Fixed vs variable costs: the difference, examples, and how to split a mixed cost

Fixed vs variable costs comes down to one question: does the total change when volume changes? Fixed costs, such as rent, salaried staff and insurance, stay the same in total within a normal range of activity; variable costs, such as materials, packaging and delivery, rise and fall with each unit. Total cost is the fixed cost plus the variable cost per unit times units, and a mixed cost can be split into the two parts from a few months of your own data.

Total cost = Fixed cost + Variable cost per unit × Units

The difference in one table

The US tax rules use the same test. The IRS full-absorption regulation, 26 CFR 1.471-11, defines fixed indirect production costs as those that "do not vary significantly with changes in the amount of goods produced at any given level of production capacity," and variable ones as those that do.

Fixed vs variable is a different split from opex vs capex, which is about whether a cost is expensed now or capitalized. A cost can be fixed opex (rent) or variable opex (delivery).

Fixed cost examples

• Rent and property taxes on warehouses, offices and plants.
• Salaried staff: supervisors, finance, management.
• Insurance premiums.
• Software licenses priced per year or per seat.
• Depreciation on equipment, on a straight-line basis.

Fixed means fixed within a range and a period. Rent is fixed until the business needs a second warehouse; salaries are fixed for the year until a new hire. Over a long enough horizon, every cost can change.

Variable cost examples

• Materials used in each unit.
• Packaging per unit shipped.
• Delivery per stop or per parcel.
• Sales commission as a percentage of revenue.
• Card and payment fees as a percentage of each transaction.

Price minus variable cost per unit is the contribution per unit, the amount each sale adds toward the fixed costs. As a share of price it is the contribution margin ratio.

Semi-variable and step costs

Semi-variable (mixed) costs have a fixed part and a variable part. A utility bill has a standing charge plus usage; maintenance has a service contract plus repairs that grow with machine hours; a vehicle has a lease plus fuel.

Step costs are fixed over a band of activity and then jump. One supervisor covers one shift; a second shift needs a second supervisor. The cost is flat until capacity runs out, then steps up, which is why capacity is arithmetic before it is a hiring decision.

Worked example: classifying cost lines, then checking with the high-low method

A packing operation has six monthly cost lines:

Classifying the lines one by one gives fixed costs of $87,000 a month and a variable rate of $15.00 per unit. When the ledger does not split cleanly, the same answer can be read from total cost and units alone. Six months:

The high-low method takes the months with the highest and lowest activity, here April (12,000 units) and May (7,400 units):

Variable rate = (Cost at highest activity − Cost at lowest activity) / (Highest units − Lowest units)

Fixed cost = Cost at highest activity − Variable rate × Highest units

Variable rate = (267,000 − 198,000) / (12,000 − 7,400) = 69,000 / 4,600 = $15.00 per unit. Fixed cost = 267,000 − 12,000 × 15.00 = $87,000. The two methods agree.

The check that proves it

Predict each month's cost as 87,000 + 15.00 × units and compare with the actual:

Every month sits within $1,000 of the prediction, on costs around $200,000 to $270,000. April and May match exactly because they are the two points the method used.

High-low uses only two months, so cross-check it with a regression on all six. With units in B2:B7 and total cost in C2:C7:

=SLOPE(C2:C7,B2:B7)
=INTERCEPT(C2:C7,B2:B7)

SLOPE returns "the slope of the linear regression line" through the points, which is the variable rate: 15.19 per unit. INTERCEPT returns the fixed part: 85,094. At 10,000 units the regression predicts $236,967 and high-low $237,000, a gap of $33. When the two splits agree this closely, either can be used; when they do not, one of the high or low months is unusual. Excel's regression tool gives the same two figures with R squared alongside.

Why the split matters

• Break-even. Fixed costs divided by contribution per unit is the break-even volume. At a price of $25.00, contribution is $10.00 per unit and the operation breaks even at 8,700 units a month.
• Flexed budgets. A budget built as 87,000 + 15.00 × units can be restated at actual volume, so a cost overrun is measured against what the actual volume should have cost.
• Pricing. Extra volume priced above $15.00 per unit adds profit while it fits inside existing capacity, even if it is below the full cost per unit.
• Cost variance analysis. Only with the split can a variance be separated into the part caused by volume and the part caused by rate.

At a customer level, the variable costs are the ones a customer actually drives, which is the basis of cost to serve per customer.

Where it goes wrong

• No stated range. Calling a cost fixed without stating the range: rent is fixed until a second warehouse is needed.
• One unusual month. High-low uses only two months; if either is unusual (a one-time repair), the split is wrong. Check against a regression on all months.
• Salaried staff that flex. In a business that flexes its team with temporary agency labor, staffing behaves as a step cost, not a fixed one.
• Allocated overhead treated as variable. Overhead presented as a cost per unit makes extra volume look unprofitable when it adds contribution, and leads to rejecting good business. Gross margin vs contribution margin vs net margin shows where the line belongs.
• A stale split. Using last year's rate after prices or processes changed.

Splitting your own costs

Once costs are split, a variance can be split too: how much came from volume and how much from rate. Covirage's cost variance tools build that bridge from your ledger, budget and headcount files with deterministic arithmetic; the external AI model explains the result and never does the calculation. See cost variance analysis: split each cost variance into volume, rate, headcount and timing, from the files you already have. For what the split is used for, see break-even analysis; for allocating the fixed pool by activity, activity-based costing.