Quick answer: Sell-through rate is a single ratio — units shipped versus average inventory on hand. IPI is a 0–1,000 composite that includes sell-through plus stranded inventory, excess inventory, and in-stock rate. IPI is what gates your storage limits; sell-through is one of the dials you turn to move it.
Sellers often treat these as competing metrics to pick between. They're not competing — one contains the other. The useful distinction is which one to act on versus which one to watch as a consequence.
Sell-through rate: the input you control
Sell-through rate is units shipped over a trailing window (commonly 90 days) divided by the average number of units available over that window. A rate of 3 means you sold three times your average on-hand stock in the period; a rate of 0.5 means half of what you're holding didn't move.
It's a clean efficiency number, and it maps to decisions:
- Low sell-through on a SKU means you're holding too much relative to demand — a candidate for price action, a deal, ad support, or a removal order.
- High sell-through means the SKU is turning fast, which is good until it tips into stockout risk.
The FBA sell-through rate post covers the calculation and the benchmark ranges. The point here is that sell-through is a per-SKU lever — you can look at a listing, see the number, and know roughly what to do.
IPI: the score Amazon acts on
IPI, the Inventory Performance Index, is a single 0–1,000 score for your whole FBA account. It blends several factors, historically including sell-through, the share of your inventory that's stranded (units with a listing problem that can't sell), excess inventory (units well beyond forecast demand), and in-stock rate on your active, productive listings. Amazon has adjusted the exact weighting over time; the IPI explainer tracks the current picture.
What makes IPI matter is the consequence: if your score is below Amazon's threshold on the check date, your FBA storage capacity can be restricted for the next period, and FBA Capacity Manager becomes the mechanism you're negotiating within. IPI is a gate, and it moves slowly — it reflects weeks of behavior, so a last-minute scramble before a check date usually doesn't rescue it.
Why watching only one fails
Watch only sell-through and you can miss the other IPI inputs entirely. A seller with healthy sell-through across active SKUs still tanked their IPI because a batch of listings had gone stranded after a category compliance change — units sitting in fulfillment centers, invisible to the sell-through math because they weren't "available," dragging the composite down. The sell-through dashboard looked fine the whole time.
Watch only IPI and you get a number that tells you there's a problem but not which SKU caused it or what to do. IPI is diagnostic at the account level and useless at the line level. You can't reorder against it, can't price against it, can't decide a removal with it.
How to use them together
Treat IPI as the gauge and sell-through, stranded count, and excess units as the controls:
| If IPI is... | Look at... | Action |
|---|---|---|
| Below threshold | Stranded inventory report first | Fix or remove stranded units — fastest recovery |
| Drifting down | Per-SKU sell-through, sorted low | Price, promote, or remove the slow tail |
| Fine but fragile | In-stock rate on top sellers | Protect velocity leaders from stockouts |
| Healthy | Nothing urgent | Spot-check monthly |
The order matters. Stranded inventory is the highest-leverage fix because those units contribute nothing and cost storage — the stranded inventory fix walks through clearing it. Then work the low-sell-through tail. Keeping your best sellers in stock protects the in-stock-rate component and your revenue at the same time — the days-of-stock alert that matters is about catching those before they lapse.
The practical cadence
Check IPI monthly, and more often in the weeks before a known storage-limit check date — but understand you're reading a trailing average, not a live number. Check per-SKU sell-through weekly as part of normal inventory review, alongside days of stock and aged-inventory risk. If the weekly work is healthy, the monthly score takes care of itself. If the score slips anyway, the stranded and excess reports tell you where to dig.
EcomSanity surfaces per-SKU velocity, days of stock, and aged-inventory risk in one view — the SKU-level signals that move IPI before the score reacts. See the FBA sell-through rate guide for the benchmark ranges.
Frequently asked questions
What's the difference between IPI and sell-through rate?
Sell-through rate is one metric: units shipped over the trailing period divided by average available inventory. IPI is a composite score from 0 to 1,000 that blends sell-through with excess inventory, stranded inventory, and in-stock rate. Sell-through is an input; IPI is the output Amazon acts on.
Which one determines my FBA storage limits?
IPI. If your IPI sits below Amazon's threshold at the check date, your FBA storage capacity can be limited for the following period. Sell-through affects limits only through its contribution to IPI.
How do I improve IPI quickly?
Fix stranded inventory first — it's the fastest, cleanest win. Then reduce excess units on slow ASINs through price, ads, or removals, and keep your best sellers in stock. Sell-through improves as those move, and IPI follows over a few weeks.