Amazon Sales Velocity: A Formula for Inventory Decisions

Quick answer: Sales velocity equals units sold divided by days in the period, but the window you choose and whether you exclude stockout days changes the number substantially, which is why experienced operators compare 7-day, 30-day, and 90-day velocity rather than trusting one figure.

There is a point in almost every Amazon business when "we sell quite a few of these" stops being useful.

It usually happens when a supplier asks for a purchase order. Suddenly, the seller needs a real number. How many units are actually moving? Was last week normal, or did a promotion distort it? Is the product speeding up, slowing down, or just bouncing around?

That number is sales velocity. It sounds technical, but the basic version is ordinary division: units sold during a period divided by the number of days in that period. If an ASIN sold 84 units in the last 28 days, its 28-day sales velocity is 3 units per day. Simple enough. The hard part is choosing a window that reflects reality.

Why a single velocity number can mislead you

Imagine a kitchen product that normally sells 2 units a day. It appears in a deal for four days, sells 12 units a day during the promotion, then returns to normal. Calculate velocity using only the deal week and you may reorder as if the temporary spike will continue. Use a full year and the figure may be too slow to notice a recent improvement.

This is why experienced operators rarely rely on one period. They compare at least three: 7-day velocity for sudden changes, 30-day velocity for current operating pace, and 90-day velocity for a more stable baseline. A 7-day rate above the 30-day rate can mean acceleration, seasonality, or a temporary event. A 7-day rate below the 30-day and 90-day rates can mean demand is cooling, the Buy Box was lost, or inventory availability has weakened. The number is not the decision. It's the beginning of the investigation.

Revenue velocity and unit velocity answer different questions

Sellers sometimes calculate sales velocity using revenue instead of units. That can be useful, but it isn't interchangeable. Unit velocity helps with stock planning. Revenue velocity helps with cash planning and account-level performance. A product selling one $400 unit a day has lower unit velocity than a product selling ten $12 units, but its revenue contribution is much larger. For inventory decisions, start with units, then bring in contribution margin, lead time, and cash requirements.

A better formula for replenishment

Once you know daily unit velocity, a basic reorder estimate is reorder quantity equals daily velocity times coverage days. Coverage days should include supplier production or purchasing time, freight time, prep-center time, Amazon receiving time, and a safety buffer.

Suppose a product sells 4 units a day. The supplier needs 20 days, shipping takes 12, prep takes 3, and Amazon receiving typically takes 10. The operating lead time is already 45 days. Add a 10-day safety buffer and the business needs roughly 220 units of coverage. That doesn't automatically mean ordering 220 units. You still subtract sellable stock, reliable inbound stock, and any inventory held outside FBA.

Don't let stockouts rewrite the history

A common calculation mistake is dividing sales by every calendar day even when the listing was unavailable for part of the period. Suppose an ASIN sold 90 units in 30 days but was out of stock for 10 of those days. The simple velocity is 3 units a day. During the 20 in-stock days, it actually averaged 4.5 units.

Neither number is perfect. The first understates demand. The second assumes all lost days would have behaved like the in-stock days. The safest approach is to keep both, labeled clearly: calendar-day velocity (3 units) and in-stock velocity (4.5 units). Use the in-stock figure for demand potential and the calendar figure for what the business actually achieved.

Promotions and Buy Box swings need annotations

A spreadsheet full of clean numbers can hide a messy month. A reseller watched a toy ASIN jump from 1.8 units a day to 5.2 and nearly doubled the next order. A quick timeline check showed the listing had won a temporary price promotion, two competitors had stocked out, and the offer held the Featured Offer almost continuously for a week. By the time the supplier shipped, competitors were back and the price had fallen. The product hadn't become a 5-unit-a-day item; the seller had measured an unusual market condition.

The fix is simple: note major events on the timeline, including price changes, deal periods, advertising launches or pauses, Buy Box losses, stockouts, listing suppressions, review rating changes, and seasonal events. Numbers become more useful once you remember what happened around them.

Sales velocity for one-off and reseller inventory

Private-label sellers use velocity to decide how much to manufacture. Resellers often have a different problem: only a few units available from a supplier, or a catalog with thousands of ASINs purchased opportunistically. For that model, velocity helps with prioritization: which replenishable ASINs deserve more capital, which fast sellers are about to disappear from stock, which slow sellers shouldn't be bought again, and which categories are gaining momentum.

A used-book seller may own one copy of each ISBN, so "reorder 30 units" is irrelevant. Still, velocity at category or cohort level can show whether textbooks, technical manuals, or collectible media are moving faster than the rest of the catalog, covered in Amazon sales by category.

What should trigger an alert?

There's no universal threshold. A product selling 0.3 units a day can be excellent if it earns $80 per sale. A product selling 20 units a day can be dangerous if the margin is tiny and replenishment takes four months. Useful alerts focus on change and consequence: velocity fell more than 30% versus the prior comparable period, days of stock dropped below lead time plus buffer, ad spend rose while unit velocity stayed flat, velocity increased sharply and current inventory can't support it, or Buy Box share fell before velocity declined. The best alert doesn't say "sales changed." It says why the change deserves attention.

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EcomSanity calculates sales velocity from connected Amazon data and uses it alongside FBA inventory to estimate days of stock automatically across the whole catalog, so sorting by what's becoming urgent doesn't require an hour with a spreadsheet.