Days of Inventory Calculator

About the Days of Inventory Calculator

Days of Inventory (DOI), also called Days Sales of Inventory (DSI), measures how many days on average it takes to sell your entire inventory. It is the inverse expression of inventory turnover, providing a more intuitive time-based metric for inventory management.

For e-commerce sellers, every day inventory sits in a warehouse or fulfillment center costs money in storage fees, insurance, and opportunity cost. Reducing DOI directly improves cash flow and reduces the risk of obsolescence, especially for products with short shelf lives or rapidly changing trends.

This calculator accepts either a cost-based approach (average inventory / daily COGS) or a unit-based approach (units on hand / average daily sales) to give you a comprehensive view of your inventory holding period.

When This Page Helps

Days of inventory gives you a concrete number to optimize. Instead of abstract ratios, you see exactly how many days your money is tied up in stock. This helps set purchasing cadences, negotiate payment terms, and identify slow-moving products that erode profitability.

How to Use the Inputs

1. Enter your average inventory value (or units on hand).
2. Enter your annual COGS (or average daily sales in units).
3. Choose whether to calculate in dollars or units.
4. Review your DOI result and benchmark rating.
5. Target reducing DOI by optimizing purchasing frequency and quantities.
6. Compare DOI across product categories to find optimization opportunities.

Example Calculation

Tips & Best Practices

• Lower DOI generally means better cash flow, but don't go so low that you risk stockouts.
• Track DOI trends over time — increasing DOI may signal declining demand or overpurchasing.
• Compare your DOI to your payment terms with suppliers. Ideally, you sell inventory before the bill is due.
• SKUs with DOI above 90 days should be reviewed for markdown or discontinuation.
• Use DOI to set inventory holding policies by product category.
• Factor in lead times when setting DOI targets — longer lead times require higher DOI.

DOI and Cash Conversion Cycle

Days of Inventory is one component of the cash conversion cycle (CCC), which also includes days sales outstanding and days payable outstanding. A shorter CCC means you're converting investment into cash faster. Reducing DOI is often the most impactful lever for improving CCC in e-commerce.

Category-Specific DOI Benchmarks

Electronics: 20–45 days. Apparel: 45–90 days. Health and beauty: 30–60 days. Home goods: 45–75 days. Grocery: 5–20 days. Use these benchmarks as starting points but calibrate to your specific product mix and business model.

DOI Optimization Strategies

Implement ABC analysis to focus DOI reduction efforts on high-value A items. Use vendor-managed inventory where possible. Consider dropshipping for long-tail products with high DOI. Negotiate consignment arrangements for new or unproven products to keep DOI low while testing market demand.

Frequently Asked Questions

• What is a good days of inventory number?

  For e-commerce, 30–60 days is generally healthy. Fast-moving consumer goods should be under 30 days. Seasonal or specialty items may run 60–90 days. Above 90 days typically signals overstocking or weak demand that needs attention.

• What is the difference between DOI and DSI?

  They are the same metric with different names. Days of Inventory (DOI) and Days Sales of Inventory (DSI) both measure the average number of days it takes to sell through your inventory. The formula and interpretation are identical.

• How does DOI affect my Amazon FBA storage fees?

  Amazon charges monthly storage fees that increase for inventory stored over 181 days and again over 365 days. Keeping your DOI below 90 days for FBA inventory helps avoid long-term storage surcharges that can severely impact profitability.

• Should I use beginning or average inventory for DOI?

  Average inventory is preferred because it smooths out fluctuations within the period. Calculate it as (beginning inventory + ending inventory) / 2, or use the average of monthly ending values for greater accuracy.

• How do I reduce my days of inventory?

  Order smaller quantities more frequently, improve demand forecasting, run promotions on slow-moving stock, implement just-in-time ordering, and discontinue consistently slow SKUs. Each of these strategies reduces the time inventory sits unsold.

• Can DOI vary by season?

  Absolutely. Most businesses see DOI drop during peak selling seasons when inventory moves faster and increase during off-peak periods. Calculate DOI by quarter to understand your seasonal pattern and adjust purchasing accordingly.