Management Accounting · Flexible budgets

Cost Behaviour and Budget Flexing for ACCA MA

Updated 11 October 2026 · Fact-checked

Cost behaviour describes how a cost changes as activity changes. Fixed costs stay constant, variable costs change in proportion to activity, and semi-variable costs have both parts. To flex a budget, split each cost into fixed and variable parts, then recalculate the variable costs at the actual activity level and keep fixed costs unchanged.

Understand Cost Behaviour and Budget Flexing

A cost behaves in a certain way when activity changes. Activity can be units produced, units sold, labour hours or machine hours. You must know the behaviour before you can predict cost at a new activity level.

A variable cost changes in total in direct proportion to activity. Cost per unit stays the same. Direct materials are a typical example. A fixed cost stays the same in total within a relevant range of activity, so cost per unit falls as activity rises. Rent is a typical example.

A semi-variable (mixed) cost has a fixed element and a variable element. A phone bill with a line rental plus a charge per call is an example. A stepped fixed cost is fixed within a range of activity, then jumps to a higher level once that range is exceeded. Supervisor salaries are a typical example: one supervisor covers up to a set number of hours, and a second is needed beyond it.

The relevant range matters. Fixed costs are only fixed within the activity range the business normally works in. Outside it, costs may step up or down. Variable costs are also assumed linear, which is a simplification.

A flexed budget restates the original budget at the actual activity level. You do this because comparing actual results with a budget set for a different activity level is not a fair comparison. To flex, you need each cost split into its fixed and variable parts. Variable costs go up or down with activity. Fixed costs stay as budgeted, unless a step applies.

Key formulas to remember

Total cost: Total cost = Fixed cost + (Variable cost per unit × Activity level); Applies to semi-variable costs. Fixed cost is the intercept, variable cost per unit is the slope.
High-low variable cost per unit: Variable cost per unit = (Cost at highest activity − Cost at lowest activity) ÷ (Highest activity − Lowest activity); Use the highest and lowest activity levels, not the highest and lowest costs, unless they coincide.
High-low fixed cost: Fixed cost = Total cost at either level − (Variable cost per unit × Activity at that level); Check by using the other activity level. You should get the same answer.
Flexed variable cost: Flexed variable cost = Budgeted variable cost per unit × Actual activity; Equivalent to budgeted variable cost × (actual activity ÷ budgeted activity).
Flexed budget cost for a semi-variable item: Flexed cost = Fixed element + (Variable rate × Actual activity); Do not scale the whole semi-variable cost by activity. Only the variable part moves.

How to solve Cost Behaviour and Budget Flexing questions

Use this method for any question on cost behaviour or budget flexing.

1Identify the activity measure the question uses, such as units, hours or sales volume.
2Classify each cost as fixed, variable, semi-variable or stepped. Read the wording for clues such as 'per unit', 'per annum' or 'rises by ₹X for every 1,000 units'.
3Split any semi-variable cost into fixed and variable parts. Use the high-low method if you are given costs at two or more activity levels.
4Check the relevant range. If the new activity is beyond a stated step point, add the extra fixed cost.
5Calculate each variable cost at the new activity level using cost per unit × new activity.
6Keep fixed costs unchanged, apart from any step, and add up fixed, variable and semi-variable costs for the total.
7Check that cost per unit for variable costs is unchanged and that total fixed cost is unchanged. Then answer the exact question asked.

Quickest way: Two-point slope and intercept

When to use it: Use this for any objective test question that gives costs at two activity levels and asks for fixed cost, variable rate or cost at a new level.

Pick the highest and lowest activity levels and write down their costs.
Subtract: change in cost ÷ change in activity gives the variable rate.
Multiply the variable rate by one of the activity levels and subtract from that level's total cost to get fixed cost.
Cost at the new level = fixed cost + variable rate × new activity.
Sense-check: the answer should lie between the two known costs if the new activity lies between the two known levels.

Common mistakes in Cost Behaviour and Budget Flexing

Flexing the whole of a semi-variable cost in proportion to activity.
Students treat the cost as fully variable because part of it changes.
Fix: Split it first. Flex only the variable element and keep the fixed element unchanged.
Using the highest and lowest cost figures instead of the highest and lowest activity levels in the high-low method.
It feels natural to pick the extreme costs.
Fix: Always choose the periods with the highest and lowest activity. Take the costs that go with them.
Flexing fixed costs with activity.
Students apply the same percentage change to every line in the budget.
Fix: Fixed costs stay at the budgeted total within the relevant range. Only variable costs flex.
Ignoring a step in a stepped fixed cost.
The step point is in the question wording and is easy to overlook.
Fix: Compare the activity level with each step threshold before you finalise the fixed cost.
Dividing by the wrong activity level when finding cost per unit for flexing.
Students divide by actual activity instead of budgeted activity.
Fix: Budgeted variable cost per unit = budgeted variable cost ÷ budgeted activity. Then multiply by actual activity.

Worked examples

Example 1

A company's maintenance cost is semi-variable. In March, 4,000 machine hours cost $26,000. In April, 6,500 machine hours cost $33,500. In May, 5,000 machine hours cost $29,000. Using the high-low method, what is the expected maintenance cost for 5,500 machine hours?

Show the solution

Highest activity is 6,500 hours at $33,500. Lowest is 4,000 hours at $26,000.
Variable rate = ($33,500 − $26,000) ÷ (6,500 − 4,000) = $7,500 ÷ 2,500 = $3 per hour.
Fixed cost = $26,000 − (4,000 × $3) = $26,000 − $12,000 = $14,000.
Check with the high level: $33,500 − (6,500 × $3) = $33,500 − $19,500 = $14,000. This matches.
Cost at 5,500 hours = $14,000 + (5,500 × $3) = $14,000 + $16,500 = $30,500.

Answer: $30,500

Example 2

A budget for 10,000 units shows: direct materials $40,000 (variable), factory rent $18,000 (fixed), and power $9,000, of which $3,000 is fixed and the rest is variable. Actual output is 12,000 units. Prepare the flexed budget total cost.

Show the solution

Direct materials per unit = $40,000 ÷ 10,000 = $4. Flexed cost = 12,000 × $4 = $48,000.
Factory rent is fixed, so it stays at $18,000.
Power variable element = $9,000 − $3,000 = $6,000 at 10,000 units, which is $0.60 per unit.
Flexed variable power = 12,000 × $0.60 = $7,200. Power fixed element stays at $3,000, so flexed power = $10,200.
Total flexed cost = $48,000 + $18,000 + $10,200 = $76,200.

Answer: $76,200

Exam tips

Read the activity measure carefully. A question may flex on labour hours rather than units produced.
In multiple response questions, check each statement against the definition. Fixed cost per unit falls as activity rises, but total fixed cost does not change within the relevant range.
For number entry questions, give the answer in the units asked for, such as $ or ₹, and do not round until the final step.
Sketch a quick graph in your head for stepped costs. The cost is flat, then jumps, then flat again.
Always check whether the question asks for the flexed cost, the variance, or the cost per unit. Many wrong answers come from answering a different question.

Practice questions from Flexible budgets

Cost Behaviour and Budget Flexing in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

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Cost Behaviour and Budget Flexing: frequently asked questions

How do you split a semi-variable cost into fixed and variable parts?

The usual exam method is the high-low method. Take the highest and lowest activity levels and divide the difference in cost by the difference in activity to get the variable rate. Then work backwards to find the fixed element.

What is a stepped fixed cost?

It is a cost that stays fixed within a range of activity but jumps to a new level when activity passes a threshold. Supervisor salaries and rented space are common examples. When you flex a budget, check whether the new activity crosses a step.

Why do we flex a budget?

Actual activity usually differs from the budgeted level. Flexing restates the budget at actual activity so you compare like with like. This shows whether costs were controlled, not just whether volume changed.

Does fixed cost per unit change when activity changes?

Yes. Total fixed cost stays the same within the relevant range, so fixed cost per unit falls as activity rises and rises as activity falls. Variable cost per unit stays constant.