Sell-through rate is units sold in a window divided by units available to sell in that window; on Amazon, the FBA definition is units sold and delivered over the past 90 days divided by the average number of sellable units in fulfillment centers over the same 90 days (sell.amazon.com, "What is the FBA Inventory tool?"). Sales velocity is units sold per unit of time, usually per day or per week, with no denominator for stock at all. One is a ratio of sales to inventory; the other is a rate of sales against the clock.
The two numbers answer different questions and get swapped for each other constantly. Velocity tells you how fast a product leaves the building, which is what a reorder needs. Sell-through tells you how much of what you are holding is moving, which is what Amazon's Inventory Performance Index reads and what a liquidation decision needs. A seller who reorders on sell-through under-buys after a big inbound; a seller who liquidates on velocity keeps dead stock because "it still sells twenty a day."
They also fail in opposite directions. Velocity goes quiet during a stockout and says nothing about lost demand. Sell-through goes wild during a stockout, because the denominator empties, and says nothing about how much you could have sold.
Illustrative numbers. A stainless travel mug, one SKU, tracked across three consecutive 90-day windows.
Window one, steady state. 1,800 units sold. Average sellable units 2,000. Sell-through: 1,800 divided by 2,000 equals 90 percent. Velocity: 1,800 divided by 90 days equals 20 units a day. Days of supply at the end of the window with 1,700 on hand: 85.
Window two, a large inbound lands on day 60. Sales hold at 1,800 units, so velocity is still 20 a day. But 6,000 units arrived on day 60. Average sellable units for the window: 60 days at about 1,000 and 30 days at about 6,500, so (60 times 1,000 plus 30 times 6,500) divided by 90 equals 2,833. Sell-through: 1,800 divided by 2,833 equals 63.5 percent. The product did not slow down at all. Sell-through fell 26.5 points because the denominator grew. Amazon's score reads the drop; your reorder logic should not.
Window three, a stockout on day 45. 900 units sold in the first 45 days at the same 20 a day, then nothing. Average sellable units: 45 days at about 450, then 45 days at zero, so 225. Sell-through: 900 divided by 225 equals 400 percent. Velocity, computed over 90 calendar days: 900 divided by 90 equals 10 a day, half the true rate. Computed over 45 in-stock days: 20 a day, the true rate. Two metrics, both distorted by the same event, in opposite directions. The true cost of the stockout is the 900 units that did not sell in the second half and never appear in either number.
A reorder point is velocity times lead time plus safety stock. Use velocity on in-stock days, from your own sales history, and adjust for the season you are ordering into rather than the season you are ordering from. Window three's 10-a-day calendar velocity would set a reorder point half as high as it should be, and the next stockout arrives on schedule. Setting reorder points for Q4 works the seasonal adjustment. A restock report that factors sales history, velocity, lead times, and inbound stock is doing this arithmetic per SKU; the inventory tracking behind it has to carry in-transit units as in-transit, or the inbound in window two looks like stock you can sell today.
Liquidation is a sell-through question with a days-of-supply answer. Window two's 6,000-unit inbound at 20 a day is 300 days of supply, more than three times Amazon's 90-day excess line. Velocity says the product is healthy. Sell-through and days of supply say 4,200 of those units will pay storage for months and the oldest will cross the aged-inventory surcharge at 181 days. The decision is whether to discount, remove, or hold the excess, and it runs on the cash each path returns from here, since the units are already paid for.
Sell-through, on Amazon's 90-day rolling definition, along with excess inventory, stranded inventory, and in-stock rate. Velocity feeds the score only indirectly, through the excess and in-stock inputs.
There is no single figure worth quoting. Amazon's FBA Inventory tool shows a recommended range for your account, and the useful comparison is a SKU against its own history and against the rest of your catalog. A rate that looks strong can be a stockout in disguise.
Daily for reorder math, because lead times are in days. Weekly for reading trends, because daily counts are noisy. Either way, divide by in-stock days.
Because the denominator jumped. A large inbound raises average sellable units for the whole 90-day window while sales stay the same. It recovers as the window rolls forward, as long as the product keeps selling at the same rate.
Crunch, the analytics AI inside ConnectBooks, answers "should I liquidate, remove, or hold this inventory ahead of Q4 storage fees" from reconciled data, weighing sell-through, velocity, future storage and aged-inventory fees, selling fees, and expected cash recovery, and ranking SKUs as hold, discount, liquidate, or remove. It cannot know that a wholesale buyer is about to take 3,000 units, or that a listing was suppressed for a week and the velocity dip is not demand. Tell it, or read the ranking with that in mind.
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