Amazon changed the rules again — and sellers live between two nightmares: running out of stock, and drowning in it. "Just right" is a narrow window, and most of us are just guessing.
I used to be a gut-feel restocker: sales spiking? Ship a container. Sales slowing? Pump the brakes. The result? A 9-day stockout during peak season that knocked my listing down seven spots, and 4,200 units gathering dust in an FBA warehouse for three months, eating my cash flow alive.
Two years ago I switched to doing math instead of fortune-telling. Here's the days-of-supply restock formula I actually use on my own account. Fair warning: the formula isn't magic. The magic is knowing your real daily sales — and most sellers don't.
The Formula: Three Inputs, One Subtraction
Reorder quantity = Daily sales velocity × (Lead time + Target days of supply) − Current inventory (on-hand + in-transit)
Plain-English breakdown:
1. Daily sales velocity. Not last month's average — a weighted average of the last 30 days. Why? Because your data from three months ago means nothing heading into peak season. My weighting: last 7 days × 60% + last 30 days × 40%. New product with no history? Back into it from ad conversion data, or start with a small conservative test batch.
2. Lead time. Days from factory order to units available for sale on Amazon — factory production, freight, plus Amazon's own receiving queue. Ocean freight alone runs 30–40 days normally, and don't forget Amazon's inbound appointment backlog. Last October one of my shipments sat outside the fulfillment center for 6 full days waiting for a check-in slot.
3. Target days of supply. How many days you want this shipment to last — 30, 45, or 60. There's a decision framework below.
4. Current inventory. Subtract FBA on-hand AND in-transit. People who ignore in-transit always end up realizing the container is still at sea after they've already placed the order.
A real example from my own account:
| Item | Value |
|---|---|
| Daily velocity (weighted 30-day) | 45 units/day |
| Lead time | 22 days |
| Target days of supply | 45 days |
| FBA on-hand | 300 units |
| In-transit | 500 units |
| Reorder quantity | 45 × (22+45) − (300+500) = 2,215 units |
Once you have the number, execute. The worst habit in restocking is running the math and then second-guessing it anyway.
Two Times I Got Burned: One Stockout, One Overstock
Case one: October 2024, a 9-day stockout. My flip-top outdoor water bottle started climbing after Prime Day — daily sales went from 60 to 110. I restocked on the 60-unit baseline for 30 days, thinking "that should cover it." Then I delayed the order a week to consolidate it with another factory batch, ocean freight got stuck 4 more days, and I stocked out for 9 days. Organic ranking fell from #7 to #14 in the category. After recovery, my ad ACoS ran about 8 points higher for weeks to win the spot back. At 110 units a day, that's over $5,000 in lost profit.
Lesson carved in stone: when the formula says order, order. Don't delay a week to "save on freight." A late shipment can cost you a quarter.
Case two: early 2025, 4,200 units overstocked. Swinging to the other extreme after the stockout scare. Q4 sold great, so in January I stocked 60 days at Q4's peak velocity of 180 units/day. Q4 ended, sales fell back to 70 a day, and 4,200 units sat in FBA for three full months. Storage fees were only part of it — I won't quote Amazon's long-term storage fee numbers since those policies change (check the latest in Seller Central). The real pain was the trapped cash: over $30,000 tied up while I had no budget left to stock spring products. I finally moved inventory to a third-party warehouse to stop the bleeding.
The harsher lesson: peak-season numbers belong to peak season — don't budget the off-season with them. I now flag November–December data separately and discount it by 30% in everyday calculations.
Counterintuitive Truth: More Stock Does Not Mean More Safety
Beginners think: stock up, sleep easy. Wrong.
FBA storage has costs and caps. Amazon enforces storage capacity limits per seller (check the latest limits in Seller Central), so when you're capped, you can't ship even if you want to. Units that sit too long face long-term storage fees. Stocking 90 days' worth looks safe; in reality it converts your cash flow into rent paid to Amazon's warehouse.
And overstock has a silent killer: returns. My 4,200 dust-collecting bottles had packaging that absorbed moisture — the return rate ran 2 points above normal. Inventory left sitting can literally degrade.
Real safety is fast turnover, not fat inventory. My hard rule: unless you're a stable top-3 bestseller in your category, cap target coverage at 45 days. Push to 60 two weeks before a big sale, drop back the moment it ends. Money in motion beats a mountain in a warehouse.
How Many Days to Target: A 30 / 45 / 60 Decision Framework
| Scenario | Target days of supply | When it fits |
|---|---|---|
| Stable everyday product | 30–45 days | Mature listing, steady velocity, repeat purchases |
| Pre-peak stocking | 60 days | Two weeks before a major sale, high traffic certainty |
| New product testing | 15–20 days | Thin data, small batches, fast cycles |
| Unreliable freight / long factory lead times | 45–60 days | Build road uncertainty into the buffer |
One principle: your days-of-supply target equals your tolerance for uncertainty on the road. The shakier the shipping and the factory, the longer the target — but cap it at 60 days, don't just keep adding.
Four Traps the Formula Won't Catch
1. Adjust velocity before big sales. Two weeks before Prime Day or Black Friday, I multiply daily velocity by 1.5–2×. That's my own five-year observed coefficient — it varies a lot by category, so calibrate it against your own history first.
2. Don't formula your first two batches. With no data, the formula is decoration. Batch one: 200–300 units to test conversion. Only scale with the formula once conversion hits my bar (15%+).
3. Account for return shrinkage. I calculate on gross sales × 0.97. Returns that come back sellable — using net sales would leave you understocked.
4. Set an automatic reorder alert. My spreadsheet flags the row red when on-hand ÷ daily velocity < lead time. Red means order now. Procrastination is a seller's worst enemy.
Restocking is just math. Nail your daily velocity, nail your lead time, lock your coverage target, and let execution do the rest. Stockouts and overstocks are both symptoms of "fuzzy math plus procrastination."
Two years on this system: stockouts went from 5 a year to 1, and trapped inventory capital dropped by nearly half. This one's battle-tested — it works.
By Ah K, E-commerce Seller (5 yrs)
