Fixed, Variable, and Mixed Costs

The Columbus line that ran half empty

Columbus Plant 3 produces Heat & Eat Chicken Bowls on a blast-freeze line built for 320,000 units per month. In May, demand delivered only 248,000 bowls. Utilities still ran, salaried supervisors still clocked in, and depreciation on freezers still posted. Variable ingredient and tray costs fell with volume, but fixed plant costs did not shrink proportionally.

James Okoro asked Priya Shah a blunt question: "If we run 248,000 instead of 320,000, what is cost per bowl?" The answer depends on separating fixed costs (unchanged in total within the relevant range), variable costs (move in total with output), and mixed costs (part fixed, part variable). Without that split, CVP (cost-volume-profit) analysis in Unit 3 and flexible budgets in Unit 4 will be wrong before the first spreadsheet row.

Northwind Foods is a mid-size packaged foods manufacturer selling through grocery and food-service channels and the anchor company for ACC 102. Annual revenue is approximately $420M across 3 plants and 180 SKUs. CFO Maria Chen, VP Operations James Okoro, and Plant Controller Priya Shah rely on standard costing, contribution margin, and budget variance analysis to run Omaha (dry goods and granola (Plant 1)), Fresno (sauces and condiments (Plant 2)), and Columbus (frozen Heat & Eat meals (Plant 3)).

ACC 101 (Financial Accounting) taught GAAP external reporting: income statement COGS, inventory on the balance sheet, and audited totals. ACC 102 uses overlapping facts for internal decisions: product-level costs, contribution margin, budgets, and variances managers act on before GAAP closes the quarter. James uses cost behavior maps to decide overtime, shift count, and whether to pause a sauce SKU to free kettle time.

Fixed costs within a relevant range

Fixed costs stay constant in total over a relevant range of activity. Columbus monthly fixed manufacturing costs (lease, salaried supervision, depreciation on blast freezers) approximate $1.8M whether output is 200,000 or 320,000 bowls. Fixed cost per unit rises when volume falls: $1.8M ÷ 248,000 = $7.26/bowl versus $1.8M ÷ 320,000 = $5.63/bowl at capacity.

Fixed does not mean forever. Outside the relevant range (adding a third shift, opening a new line), fixed costs step up. Managers who treat fixed as variable under-price promos; managers who treat variable as fixed overstate break-even.

Variable costs and unit economics

Variable costs change in total in proportion to volume within the relevant range. For bowls, variable manufacturing cost is approximately $3.85 per unit (protein, rice, tray, packaging). Total variable cost at 248,000 units = $954,800; at 320,000 units = $1,232,000. Variable cost per unit stays near $3.85 if yields are stable.

Northwind's SKU cards show variable material, variable labor, and variable packaging. Fixed manufacturing overhead is applied separately using predetermined rates from Unit 1 lessons.

Unit CM supports short-run pricing and mix; it excludes fixed plant and S&A (selling, general, and administrative) until coverage is tested.

Mixed costs and separation techniques

Mixed costs contain both fixed and variable elements. Columbus utilities might include a $42,000/month base service fee plus $0.018 per kWh above a threshold. Maintenance contracts often combine annual retainers with per-call charges.

Accountants separate mixed costs with the high-low method (cost at highest volume minus cost at lowest volume, divided by volume difference) or regression on monthly data. Priya uses twelve months of production hours and utility bills on Fresno kettles to estimate variable rate before budgeting.

Total cost, average cost, and marginal cost

Total cost = fixed + variable. Average cost = total ÷ units; it blends fixed and variable and falls with volume when fixed dominates. Marginal cost is the cost of one more unit, close to variable cost in the short run.

James evaluates an incremental 20,000-case club order on marginal/variable cost, not average cost that embeds idle capacity. Maria evaluates whether the plant earns enough contribution to cover $3.2M/month company-wide fixed manufacturing overhead across three plants.

Manufacturing versus S&A behavior

Northwind separates manufacturing cost behavior (plant floor) from S&A (HQ, sales force, brand advertising ~$1.85M/month). Both contain fixed and variable pieces, but GAAP treats most manufacturing as product cost while S&A is period expense.

A national TV campaign is largely fixed S&A. Broker commissions on incremental cases are variable S&A. Promo decisions need both plant variable cost and variable selling cost to compute true incremental CM.

Priya needs a variable utility rate for Columbus flexible budgets. She pulls the five highest and lowest production months (bowls produced versus utility cost).

Part A: High-low data

Check: Oct − Feb volume = 113,000 bowls ✓

Part B: Variable and fixed components

Variable rate = ($186,400 − $142,600) ÷ (318,000 − 205,000) = $43,800 ÷ 113,000 = $0.3876 per bowl (approximate).

Fixed component = $186,400 − ($0.3876 × 318,000) = $63,143/month (rounded).

Check: $63,143 + ($0.3876 × 248,000) ≈ $159,268 expected at May volume ✓

Part C: Cost per bowl at two volumes

At 248,000 bowls (May): Variable mfg $3.85 + allocated fixed plant $7.26 + utilities ~$0.64 = ~$11.75 full manufacturing (simplified; excludes allocated corp OH).

At 320,000 bowls (capacity): Fixed plant per bowl $5.63; utilities per bowl lower. Average cost falls ~$1.40/bowl with volume recovery.

James uses this to show why underutilization hurts margin even when commodity costs are flat.

Part D: Managerial read

Do not use February utility cost per bowl as the standard for a July heat wave without re-estimating the variable kWh rate. Fixed supervision is controllable only over multi-year horizons; James should not treat salaried headcount as variable when evaluating a two-month promo.

Fresno produced 680,000 sauce cases in March with $1.42 variable cost per case. April pushed to 712,000 cases with overtime; variable cost rose to $1.48 per case because labor premium and energy surcharges are volume-linked but not linear. Priya flags the relevant range: up to 690,000 cases, standard labor rate holds; above that, variable rate steps up $0.04/case. James schedules April volume at 705,000 knowing the step.

Check: incremental 25,000 cases above 690,000 × $0.04 = $1,000 extra variable; remainder of $0.06 increase driven by paste price ✓

Omaha granola fixed manufacturing costs $980,000/month; variable cost $2.18/case. Actual volume 395,000 cases; capacity 450,000 cases.

(1) Compute fixed cost per case at actual and at capacity. (2) Compute total manufacturing cost at actual volume. (3) If volume rises by 30,000 cases, approximate incremental cost per additional case.

Solution

Fixed per case actual: $980,000 ÷ 395,000 = $2.48. At capacity: $980,000 ÷ 450,000 = $2.18.

Total mfg cost: ($2.18 + $2.48) × 395,000 = $1,835,700 (or $980,000 + $2.18×395,000).

Incremental 30,000 cases ≈ $2.18 variable each in short run; fixed total unchanged → $65,400 incremental cost.

Check: $1,835,700 ÷ 395,000 = $4.66 average ✓

Key takeaways

• Fixed costs are constant in total within the relevant range; per-unit fixed moves inversely with volume.
• Variable costs scale with output; unit variable cost is the short-run marginal cost proxy.
• Mixed costs require high-low or regression before flexible budgeting.
• Northwind SKU CM uses variable manufacturing; promos may also need variable S&A.
• Average cost blends behavior types; incremental decisions use marginal cost logic.

1. Plot six months of a mixed cost at your firm. Estimate variable rate with high-low.
2. For one SKU, compute unit CM and fixed cost per unit at 80% and 100% of capacity.
3. Continue to Lesson 3: Direct and Indirect Costs.

Fixed, Variable, and Mixed Costs in Northwind's operating cadence

Northwind's Columbus frozen line carries fixed costs (lease, salaried supervisors, depreciation on blast freezers) near $1.8M per month whether the line runs 200,000 or 320,000 bowls. Variable costs (chicken, rice, tray packaging) move at roughly $2.41 per bowl at current commodity prices. Mixed costs (utilities, maintenance contracts) contain a fixed service fee plus usage-sensitive components. High-low or regression on twelve months of production and utility bills separates the sticky base from the volume-sensitive portion before CVP (cost-volume-profit) analysis in Unit 3.

CFO Maria Chen, VP Operations James Okoro, and Plant Controller Priya Shah review cost classification and decision relevance in monthly plant controller meetings before data hardens into GAAP quarter-close. Priya Shah's team posts standard cost updates, volume variances, and mix effects to shared folders James Okoro's operators can action within 48 hours. Maria Chen uses the same underlying transactions ACC 101 will later classify for external statements, but managerial reports may show segment margin, transfer prices, and flexible budget comparisons not required in the 10-K (annual SEC filing).

Walk the arithmetic habit every controller expects. When fixed, variable, and mixed costs produces a rate, ratio, or variance, show the numerator definition, denominator definition, period, and plant scope. If Omaha and Columbus use different allocation bases, state why (machine intensity vs labor intensity). A single blended rate is simpler but can misprice SKUs; ABC (activity-based costing) fixes that complexity with more measurement cost.

Extended scenario: cross-plant read for Fixed, Variable, and Mixed Costs

Picture a Tuesday S&OP (sales and operations planning) review. Grocery sales beat forecast on NorthWind Granola 12oz by 6% while food-service sauce lagged. Contribution margin dollars rose roughly $71K on granola alone at $2.81 unit CM, but Fresno faced overtime on sauce kettles and Columbus cold storage approached 96% utilization. Fixed, Variable, and Mixed Costs is how leadership decides whether to pull forward Omaha oven maintenance, expedite tomato paste, or reprice a low-CM promotional pack.

Reconcile before recommending. Fixed manufacturing overhead budget $3.2M per month must be covered by portfolio CM after variable costs. At current granola CM ratio 56.3%, price cuts require explicit volume lift calculations; see Unit 3 CVP. Budget variances (Unit 4) will later decompose whether misses were volume, price, or efficiency.

Stakeholder tension is normal. James Okoro protects line reliability and food safety audits. Maria Chen protects covenant headroom and EPS (earnings per share) guidance. Commercial leads protect slotting and brand share. Fixed, Variable, and Mixed Costs gives shared vocabulary so debate targets assumptions (standard oat price, changeover minutes, transfer price) instead of personalities.

Mechanics checklist: Fixed, Variable, and Mixed Costs

Use the same checklist Priya posts on every analysis deck: (1) Cost object defined (SKU, job, plant, customer). (2) Time horizon labeled short-run vs long-run; capacity decisions differ. (3) Relevant costs isolated; sunk and allocated corporate charges scrutinized. (4) Denominator for any rate shown (machine hours, cases, labor dollars). (5) Check line ties detail to control totals within $1,000 unless immateriality policy says otherwise.

Spreadsheet replication: separate data (volumes, prices, hours) from formulas (rates, variances, CM). Color inputs blue; never embed hard-coded totals in CM formulas. Tie units × unit CM = total CM and fixed + variable = total manufacturing cost on every tab. Northwind rejects decks where margin percent disagrees with dollar CM due to mixed rounding.

For cost classification and decision relevance, link forward and back. Earlier cost classification lessons explain why a cost is fixed or indirect; later variance and decision lessons consume the same standard cost database. Breaking the chain (e.g., changing oat standard without updating budget and transfer price) creates silent contradictions across plants.

ACC 101 bridge and external reporting

Financial accounting in ACC 101 answered: what happened, in GAAP language, for outsiders? Managerial accounting answers: what should we do next quarter, with product and plant detail? Northwind's inventory on the balance sheet equals capitalized product cost; COGS on the income statement releases those costs when customers take title. Period costs (HQ, ads) never inventory.

Differences are legitimate. Managerial standard costs may differ from actual GAAP costs until variances close at period end. Overhead allocation choices for pricing can include discretionary marketing sub-pools excluded from inventory capitalization under GAAP. Maria insists teams label GAAP view vs managerial view on every slide to prevent audit committee confusion.

When fixed, variable, and mixed costs touches inventory or COGS, articulate the flow: beginning FG (finished goods) + COGM (cost of goods manufactured) − COGS = ending FG. Weighted-average process costing at Fresno must match pounds of sauce in tanks to financial pounds shipped.

Practice extension: self-check without peeking

Open a blank workbook tab. Row 1: write the Northwind decision Fixed, Variable, and Mixed Costs informs this month. Row 2: list three variable and three fixed costs for the relevant plant. Row 3: compute unit CM for NorthWind Granola 12oz at price $4.99 and variable $2.18. Row 4: state one relevant and one irrelevant cost for a hypothetical SKU drop decision. Row 5: define the check line you would show Maria.

Compare your rows to this lesson's worked examples. Gaps mark what to re-read. If you work outside manufacturing, map plant → team, SKU → product line, and OH → shared services; the logic survives.

Executive questions on Fixed, Variable, and Mixed Costs

"How sure are we?" Show assumptions, sensitivity on volume ±5%, and whether data is actual, flexed budget, or forecast. "What is the dollar impact?" Translate units to CM dollars and fixed coverage. "What changes next month?" Name owners: purchasing for price variances, maintenance for downtime, sales for mix. "Does this match GAAP?" Flag timing differences between managerial standards and financial close.

Northwind's credible narrative is four bullets: recommendation, quantified CM or variance impact, key assumption, and metric that would falsify the view within 30 days. Fixed, Variable, and Mixed Costs is operational only when those bullets reference this lesson's mechanics, not generic strategy language.

Numerical reconciliation drill (Fixed, Variable, and Mixed Costs)

Month-end tie-out Priya runs: (A) sum of SKU margins reconciles to plant contribution within 0.3%. (B) OH applied at standard rate reconciles to actual OH pool ± under/over-applied balance. (C) Units produced × standard hours per unit reconciles to payroll hours ± overtime flag. (D) Pounds issued from warehouse reconciles to BOM (bill of materials) allowance ± scrap ticket.

Document materiality. Northwind sets $25,000 investigation threshold for single-plant variances unless food safety or retailer OTIF is implicated. Smaller variances roll into trend charts for cost classification and decision relevance. This discipline prevents chasing noise while catching structural drift in fixed, variable, and mixed costs drivers.