DOI formula, category benchmarks, color-coded thresholds, and reorder point strategies. The single metric that controls both stockout risk and storage costs.
Mar 11, 2026 · 15 min · 4,000 words
Days of Inventory (DOI) tells you exactly how many days your current stock will last at your current sell-through rate. It's the simplest, most actionable inventory metric for Amazon sellers. Yet most sellers either don't track it or rely on gut feel instead of data. That's how you end up with $30K in aged inventory surcharges or a best-seller that stocks out for two weeks. We pulled the playbook below from cohort reviews with brands and agencies, not from a slide deck.
This guide covers the DOI formula, category-specific benchmarks, how to set reorder points using DOI, and the direct connection between DOI and your IPI score. You'll also learn how color-coded DOI thresholds transform inventory management from a weekly spreadsheet exercise into a daily 2-minute check.
DOI = Current FBA Units ÷ Average Daily Sales. For most Amazon categories, the sweet spot is 30-60 days. Below 14 days, you're at high stockout risk. Above 90 days, you're paying unnecessary storage fees and dragging down your IPI score. The goal: every ASIN in the green zone.
DOI is calculated as:
DOI = Current FBA Units ÷ Average Daily Unit Sales
Where average daily sales = total units sold over last 30 days ÷ 30
If you have 450 units in FBA and sell an average of 15 units per day, your DOI is 30 days. Simple. But the power comes from tracking DOI across your entire catalog and acting on outliers.
| ASIN | FBA Units | Daily Sales | DOI | Status |
|---|---|---|---|---|
| Kitchen Gadget A | 600 | 20/day | 30 days | Healthy |
| Kitchen Gadget B | 150 | 12/day | 12.5 days | At Risk |
| Kitchen Gadget C | 2,400 | 8/day | 300 days | Excess |
| Kitchen Gadget D | 900 | 18/day | 50 days | Healthy |
In this example, Kitchen Gadget B needs an urgent restock (12.5 days of supply at current velocity). Kitchen Gadget C is a serious problem: 300 days of inventory means it'll hit aged surcharges within the quarter. Without DOI tracking, you might not catch either issue until it's too late.
A simple 30-day average works for stable products, but it misses trends. If sales jumped from 10/day to 20/day last week (new PPC campaign, seasonal spike), your DOI is actually half what the 30-day average suggests. Use a 7-day weighted average alongside the 30-day average to catch velocity changes early. Nova's demand velocity metric does this automatically.
The "right" DOI depends on your replenishment lead time, category seasonality, and margin structure. According to Statista's marketplace data and our analysis of seller performance across categories, here are practical benchmarks:
| Category | Target DOI | Min (Stockout Risk) | Max (Excess) | Lead Time Factor |
|---|---|---|---|---|
| Consumables / Supplements | 30-45 days | 14 days | 60 days | Short (domestic sourcing) |
| Home & Kitchen | 45-60 days | 21 days | 90 days | Medium (mixed sourcing) |
| Electronics / Tech | 30-45 days | 14 days | 60 days | Short to medium |
| Toys & Games | 30-45 days | 14 days | 60 days | Long (overseas, seasonal) |
| Apparel | 45-60 days | 21 days | 90 days | Long (overseas, size variants) |
| Oversized / Heavy | 21-30 days | 7 days | 45 days | Short (higher storage cost) |
Oversized products occupy significantly more cubic footage, so storage costs per unit are much higher. A 2-cubic-foot oversized product at 60 DOI costs roughly 4x more in storage than a 0.25-cubic-foot standard product at the same DOI. For oversized items, keep DOI under 30 days and use AWD or 3PL storage for buffer stock.
The fastest way to manage inventory across hundreds of SKUs is a color-coded threshold system. Instead of scanning spreadsheets for numbers, you look for colors. Here's the framework that works for most Amazon sellers:
0-7 days
Imminent stockout. Emergency restock or has already stocked out.
8-21 days
Reorder now. May stock out before inbound arrives.
22-60 days
Optimal zone. Good balance of availability and cost.
60+ days
Overstocked. Drags IPI, risks aged surcharges.
Nova's FBA Inventory module applies exactly this color-coding to every ASIN in your catalog. Your morning inventory check becomes: scan for red (action needed today), check amber (action needed this week), ignore green (healthy). A 500-SKU catalog becomes manageable in 5 minutes.
DOI tells you where you are. Reorder points tell you when to act. The formula:
Reorder Point = (Lead Time in Days × Daily Sales) + Safety Stock
When your current FBA units hit this number, trigger a replenishment order
| Variable | Value | Source |
|---|---|---|
| Average daily sales | 15 units/day | 30-day average from Seller Central |
| Lead time | 21 days | Supplier production + shipping + FBA check-in |
| Safety stock | 7 days × 15 = 105 units | Buffer for demand spikes |
| Reorder point | (21 × 15) + 105 = 420 units | When FBA hits 420 units, reorder |
| Equivalent DOI | 28 days | 420 ÷ 15 = 28 days of supply |
So when this product's DOI drops to 28 days, you trigger a replenishment. By the time the new stock arrives (21 days later), you'll have 7 days of safety stock left. You never stock out, and you never hold more than 45-50 days of inventory.
A pet supplies brand with 85 SKUs was averaging 90-day DOI across their catalog. They were paying $4,200/month in storage fees with 12 ASINs approaching aged surcharge thresholds. By implementing DOI-based reorder points (targeting 40-day average) and clearing 8 dead SKUs, they reduced monthly storage fees to $1,800 within 60 days. Their IPI climbed from 380 to 540 in the same period, unlocking unlimited restock capacity for Q4.
Amazon's IPI score uses sell-through rate. Many sellers track DOI. They're related but not identical. Here's how they compare:
| Metric | DOI (Days of Inventory) | Sell-Through Rate |
|---|---|---|
| Formula | Units on hand ÷ daily sales | Units sold (90d) ÷ avg units on hand |
| Output | Days of supply remaining | Turnover ratio (higher = better) |
| Best for | Reorder decisions, stockout prevention | IPI optimization, Amazon compliance |
| Actionability | High (directly maps to reorder timing) | Medium (tells efficiency, not timing) |
| Used by | Inventory managers, ops teams | Amazon's IPI algorithm |
The answer: track both, but use DOI for daily decisions. DOI tells you "this product runs out in 12 days." Sell-through tells you "this product turns 4x per quarter." The first drives action. The second tells you if your overall strategy is working. Both improve when you optimize your restock cadence.
DOI is the single metric that influences both your storage costs and your IPI score. Here's the direct connection:
Linear
2x DOI = roughly 2x storage cost (plus aged surcharges after 271 days)
Inverse
Lower DOI = higher sell-through = higher IPI
Compound
Lower DOI → better IPI → more restock capacity
Nova's FBA inventory tracker is built around DOI as the core metric. Every ASIN shows current DOI with color-coded thresholds (red/amber/green), demand velocity trends, stock position (available, reserved, inbound), and pipeline projections. You see at a glance which products need attention and which are healthy. For broader storage-fee and aging-inventory visibility, the full Amazon inventory management software view ties this into P&L tracking, so you can calculate the true contribution margin impact of holding 90 days vs 45 days of any given product.
How DOI directly impacts your monthly and aged inventory storage costs.
How sell-through rate (inverse of DOI) feeds into your IPI calculation.
How DOI management prevents you from hitting capacity ceilings.
The complete framework for cash flow, forecasting, and reorder systems.
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