivered or sold. The test is one question: if we deliver one more unit, does this cost grow? In digital businesses they come in three shapes: per unit (a tutor's or courier's payout), per currency unit of sales (a payment fee of 2.5% of the price) and per use (video minutes, AI inference, SMS, cloud usage).
Per unit, variable costs stay roughly stable, while the monthly total rises and falls with volume. That is why they are best compared per unit: add up the variable columns and divide by units delivered. Price minus variable costs per unit is the contribution margin, the number that drives break-even and most pricing decisions.
Two details are easy to miss. Usage-based costs follow usage, not units: a heavy user or a longer session costs more even at the same price. And costs billed in a foreign currency, such as video or AI priced in USD, also move with the exchange rate, so a variable cost per unit can change without anyone changing a supplier.
Example
In November 2025 Klasna paid tutors 2,495,460 UAH, payment fees of 96,558 and 194,370 for video and AI, for 6,270 lessons: 2,786,388 ÷ 6,270 = 444.40 UAH per lesson.
Over the year the total swings from 1,169,090 UAH in July (2,600 lessons) to 4,315,532 in May (9,620 lessons), while the cost per lesson stays between 442 and 450 UAH. The total follows lessons; the rate barely moves.
Common mistakes
- Calling anything that moves with the season variable. A marketing budget raised for peak months is a decision, fixed within the month.
- Forgetting the payment fee. It is a percentage of the price, so it shrinks with discounts and grows with price rises.
- Assuming usage costs are per unit. Usage per unit can grow quietly, for example when an AI feature gets popular.
- Ignoring currency. USD-billed costs change in UAH when the exchange rate moves.
Learn it in the course
- Cost behavior: fixed, variable and step costs · Cost behavior on a real year of costs: which costs move with lessons, which don't, and which jump at a threshold.
- Contribution margin per unit: what one lesson leaves · Contribution margin per unit, step by step: price minus tutor payout, fees and video minutes, and what a lesson and a package really earn.
- Break-even point in units and margin of safety · Break-even point in units: how many lessons a month cover the fixed costs, and how far sales can fall before a loss.
- The allocation trap: should we drop programming? · A subject that looks loss-making only because of the shared costs it carries.
- Relevant, sunk and opportunity costs · Which numbers belong in a decision and which only feel like they do.