Fulfillment Lexicon

Days of Inventory

Days of Inventory (DOI) is the number of days current inventory will last at the expected sales rate. Also called days of supply or days inventory outstanding.

Topic: Metrics & Unit Economics Entry KPI-068

Why it matters for ecommerce brands

Amazon and many 3PLs price long-term storage by age, so tracking DOI per SKU avoids aged-inventory surcharges.

Formula and calculator

DOI = units on hand ÷ average daily units sold

Finance version: (average inventory ÷ COGS) × 365.

Units version

The example values are filled in. Change any number to see your own result.

units
units/day

Days of inventory = 30.0 days

Finance version

The example values are filled in. Change any number to see your own result.

$
$

Days of inventory = 60.8 days

Common questions

How do you calculate days of inventory for one SKU?
Divide units on hand by average daily units sold. A SKU with 900 units on hand selling 30 units a day has 30 days of inventory.

Cite this entry

"Days of Inventory (DOI)." Fulfillment Lexicon, last reviewed October 16, 2026. https://fulfillment.wiki/glossary/days-of-inventory/

Written by Fulfillment Lexicon Editorial Team. Last reviewed October 16, 2026. How we write definitions.

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