Accounting Ver10. “Investment Feasibility Should Be Evaluated Using Contribution Margin — Not Gross Margin” ── Why relying on Gross Margin will always lead to wrong decisions

shigenoritanaka3
4月22日
読了時間: 3分

　　　　　　　　　　　　　　　　　　　　　　　　　　　　Apr 22, 2026

Thank you for reading.

In this article, I will explain a common mistake many companies make when evaluating investments: using Gross Margin (GM) as the basis for feasibility analysis.

🟦 Why Gross Margin leads to incorrect investment decisions

To be clear: GM should not be used for investment evaluation.

The reason is simple. The Cost of Sales (COS) contains fixed costs, such as:

Direct labor
Indirect labor
Factory fixed costs
HQ allocations
Depreciation

Because fixed costs are mixed into COS, GM does not reflect the true cost structure that changes with investment.

For more detail, please refer to:

Accounting Ver05. “Why Financial Accounting Alone Cannot Support Sound Decision-Making — The Structural Issue of Fixed Costs Embedded in Gross Margin and the Importance of a Management Accounting View

🟦 What happens when you use GM for investment decisions

✔ For capacity expansion investments

GM underestimates the return. Because COS includes fixed costs that do not change with increased production, GM makes it appear as if costs increase more than they actually do.

✔ For efficiency-improvement or cost-reduction investments

COS overestimates the savings. It makes fixed costs - such as labor or factory overhead - look “reducible,” even though they will not decrease without structural changes.

In both cases, using GM distorts the economics of the investment.

🟦 Investment feasibility must be evaluated using Contribution Margin (CM)

Contribution Margin is defined as:

Sales − Variable Cost = Contribution Margin

Key points:

Variable Cost = costs that change in proportion to sales or production
Labor should be treated as a fixed cost unless the investment directly reduces headcount.
CM shows how much the business contributes to covering fixed costs

In other words:

CM isolates only the costs that change with the investment and shows how much the investment contributes to fixed-cost coverage.

This is why CM is the correct metric for investment feasibility.

🟦 Understanding CM through the cost structure: fixed cost, variable cost, and break-even point

When you visualize fixed costs, variable costs, and the break-even point, it becomes immediately clear why GM is inappropriate and CM is essential.

Please see the diagram below:

🟦 Marginal Profit vs. Contribution Margin

1. Marginal Profit

= Sales - Direct Material - Direct Outsourcing Cost

Captures only part of the variable costs
Better than GM because it excludes fixed costs
But still insufficient for investment decisions because it does not capture the full set of variable costs

2. Contribution Margin

=Sales - All variable costs included in manufacturing

Examples: