What is the difference between days of supply and days of inventory?

Days of supply looks forward: it divides current stock by sales velocity to estimate how many days until you run out. Days of inventory looks backward: it measures how many days your average inventory took to sell over a past period. One is for restock timing; the other is for efficiency and accounting.

How do you calculate days of supply?

Divide your current available units by your daily sales velocity. If you have 600 units and sell 20 a day, you have 30 days of supply. Compare that number to your total lead time to know whether there is still time to reorder before you run out.

Which metric should I use to decide when to reorder?

Days of supply, because it is forward-looking and directly comparable to your lead time. When days of supply falls toward your total lead time, it is time to order. Days of inventory is better for spotting slow movers and measuring how efficiently capital is moving.

February 1, 20266 min readInventory Planning

Days of Supply vs Days of Inventory on Amazon FBA

Days of supply vs days of inventory: what each measures, how to calculate them, and which one tells an Amazon FBA seller when to reorder.

T. Brian Jones

Co-founder & CTO, Inventory Hero

Frequently Asked Questions

What is the difference between days of supply and days of inventory?: Days of supply looks forward: it divides current stock by sales velocity to estimate how many days until you run out. Days of inventory looks backward: it measures how many days your average inventory took to sell over a past period. One is for restock timing; the other is for efficiency and accounting.
How do you calculate days of supply?: Divide your current available units by your daily sales velocity. If you have 600 units and sell 20 a day, you have 30 days of supply. Compare that number to your total lead time to know whether there is still time to reorder before you run out.
Which metric should I use to decide when to reorder?

Written by

T. Brian Jones

Co-founder & CTO, Inventory Hero

T. Brian Jones is co-founder and CTO of Inventory Hero. He leads the engineering behind its Amazon data pipeline, demand forecasting, and the AI platform that lets sellers talk to their live inventory, sales, and supplier data in plain language.

Related reading

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What is days of supply?

Days of supply is how many days your current inventory will last at the current sales pace:

Days of supply = current available units / daily sales velocity

If you hold 600 sellable units and sell 20 a day, you have 600 / 20 = 30 days of supply. It is forward-looking, which is exactly what restock timing needs. See the days of supply definition.

Seller Central shows its own "Days of Supply" figure in the FBA inventory dashboard. It is a useful glance, but it is not always the number to plan on: it can fold in reserved inventory and uses Amazon's own velocity window, so it may differ from a figure you calculate on sellable units over a window you choose. Know which one you are reading before you make a reorder call on it.

The key comparison is against your total lead time. When days of supply falls toward lead time, you are at the edge of being able to reorder in time; once it drops below lead time, a new order cannot arrive before you stock out. That is why days of supply, sales velocity, and the reorder point are really three views of the same decision.

What is days of inventory?

Days of inventory, also called days inventory outstanding (DIO), measures how long your inventory took to sell over a past period:

Days of inventory = (average inventory / cost of goods sold) x days in period

It is backward-looking, an efficiency and accounting metric. A high days-of-inventory number means capital sat in stock a long time before converting to sales, which flags slow movers and cash tied up. It is the inverse view of inventory turnover.

When to use which

Question	Metric
When do I reorder this SKU?	Days of supply
Will a new order arrive in time?	Days of supply vs lead time
How efficiently is my cash moving?	Days of inventory
Which SKUs are slow movers?	Days of inventory

Question

Metric

When do I reorder this SKU?

Days of supply

Will a new order arrive in time?

Days of supply vs lead time

How efficiently is my cash moving?

Days of inventory

Which SKUs are slow movers?

Days of inventory

The mistake is using a backward-looking efficiency metric to make a forward-looking timing decision. Days of inventory tells you a SKU has been slow; days of supply tells you whether you are about to run out. Track your forward cover in the reorder point calculator.

The bottom line

Days of supply is the forward-looking cover number you use to time reorders, calculated as current stock over velocity and read against your lead time. Days of inventory is the backward-looking efficiency number you use to spot slow movers and tied-up cash. Use the first to stay in stock and the second to stay efficient. For the full system, see Amazon FBA restock planning.

Days of Supply vs Days of Inventory on Amazon FBA

Frequently Asked Questions

T. Brian Jones

Related reading

Amazon FBA Inventory Statistics That Actually Matter

Inventory Metrics: Leading vs Lagging Indicators

The Inventory KPIs That Matter Most for Amazon FBA

What is days of supply?

What is days of inventory?

When to use which

The bottom line