Monthly Inventory Turnover Formula
Learn how monthly inventory turnover, average inventory, and days inventory on hand are calculated from inventory balances and cost of goods sold.
Monthly inventory turnover measures how often the average inventory held during a month was sold or used at cost. It helps put inventory movement into context, while days inventory on hand translates that turnover rate into an estimated number of days of stock held.
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Monthly inventory turnover
Where:
First calculate the average inventory value for the month. Then divide the month's cost of goods sold by that average value to estimate how many times inventory turned over.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| COGS - Monthly cost of goods sold | The cost value of inventory sold or consumed during the month, excluding sales margin. | currency |
| BI - Beginning inventory | The inventory value at the start of the month, measured using the same valuation basis as cost of goods sold. | currency |
| EI - Ending inventory | The inventory value at the end of the month, measured using the same valuation basis as cost of goods sold. | currency |
| AI - Average inventory | The simple mean of beginning and ending inventory values. | currency |
| D - Days in month | The number of calendar days in the reviewed month. | days |
| T - Monthly inventory turnover | The number of times average inventory was sold or used during the month. | number |
| DIO - Days inventory on hand | The estimated average number of days inventory remained on hand during the month. | days |
Step-by-Step Calculation
Record comparable inventory values
Use beginning and ending balances prepared on the same inventory valuation basis as monthly cost of goods sold.
BI = beginningInventory; EI = endingInventory
Calculate average inventory
Add the opening and closing inventory values, then divide by two.
averageInventory = (beginningInventory + endingInventory) / 2
Calculate monthly turnover
Divide the month's cost of goods sold by average inventory. The result is expressed as times per month.
inventoryTurnover = costOfGoodsSold / averageInventory
Estimate days inventory on hand
Divide the number of days in the month by turnover to estimate the average stock days on hand.
daysInventoryOnHand = daysInMonth / inventoryTurnover
Example: inventory turnover for a 30-day month
Calculate average inventory
($50,000 + $40,000) / 2
$45,000
Calculate monthly turnover
$180,000 / $45,000
4.00 times
Calculate days inventory on hand
30 / 4.00
7.5 days
Final Result
Monthly inventory turnover is 4.00 times, average inventory is $45,000, and estimated days inventory on hand is 7.5 days.
Assumptions
- ✓Beginning inventory, ending inventory, and cost of goods sold are measured using the same inventory valuation basis.
- ✓Cost of goods sold represents inventory sold or consumed in the selected month, not sales revenue.
- ✓Average inventory is estimated using only the beginning and ending balances.
- ✓The selected number of days reflects the calendar length of the month.
Limitations
- !A simple beginning-and-ending average may not reflect large inventory fluctuations within the month.
- !Returns, write-downs, transfers, stock corrections, and unusual purchases can affect interpretation.
- !A higher turnover rate is not automatically better if it results from stock shortages or missed demand.
- !Comparisons between businesses can be misleading when product mix, inventory methods, seasonality, or operating models differ.
Common Mistakes to Avoid
Using sales revenue instead of cost of goods sold, which can overstate turnover because revenue includes margin.
Combining inventory values and cost of goods sold prepared under different valuation methods.
Using an annual cost of goods sold figure with a monthly average inventory balance.
Comparing a seasonal peak month with a normal month without accounting for demand and stocking patterns.
Treating days inventory on hand as the exact age of every item rather than an average estimate.
Related Formulas
Frequently Asked Questions
What is the formula for monthly inventory turnover?
Monthly inventory turnover equals monthly cost of goods sold divided by average inventory. Average inventory is beginning inventory plus ending inventory, divided by two.
How do you calculate average inventory for one month?
Add the beginning inventory value and ending inventory value, then divide the total by two. This calculator uses that simple average.
Why is cost of goods sold used instead of sales revenue?
Inventory and cost of goods sold are normally measured at cost. Sales revenue includes markup or margin, so it does not provide a like-for-like turnover measure.
How are days inventory on hand calculated?
Days inventory on hand equals the number of days in the month divided by monthly inventory turnover. In the example, 30 divided by 4 equals 7.5 days.
What does 4 times monthly inventory turnover mean?
It means monthly cost of goods sold was four times the average inventory value held during that month. It is an accounting movement measure, not a count of physical replenishment orders.
Can monthly inventory turnover be compared with annual turnover?
It can be compared only after aligning the periods and inputs. Monthly turnover uses one month's cost of goods sold and inventory average, while annual turnover uses annual figures.
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