Inventory Costing Methods: Fundamentals

TL;DR

Inventory costing methods determine how you value your inventory and the Cost of Goods Sold (COGS). The main methods are FIFO, LIFO, and Weighted-Average, each impacting your financial statements differently. Choosing a method depends on your business goals and accounting standards.

1. The Mental Model

Imagine you have a pile of identical t-shirts bought at different prices over time. When you sell one, which price do you assume it came from? That assumption is your inventory costing method.

2. The Core Material

When a company buys inventory at different times and prices, it needs a systematic way to assign a cost to the items sold (Cost of Goods Sold or COGS) and the items remaining (Ending Inventory). This is crucial because it directly affects your reported profit and asset value.

The three primary methods are:

2.1 First-In, First-Out (FIFO)

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Concept: Assumes that the first goods purchased are the first ones sold.
Impact:
* COGS: Uses the oldest (often lowest, in rising price environments) costs.
* Ending Inventory: Consists of the most recently purchased (often highest) costs.
* Net Income: Tends to be higher in periods of rising costs because lower COGS leads to higher gross profit.
* Balance Sheet: Ending inventory values usually reflect current market prices better.
Analogy: A grocery store, where older produce is sold first to prevent spoilage.

2.2 Last-In, First-Out (LIFO)

Concept: Assumes that the last goods purchased are the first ones sold.
Impact:
* COGS: Uses the most recent (often highest, in rising price environments) costs.
* Ending Inventory: Consists of the oldest (often lowest) costs.
* Net Income: Tends to be lower in periods of rising costs because higher COGS leads to lower gross profit.
* Balance Sheet: Ending inventory values can be significantly understated compared to current market prices.
Important: LIFO is not permitted under IFRS (International Financial Reporting Standards). It is allowed under U.S. GAAP.
Analogy: A pile of bricks, where you take the ones on top first because they're easiest to reach.

2.3 Weighted-Average Method

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Concept: Calculates the average cost of all available inventory for sale and uses this average for both COGS and ending inventory.
Impact:
* COGS & Ending Inventory: Uses the same average cost per unit.
* Net Income: Produces a net income figure that falls between FIFO and LIFO during periods of changing prices.
* Simplicity: Often simpler to apply, especially in a perpetual inventory system if calculated after each purchase.
Formula: Average Cost per Unit = (Total Cost of Goods Available for Sale) / (Total Units Available for Sale)

2.4 Inventory Systems

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The choice of costing method often interacts with the inventory system used:
* Perpetual System: Inventory records are continuously updated after each purchase and sale. COGS is recorded with each sale.
* Periodic System: Inventory is physically counted at the end of an accounting period. COGS is calculated at the end of the period.

Here's a quick look at how the methods generally compare:

graph TD
    A["Inventory Costing Methods"] --> B["FIFO (First-In, First-Out)"]
    A --> C["LIFO (Last-In, First-Out)"]
    A --> D["Weighted-Average"]

    B --> B1["Oldest Costs --> COGS"]
    B --> B2["Newest Costs --> Ending Inventory"]

    C --> C1["Newest Costs --> COGS"]
    C --> C2["Oldest Costs --> Ending Inventory"]
    C1 --- LIFO_NOT_IFRS_NOTE["(Not allowed under IFRS)"]

    D --> D1["Average Cost --> COGS"]
    D --> D2["Average Cost --> Ending Inventory"]

    B1 & C1 & D1 --> E["Impacts Net Income"]
    B2 & C2 & D2 --> F["Impacts Balance Sheet (Assets)"]

3. Worked Example

Let's use the following purchases and sales data for a product called "Widget" for January:

• Jan 1: Beginning Inventory: 10 units @ $10 each = $100
• Jan 5: Purchase: 20 units @ $12 each = $240
• Jan 15: Sale: 15 units
• Jan 20: Purchase: 15 units @ $13 each = $195
• Jan 25: Sale: 20 units

We need to calculate COGS and Ending Inventory for January using each method under a periodic inventory system (meaning we calculate at month-end).

Total Goods Available for Sale:
Units: 10 + 20 + 15 = 45 units
Cost: $100 + $240 + $195 = $535

Total Units Sold: 15 + 20 = 35 units
Ending Inventory Units: 45 - 35 = 10 units

1. FIFO Method:

• COGS (35 units):

  • 10 units from Jan 1 @ $10 = $100
  • 20 units from Jan 5 @ $12 = $240
  • 5 units from Jan 20 @ $13 = $65 (The remaining 5 units needed to make 35 total)
  • Total COGS = $100 + $240 + $65 = $405

• Ending Inventory (10 units):

  • These are the latest units purchased.
  • 10 units from Jan 20 @ $13 = $130
  • Total Ending Inventory = $130

2. LIFO Method:

• COGS (35 units):

  • 15 units from Jan 20 @ $13 = $195
  • 20 units from Jan 5 @ $12 = $240
  • Total COGS = $195 + $240 = $435

• Ending Inventory (10 units):

  • These are the earliest units purchased.
  • 10 units from Jan 1 @ $10 = $100
  • Total Ending Inventory = $100

3. Weighted-Average Method:

• Weighted-Average Cost per Unit:

  • Total Cost of Goods Available = $535
  • Total Units Available = 45 units
  • Average Cost per Unit = $535 / 45 = $11.8889 (approx.)

• COGS (35 units):

  • 35 units * $11.8889 = $416.11 (approx.)
  • Total COGS = $416.11

• Ending Inventory (10 units):

  • 10 units * $11.8889 = $118.89 (approx.)
  • Total Ending Inventory = $118.89

Summary for January:

Notice how FIFO results in the lowest COGS (and thus highest gross profit) and highest ending inventory, while LIFO results in the highest COGS (and lowest gross profit) and lowest ending inventory during this period of rising prices. Weighted-average falls in between.

4. Key Takeaways

• FIFO assumes oldest costs are expensed first, leaving newest costs in inventory.
• LIFO assumes newest costs are expensed first, leaving oldest costs in inventory.
• Weighted-Average uses an average cost for both COGS and ending inventory.
• In periods of rising prices, FIFO generally leads to higher net income and higher ending inventory values.
• In periods of rising prices, LIFO generally leads to lower net income and lower ending inventory values (if allowed).
• The choice of method must be consistently applied and impacts financial ratios significantly.
• LIFO is permitted under U.S. GAAP but explicitly prohibited under IFRS.

Common Mistakes to Avoid:
- Inconsistent Application: Once you choose a method, you must stick with it unless there's a very good reason to change, and disclose the change.
- Ignoring Price Trends: Not understanding how price changes (inflation/deflation) will affect reported profit under each method.
- Confusing Perpetual vs. Periodic: The calculations for COGS and ending inventory can differ slightly between inventory systems, especially for Weighted-Average and LIFO.
- Applying LIFO under IFRS: This is a big no-no for companies reporting under IFRS.

5. Now Try It

Using the same data from the Worked Example:
* Beginning Inventory: 10 units @ $10
* Purchase: 20 units @ $12
* Sale: 15 units
* Purchase: 15 units @ $13
* Sale: 20 units

Imagine the prices were decreasing:
* Jan 1: Beginning Inventory: 10 units @ $13 each
* Jan 5: Purchase: 20 units @ $12 each
* Jan 15: Sale: 15 units
* Jan 20: Purchase: 15 units @ $10 each
* Jan 25: Sale: 20 units

Calculate the COGS and Ending Inventory for January using FIFO, LIFO, and Weighted-Average (periodic system).

What success looks like: You'll have three sets of COGS and Ending Inventory figures, and you should notice how the relationship between FIFO, LIFO, and Weighted-Average changes when prices are decreasing compared to when they were increasing.