Days of inventory cover is how long your current stock will last at your current rate of sale: units on hand divided by average daily units sold. If you hold 120 units and sell 8 a day, you have 15 days of cover. Track it per SKU and it becomes your earliest stockout warning.
The formula, and the one decision inside it
Days of cover = units on hand / average daily units sold
The only real decision is the averaging window for daily sales:
- 7-day average reacts fast, catches promotions and spikes, but gets noisy for slow movers.
- 30-day average is stable and better for steady sellers, but lags when demand shifts.
- Blended (weight recent days higher) is what most planning tools converge on.
Worked example: you hold 45 units of a SKU. It sold 63 units in the last 30 days, so 2.1 per day. Days of cover = 45 / 2.1 = about 21 days. If your supplier takes 14 days to deliver, you have a 7-day decision window, not a 21-day one. That difference is the entire discipline.
Why multichannel changes the math
When you sell on one store, the store’s own velocity is the whole picture. When the same stock pool serves Amazon, eBay, and your web store, the number that matters is combined daily velocity across every channel, and it can shift channel by channel week to week. A SKU doing 1 a day on your store and 4 a day on Amazon has 5-a-day drain on one shared pool.
Two practical consequences:
- Compute cover against total cross-channel velocity, never per channel in isolation. Per-channel numbers understate the drain and overstate your runway.
- Watch the channel mix. If Amazon’s share of a SKU’s sales jumps from 40 percent to 70 percent, your cover shortens even though your store’s own sales look flat. Unifystock’s channel-mix analytics exist for exactly this: seeing where each SKU actually sells.
From days of cover to a reorder point
Days of cover becomes actionable the moment you compare it with supplier lead time:
- Lead time: how many days from placing a purchase order to sellable stock on the shelf. Be honest; include receiving time.
- Reorder trigger: when days of cover drops to lead time plus a safety margin, order. Cover 21, lead time 14, margin 4: your trigger is 18 days of cover.
- Order quantity: enough to restore your target cover, often 30 to 60 days for steady SKUs, less for seasonal or fashion-risk items.
The safety margin exists because both inputs wobble: sales spike and suppliers slip. If you would rather hold that margin as units instead of days, that is safety stock, sized properly here.
What a good number looks like
There is no universal target; there is a healthy range per business model:
- Under lead time: red. You will stock out before replenishment can land, and on marketplaces a stockout also means losing rank and the sale-after-the-sale.
- Lead time to roughly 2x lead time: the working zone for most steadily-selling SKUs.
- 90+ days: your cash is sleeping on a shelf. Long cover is not safety, it is inventory-holding cost plus obsolescence risk.
The distribution matters more than the average. A catalog whose average cover is 40 days can still contain the five SKUs at 3 days that produce next week’s stockouts, which is why per-SKU visibility beats any summary number.
Automating the forecast
Doing this in a spreadsheet works for 20 SKUs and falls apart at 200: velocities change weekly, channel mix drifts, and manual recalculation happens exactly as often as nobody is busy. The pieces worth automating:
- Per-SKU days-of-cover computed from live cross-channel sales, refreshed continuously rather than on export day.
- Stockout forecasting: the projected run-out date per SKU, sorted soonest-first, so the reorder list writes itself.
- Low-stock alerting at thresholds you choose, so the SKUs entering the red zone come to you.
Unifystock’s dashboard ships days-of-cover and stockout forecasting per SKU, computed from your real sales across every connected channel (Amazon, eBay, Shopify, WooCommerce, Etsy, and OpenCart at launch), with low-stock thresholds you set once. It pairs with oversell-risk detection, because the same thin-stock SKUs that stock out are the ones that double-sell on the way down.
Common questions
Is days of inventory cover the same as days sales of inventory (DSI)?
They answer the same question with different inputs. DSI is the accounting version, computed from inventory value and COGS over a period. Days of cover is the operational version, computed from units and unit velocity. For replenishment decisions, use units; for financial reporting, use DSI.
How often should I recalculate it?
Continuously, in practice. Any tool computing it for you should refresh as sales land. If you are doing it manually, weekly is the minimum for active SKUs, and always before placing a purchase order.
What about seasonal products?
Use a velocity window that matches the season you are entering, not the one you are leaving. Last month’s average understates December demand for a seasonal SKU. This is where blended or forward-looking estimates earn their keep.
Does days of cover replace safety stock?
No, they cooperate. Days of cover tells you when to reorder; safety stock decides how much cushion you hold for the unexpected. Size them separately and your buffer stops being a guess.
See your run-out dates before they happen
Connect your channels to Unifystock and get low-stock alerts computed from live cross-channel sales on every plan, with per-SKU days of cover and stockout forecasts on the plans above. Plans start at $49/month with unlimited orders. See pricing.