Fixed, Variable, and Mixed Costs

In the previous chapter, "The Three Elements of Cost," we looked at what costs are made of: materials, labour, and overheads. We sliced the business expenses based on their nature. But knowing what a cost is made of is only half the battle. To actually run a business, price a product, or plan for the future, you need to know how those costs will behave when things change.

Imagine you run a factory making Kashmiri willow cricket bats. If you get a massive order and need to double your production next month, which of your costs will double? Which ones will stay exactly the same? And which ones will go up, but only by a little bit?

This is the study of Cost Behaviour. It is the foundation of all management decision-making. If you do not understand how your costs react to changes in production volume, you cannot accurately price your products, you cannot calculate your profit margins, and you certainly cannot predict your break-even point.

In this chapter, we are going to divide all costs into three behavioral categories: Fixed, Variable, and Mixed. We will also look at a special category called Step Costs, and learn a simple mathematical trick to split mixed costs into their fixed and variable parts.

The Golden Rule: The Relevant Range

Before we define any cost, we have to establish a boundary condition called the Relevant Range.

In the real world, no cost is truly fixed forever. If your bat factory's rent is ₹50,000 a month, you might call that a fixed cost. But what if your business grows so much that you need to rent a second building? Your rent just doubled. Does that mean rent wasn't a fixed cost?

No, it means you stepped outside your relevant range.

When we say a cost is "fixed," we always mean "fixed within the relevant range of our current factory capacity." If your factory can make anywhere from 0 to 5,000 bats a month, that 0–5,000 is your relevant range. We assume all our cost rules apply only as long as we stay within that capacity.

Fixed Costs: The Heavy Anchor

A fixed cost is the stubborn, unmoving anchor of your business. It does not care if you are having a great month or a terrible month. It does not care if your machines are running 24/7 or gathering dust. The bill arrives, and the amount is exactly the same.

Common examples of fixed costs include:

• Factory Rent: The landlord charges ₹50,000 a month whether you make 10 bats or 4,000 bats.
• Insurance Premiums: Fire insurance for the building costs the same annually regardless of output.
• Salaries of Permanent Staff: The Factory Manager's monthly salary of ₹80,000 does not fluctuate with daily production quotas.
• Depreciation (Straight-Line): If a machine loses ₹10,000 of value every year based on time, that is a fixed cost.

The "Per Unit" Illusion of Fixed Costs

Here is where students and new business owners make their biggest mistake. They look at fixed costs in total, which is easy. But you also have to understand how fixed costs behave per unit of production.

Let's look at the factory rent of ₹50,000.

• If you make 1 bat this month, the rent cost attached to that one bat is ₹50,000.
• If you make 100 bats, the rent cost per bat drops to ₹500 (₹50,000 / 100).
• If you make 1,000 bats, the rent cost per bat drops to ₹50 (₹50,000 / 1,000).
• If you make 5,000 bats, the rent cost per bat is just ₹10.

Notice what is happening? The Total Fixed Cost stays perfectly flat. But the Fixed Cost Per Unit goes down as production goes up. It spreads out over more units, becoming thinner and thinner.

This is why mass production makes things cheaper. By producing millions of units, massive companies spread their massive fixed costs so thin that the fixed cost per unit becomes almost zero.

Variable Costs: The Loyal Shadow

If fixed costs are a stubborn anchor, variable costs are a loyal shadow. They follow your production volume perfectly. If you make more, they go up. If you make less, they go down. If you stop production entirely, they drop to zero.

Common examples of variable costs include:

• Direct Materials: The willow wood and rubber grips needed for the bats. If one bat needs ₹600 worth of wood, ten bats need ₹6,000 worth of wood. Zero bats need ₹0 of wood.
• Direct Labour (Wages): If you pay workers ₹200 for every bat they assemble (piece-rate), the total wage bill moves perfectly with production.
• Sales Commissions: If a salesman gets ₹50 for every bat sold, the total commission expense varies directly with sales volume.
• Packaging: The cardboard boxes used to ship the bats.

The "Per Unit" Reality of Variable Costs

Let's look at the wood required for the bats, which costs ₹600 per bat.

• If you make 1 bat, the total wood cost is ₹600. The cost per unit is ₹600.
• If you make 100 bats, the total wood cost is ₹60,000. The cost per unit is still ₹600.
• If you make 1,000 bats, the total wood cost is ₹6,00,000. The cost per unit is still ₹600.

Notice the exact opposite behavior compared to fixed costs. The Total Variable Cost climbs higher and higher as you produce more. But the Variable Cost Per Unit stays perfectly flat and constant.

Mixed Costs (Semi-Variable Costs): The Hybrid

In the real world, not every cost is purely fixed or purely variable. Many costs have a split personality. They have a baseline fixed amount that you have to pay no matter what, plus a variable amount that kicks in based on how much you use.

We call these Mixed Costs or Semi-Variable Costs.

The most classic example of a mixed cost is your electricity bill. Even if you lock the factory doors and go on a month-long holiday, the power company will still send you a bill. Why? Because there is a fixed "meter rent" or "fixed connection charge" (say, ₹1,000). That is the fixed component. But when you open the factory and start running the heavy machinery, the meter spins. You are charged an additional ₹8 for every unit (kWh) of electricity consumed. That is the variable component.

Other examples include:

• Telephone/Internet Bills: A fixed monthly line rental plus charges for extra data or international calls.
• Maintenance Costs: A fixed annual contract fee to a mechanic to be on call, plus an hourly rate for the actual hours they spend fixing a broken machine.
• Salesman Compensation: A fixed base salary of ₹15,000 a month, plus a ₹50 commission for every unit sold.

Why Mixed Costs are a Problem

Mixed costs are a headache for accountants. If you want to predict next month's expenses, or if you want to calculate your break-even point (which we will do in Marginal Costing later), you must know exactly what your total fixed costs are and what your variable cost per unit is.

You cannot leave a cost as "mixed." You have to take a surgical knife and split the mixed cost into its two pure components: the fixed part and the variable part.

How do we do that? We use a mathematical technique called the High-Low Method.

Separating Mixed Costs: The High-Low Method

The High-Low Method is a simple, practical way to separate a mixed cost into its fixed and variable elements using historical data. It looks at the highest and lowest levels of activity over a period and compares the costs at those two points.

The logic is simple: Since the fixed cost is the same at both the high point and the low point, any difference in the total cost between these two points must be entirely due to the variable cost of producing the extra units.

Here is the step-by-step process:

Let's make this concrete with a worked example.

Worked Example: Splitting a Mixed Cost

Suppose your factory has recorded the following machine maintenance costs over the last five months. Maintenance is a mixed cost.

• January: 1,200 machine hours run -> Total Cost: ₹18,000
• February: 1,500 machine hours run -> Total Cost: ₹21,000
• March: 2,400 machine hours run -> Total Cost: ₹30,000
• April: 800 machine hours run -> Total Cost: ₹14,000
• May: 1,800 machine hours run -> Total Cost: ₹24,000

Goal: Find the variable cost per machine hour, and the fixed monthly maintenance cost.

Step 1 & 2: Identify the High and Low points Warning: Always pick the high and low based on the ACTIVITY LEVEL (machine hours), not the cost in rupees.

• Highest Activity: March (2,400 hours, Cost: ₹30,000)
• Lowest Activity: April (800 hours, Cost: ₹14,000)

Step 3: Calculate the differences

• Difference in Activity (Hours) = 2,400 - 800 = 1,600 hours
• Difference in Cost (Rupees) = ₹30,000 - ₹14,000 = ₹16,000

Step 4: Calculate Variable Cost Per Unit

• Formula: (Change in Cost) ÷ (Change in Activity)
• ₹16,000 ÷ 1,600 hours = ₹10 per machine hour

We now know the variable component is ₹10 per hour.

Step 5 & 6: Calculate the Fixed Cost Take either the High point or the Low point (the math works out exactly the same). Let's use the High point (March).

• Total Cost in March = ₹30,000
• We know March had 2,400 hours.
• Total Variable Cost in March = 2,400 hours × ₹10/hour = ₹24,000
• Total Cost (₹30,000) - Variable Cost (₹24,000) = Fixed Cost (₹6,000)

Let's verify it using the Low point (April) just to be sure:

• Total Cost in April = ₹14,000
• Total Variable Cost in April = 800 hours × ₹10/hour = ₹8,000
• Total Cost (₹14,000) - Variable Cost (₹8,000) = Fixed Cost (₹6,000)

Conclusion: The mixed maintenance cost is actually a fixed cost of ₹6,000 per month, plus a variable cost of ₹10 per machine hour. Now, if the manager asks, "What will maintenance cost next month if we run for 2,000 hours?", you can easily calculate it: ₹6,000 + (2,000 × ₹10) = ₹26,000.

Step Costs (Semi-Fixed Costs): The Staircase

There is one final, slightly tricky category of cost behavior that bridges the gap between fixed and variable. These are called Step Costs (or Semi-Fixed Costs).

A step cost is fixed for a small, specific range of activity, but then jumps up to a higher fixed level once that range is exceeded. If you graph it, it looks exactly like a staircase.

The Classic Example: Quality Control Supervisors Imagine one supervisor can effectively monitor up to 15 factory workers. You pay the supervisor a fixed salary of ₹30,000 a month.

• If you have 5 workers, you need 1 supervisor (Cost: ₹30,000).
• If you have 12 workers, you still need 1 supervisor (Cost: ₹30,000).
• But the moment you hire your 16th worker, one supervisor is no longer enough. You must hire a second supervisor.
• Suddenly, your cost jumps from ₹30,000 to ₹60,000. It will stay at ₹60,000 until you hire your 31st worker, at which point it steps up again to ₹90,000.

Step costs remind us why the "Relevant Range" is so important. Within the narrow range of 1 to 15 workers, the supervisor's salary behaves exactly like a fixed cost. But over a wider range, it steps up, behaving a bit like a clunky variable cost.

When planning for the future, managers must keep a close eye on step costs. If you are currently at 15 workers, hiring just one more worker might bring in a little extra revenue, but it will trigger a massive ₹30,000 jump in step costs, potentially destroying your profit margin for that month.

Quick Revision

• Cost Behaviour is the study of how costs change in response to changes in production volume or activity levels.
• Relevant Range is the normal operating capacity within which our assumptions about fixed and variable costs hold true.
• Fixed Costs remain constant in total, but the fixed cost per unit decreases as production increases. (e.g., Rent, Insurance).
• Variable Costs increase in total as production increases, but the variable cost per unit remains constant. (e.g., Raw Materials, Direct Wages).
• Mixed Costs (Semi-Variable) have both a fixed base and a variable component (e.g., Electricity bill with a meter rent + usage charge).
• The High-Low Method is used to separate mixed costs. Formula for Variable Cost per unit: (Highest Cost - Lowest Cost) / (Highest Volume - Lowest Volume).
• Step Costs remain fixed over a small range of activity and then jump to a new fixed level when capacity is breached (e.g., hiring an additional supervisor for every 15 workers).