Days of supply counts the shipment before it lands

Days of supply is the stock you hold divided by what you sell per day. The figure is safe only when every unit it counts is already on the shelf. Add inbound and it can say 100 days while the shelf empties on day 30. Inbound only covers sales from the day it lands, so check cover week by week.

Stock that lands on day 43 cannot be sold on day 31.

What the number divides

One product, selling 10 units a day. You hold 300 at FBA, and a shipment of 700 lands at the start of week 7, day 43. There are two ways to state its days of supply:

Both are correct divisions. Neither contains a date. The 100-day figure assumes the 700 units are on the shelf today, and they will not be for six weeks.

Week by week, the same product

At 70 a week, the line runs like this. Sales a week cannot exceed what is on the shelf, so the last column counts what demand was there and could not be sold:

The shelf empties on day 30, two days into week 5. The shipment lands on day 43. For the 12 days in between, 120 units of demand go unserved. At a $25 price that is $3,000 of sales, and the 100-day figure looked comfortable the whole time.

Why the total hides it

Days of supply answers "how long would this stock last if it were all here". A stockout is a question about a particular week. Adding inbound to the total turns a timing question into a quantity one, and the quantity is fine: 1,000 units really is 100 days of sales. The problem sits entirely in the order they arrive.

So there are two checks, and they use different numbers. To decide how much to order, count every unit already coming, whenever it lands: the reorder quantity formula does that. To decide whether you run out, put each inbound shipment in the week it lands and read down the weeks for the first one at zero.

Compare cover with lead time, not with a good number

A days-of-supply figure has no good value of its own. It is good or bad against the time a new order takes to arrive, plus the buffer you want to keep. Two SKUs with 600 units each, both on a 10-week lead time with a 4-week buffer, so 14 weeks of lead time plus buffer:

Same units, opposite problems. A needs cash now and probably faster freight for part of the order. B needs no cash at all for months, and any reorder on it adds to $2,560 that is already waiting. Sorting a catalog by units puts them side by side; sorting by cover against lead time puts them at opposite ends.

The comparison has four outcomes worth naming. Cover shorter than the lead time means a new order cannot arrive before you run out. Cover between the lead time and the lead time plus buffer means an order today arrives in time but eats into the buffer. Cover beyond lead time plus buffer is covered, and cover far beyond it, like B's, is cash waiting on the shelf. Vendlen's weekly stock projection gives each Sales SKU on each channel a similar status in its Coverage column: a lead-time risk, a buffer gap, covered, or well above the target.

Closing a gap you have found

Finding the gap in week 1 rather than in week 5 leaves arithmetic you can still use. With the example's numbers:

• Ship part of it sooner. The gap is 120 units. A smaller shipment of at least 120 that lands by day 30 covers it, and the rest can follow by the slower route.
• Stretch what you hold. 300 units have to last 42 days, which is about 7 a day instead of 10. Slowing sales by about 30% for six weeks, through price or advertising, keeps the listing in stock until the shipment lands. It does not win back the 120 units; it trades them for a higher price or lower ad spend on the units you do sell, and you can estimate that trade before you make it.
• Move the arrival. Every week the shipment lands earlier removes 70 units from the gap.

Where this still needs a person

An arrival week is an estimate until the goods are received. A shipment that slips moves the gap with it, and a projection that still shows the old date is optimistic about every week after it. Vendlen does not drop a lot whose arrival week has passed: it moves it to next week and marks it overdue, so the row stays honest until someone updates the date.

The table projects the current selling rate forward. It is not a forecast, so a peak you know is coming has to go into the rate by hand, and a rate measured during an earlier stockout will understate demand: calculate sales velocity on in-stock days before trusting the line.

And the check is per channel. Units at another channel's warehouse do not fill this channel's weeks, which is why the projection in Inventory and Supply shows one channel at a time.

What to do with this

1. List what you hold of the SKU on this channel, and every inbound shipment with the week it is expected to land.
2. Build eight to twelve rows: start, lands, sold at your weekly rate, ends with. Sales cannot go below the stock on hand.
3. Find the first week that ends at zero and compare it with the next arrival. If the arrival is later, the demand the shelf could not meet in between, the Missed column, is the size of the gap.
4. Close it while there is still time: a smaller, faster shipment, slower sales, or an earlier arrival.
5. Across the catalog, sort by weeks of cover against each SKU's lead time plus buffer, not by units, and put cash where cover is shorter than the lead time first.

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