Weeks of supply equals inventory on hand divided by average weekly sales, both in units. If you hold 800 units and sell 100 a week, you have 8 weeks of supply. It tells you how long your current stock will last at your recent sales pace, which is the basis for deciding when to reorder before you run out.
How is weeks of supply different from days of supply?
They measure the same thing on different time scales. Weeks of supply uses average weekly sales; days of supply uses average daily sales. To convert, multiply weeks of supply by 7 to get days, or divide days by 7 to get weeks. Sellers with longer lead times often think in weeks because it maps more naturally to supplier and shipping timelines.
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.
What is a good weeks of supply for an Amazon seller?
There is no universal target; the right level is your replenishment lead time plus a safety buffer. If it takes 6 weeks to reorder and receive stock and you want a 2-week buffer, you should reorder when you hit about 8 weeks of supply. Too many weeks ties up cash and risks storage fees; too few risks a stockout. Set the target from your own lead time and demand variability.
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The weeks of supply formula is inventory on hand divided by average weekly sales, in units. The short version: it tells you how many weeks your current stock will last at your recent selling pace, it is the same idea as days of supply just measured in weeks, and you use it by comparing it against your lead time plus a buffer to time reorders. Below is how to calculate it, read it, and act on it.
Weeks of supply is a simple ratio, both terms in units:
Weeks of supply = inventory on hand / average weekly sales
Inventory on hand is the units you can count on: available at FBA plus confirmed inbound (already shipped, not yet received). Include the inbound, because for an overseas seller it is often weeks of stock in transit; leaving it out makes weeks of supply read low and pushes you to reorder too early and overstock.
Average weekly sales is your recent units sold per week, usually a trailing 4 to 8 week average to smooth out spikes.
Pull the inputs from Seller Central: on-hand units from Manage FBA Inventory, and weekly unit sales from Business Reports, under Detail Page Sales and Traffic by ASIN. One caution on the sales figure: exclude any out-of-stock days from the window, or a past stockout drags your average down and makes both your velocity and your weeks of supply read lighter than real demand.
If you hold 800 units and sell 100 a week, you have 8 weeks of supply. That is the whole calculation; the skill is in choosing a sales average that reflects your real, current demand.
Say a SKU has 1,200 units on hand and sold 600 units over the last 4 weeks:
Average weekly sales = 600 / 4 = 150 units a week.
Weeks of supply = 1,200 / 150 = 8 weeks.
So at the current pace, this SKU has 8 weeks of stock left. Whether that is comfortable or alarming depends entirely on how long it takes you to reorder, which is the next step.
Weeks of supply is only actionable next to your replenishment lead time:
Lead time is how many weeks it takes to reorder and get sellable stock back (supplier production plus shipping plus FBA receiving). For overseas sourcing this is often 12 weeks or more once you add production, ocean freight, customs, and receiving, so use your real number, not the 6 weeks in the example below.
Buffer is the extra weeks of safety stock you want against demand spikes and delays.
Reorder trigger = lead time + buffer, in weeks.
If your lead time is 6 weeks and you want a 2-week buffer, you reorder when weeks of supply drops to about 8. In the example above, 8 weeks of supply means you are right at the reorder point now. This is the same logic as a reorder point, expressed in weeks.
Weeks of supply and days of supply are the same measure on different scales:
Weeks to days: multiply by 7. Eight weeks of supply is 56 days.
Days to weeks: divide by 7.
Use whichever matches how you think about your supply chain. Sellers with long overseas lead times often prefer weeks, because supplier and shipping timelines are quoted in weeks; sellers with fast domestic replenishment may prefer days for finer resolution. The math is identical either way.
There is no universal target, but the extremes are clear:
Too few weeks (below your lead time plus buffer) means you risk a stockout before a reorder can arrive.
Too many weeks ties up cash in stock and can trigger storage fees and aging, the overstock problem.
The right band is enough to cover lead time and variability with a sensible buffer, no more.
A usable rule of thumb: reorder when weeks of supply falls to your lead time in weeks plus one to two buffer weeks. For a 6-week supplier with steady demand, that is roughly 7 to 8 weeks; for a 12-week overseas lead time with variable demand, more like 14 to 16. The higher your demand variability, the more buffer you need. Watching weeks of supply trend down toward that trigger is how you catch reorders early, and watching it stay stubbornly high is how you catch slow movers and excess.
The formula is only as good as the average weekly sales you feed it, and that choice is where sellers go wrong:
Too short a window (say one week) is jumpy; a single big day makes weeks of supply lurch, and you overreact.
Too long a window (say six months) is sluggish; it misses a real recent acceleration or slowdown until it is too late.
A trailing 4 to 8 weeks usually balances the two for a stable product, smoothing noise without lagging real changes.
Seasonality is the trap. If demand is climbing into Q4, a trailing average understates what next week will actually sell, so your weeks of supply reads higher (safer) than reality, and you reorder late. Going into a post-holiday slump, the reverse: the average overstates demand and you risk overbuying. When you know a seasonal shift is coming, weight the average toward recent weeks or forecast forward rather than trusting a flat trailing number. The formula is mechanical; judging the sales input is the skill.
Weeks of supply is inventory on hand divided by average weekly sales, converting a unit count into the time your stock will last. Calculate it on a trailing sales average, read it against your lead time plus a buffer to set a reorder trigger, and convert to days by multiplying by 7 when you need to. It is one of the core inventory KPIs for staying in stock without overbuying. Pair it with your inventory turnover to see both how long stock lasts and how fast it moves.