What is the days of cover formula?

Days of cover = usable stock on hand / average daily usage. A part with 300 usable units and an average usage of 20 a day has 15 days of cover.

Is days of coverage the same as days of cover?

Yes. Days of cover, days of coverage, days of supply and stock cover are names for the same measure: how many days your usable stock will last at your average usage.

What is a good number of days of cover?

There is no single good number. It should be at least the supplier's current lead time plus a buffer for variability on parts you reorder, and much lower on parts you are phasing out. Compare cover with lead time part by part instead of chasing one target for all items.

Is days of cover the same as days inventory outstanding?

No. Days inventory outstanding is a financial ratio for the whole inventory, based on cost of goods sold. Days of cover is an operational measure for one item, based on its own usage.

Should I use forecast or past usage?

Past usage is simpler and auditable. A forecast can be better for parts with a known change ahead, but it adds forecast error. Many teams start with past usage and override it where they know demand will change.

How often should days of cover be recalculated?

At least as often as you review purchase orders, usually daily or weekly, because stock and usage move every day.

Glossary

What is days of cover? Formula, example and how to use it

Days of cover: Days of cover is how many days the stock you can use today will last at your average daily usage.

Days of cover = usable stock on hand / average daily usage

By Roufaida Bouchamha, founder

The definition

Days of cover (also called days of coverage, days of supply or stock cover) answers one question: if nothing new arrives, how many days until this part runs out? It turns a stock quantity, which means little on its own, into time, which every planner and buyer can compare with a lead time or a delivery date.

How to calculate it

Usable stock: on-hand quantity that can actually be consumed. Exclude stock on quality hold, blocked, or reserved for another site if your ERP tracks it separately.
Average daily usage: quantity consumed or shipped over a fixed window, divided by the number of days in the window. Twelve weeks is a common window; use the same one for every part.
Divide the first by the second. A part with 300 units and usage of 20 a day has 15 days of cover.

Illustrative example, not a customer result

Two parts look similar in the ERP: 500 units of a bracket and 500 units of a seal. The bracket is used at 10 a day, so it has 50 days of cover. The seal is used at 125 a day, so it has 4 days. Same quantity, very different urgency. If the next seal delivery is due in 9 days, the seal is a shortage risk and the bracket is not.

Why it matters more than quantity or days late

Days of cover is the common unit that lets you compare a stock position with a supplier date. Put it next to the date the next order will really arrive and you know whether a part runs short. Sort a past-due purchase order list by the cover of the part behind each order, shortest first, and you get a chase list that puts the orders that can stop production at the top.

Where the number misleads

New parts have no usage history: flag them instead of showing infinite cover.
Seasonal or lumpy demand makes a 12-week average too high or too low. Check the trend before trusting the figure.
Usage not recorded (backflushing switched off, consumption booked late) inflates cover. A part that shows 200 days of cover but keeps running short usually has this problem.
Cover tells you when a part runs out, not which customer order or work order it affects. Linking the two still needs your planners and your bill of materials.

Days of cover and reorder points

A reorder point is set so that an order is placed while there is still enough cover to last the lead time. When cover falls below the supplier's current lead time and no order is on the way, the part will run short whatever you do next. That is why cover is most useful read next to the lead time suppliers quote today, not the one set in the ERP years ago (see lead-time drift).

How WERISCO uses it

WERISCO computes days of cover for every part from your stock and usage, from your ERP or from the exports you already run, compares it with the date each open order will arrive, and ranks late or undated supplier orders by severity, with each part's days of cover shown as evidence on each line. Your team decides what to chase; nothing is written back to your ERP. See it on your own data with a supply chain audit.

Questions people ask

What is the days of cover formula?: Days of cover = usable stock on hand / average daily usage. A part with 300 usable units and an average usage of 20 a day has 15 days of cover.
Is days of coverage the same as days of cover?: Yes. Days of cover, days of coverage, days of supply and stock cover are names for the same measure: how many days your usable stock will last at your average usage.
What is a good number of days of cover?: There is no single good number. It should be at least the supplier's current lead time plus a buffer for variability on parts you reorder, and much lower on parts you are phasing out. Compare cover with lead time part by part instead of chasing one target for all items.
Is days of cover the same as days inventory outstanding?: No. Days inventory outstanding is a financial ratio for the whole inventory, based on cost of goods sold. Days of cover is an operational measure for one item, based on its own usage.
Should I use forecast or past usage?: Past usage is simpler and auditable. A forecast can be better for parts with a known change ahead, but it adds forecast error. Many teams start with past usage and override it where they know demand will change.
How often should days of cover be recalculated?: At least as often as you review purchase orders, usually daily or weekly, because stock and usage move every day.