Average Inventory Formula: How to Calculate and Use It

Average inventory estimates the typical inventory value or quantity held during a period instead of relying on a single beginning or ending balance.

The simplest formula is:

Average Inventory = (Beginning Inventory + Ending Inventory) ÷ 2

Average inventory example

If a business begins the year with $80,000 of inventory and ends with $120,000:

Average Inventory = ($80,000 + $120,000) ÷ 2 = $100,000

That $100,000 estimate can then be used in metrics such as inventory turnover and DSI.

Why use average inventory?

Inventory changes throughout a period as stock is purchased and sold. Using only ending inventory can make performance metrics misleading if the closing balance is unusually high or low.

Average inventory smooths the measurement so it better reflects the inventory investment held during the period.

Average inventory and turnover

A standard inventory turnover formula is:

Inventory Turnover = Cost of Goods Sold ÷ Average Inventory

If annual COGS is $600,000 and average inventory is $100,000, turnover is six times.

Average inventory and DSI

A common Days Sales of Inventory formula is:

DSI = (Average Inventory ÷ COGS) × Number of Days

Average inventory therefore directly affects both turnover and DSI calculations.

When beginning and ending inventory are enough

The simple two-point average works best when inventory levels are relatively stable and the beginning and ending balances are reasonably representative of the period.

When the simple average can be misleading

Seasonal businesses can have major swings between periods. A retailer may build inventory heavily before the holidays and finish the year with a much lower balance. Using only January 1 and December 31 can miss the peak investment.

In that case, use monthly or weekly inventory balances:

Average Inventory = Sum of Periodic Inventory Balances ÷ Number of Balances

Monthly average inventory example

Suppose quarter-end inventory balances are:

• January: $80,000
• February: $110,000
• March: $140,000

Average = ($80,000 + $110,000 + $140,000) ÷ 3 = $110,000

Using more frequent balances can make the average more representative when inventory is volatile.

Average inventory by units vs. value

The same averaging concept can be applied to units or inventory value, but use the version that matches the metric.

Financial ratios such as turnover commonly use inventory value at cost. Operational planning may use average units by SKU or location.

Average inventory vs. current inventory

Current inventory answers what the business has now. Average inventory describes the typical level held over a historical period. Do not use average inventory to make an immediate stock-availability decision.

How average inventory connects to carrying cost

Carrying-cost analysis often uses average inventory value because holding costs accrue over time rather than only at the period end.

How to reduce average inventory safely

Reducing average inventory can improve turnover and release working capital, but cutting stock blindly can create stockouts. Better approaches include:

• Improve forecasting
• Reduce excess inventory
• Review safety stock
• Shorten supplier lead times
• Use smaller, better-timed orders
• Transfer inventory between locations before purchasing more

See excess inventory and inventory replenishment.

Average inventory by location

For multi-location businesses, average inventory can be calculated per store to compare inventory investment with sales, margin, turnover, or location-level demand.

Common average-inventory mistakes

• Using retail value with cost-based COGS
• Using only ending inventory in turnover
• Using two-point averages for highly seasonal stock
• Mixing units and value
• Comparing periods with different valuation methods

How Stash fits

Stash gives physical businesses visibility into stock and product performance across locations. Accurate historical inventory balances make metrics such as turnover, DSI, and inventory investment more useful.

Frequently asked questions

What is the average inventory formula?

The simplest formula is beginning inventory plus ending inventory divided by two.

Should average inventory use cost or retail value?

Use a valuation basis consistent with the metric. Inventory turnover based on COGS generally uses inventory valued at cost.

Can you calculate average inventory monthly?

Yes. Averaging monthly or weekly balances is often more representative for seasonal or volatile businesses.