What DSI means in FBA
Days sales of inventory (DSI), also called days inventory outstanding or inventory days, measures how many days your money sits in inventory before converting to a sale. It answers the question: on average, how long does a dollar of inventory take to become a dollar of revenue?
For Amazon FBA sellers, DSI is the right metric for financial planning and cash flow forecasting. While days of supply tells you when you will stock out (an operations metric), DSI tells you how efficiently your capital is working (a finance metric). Both matter, but they serve different audiences: your supply chain team watches DOS, your accountant and investors watch DSI.
DSI is the direct inverse of inventory turnover ratio. If your turnover is 6, your DSI is 365 ÷ 6 = about 61 days. This bidirectional relationship makes it easy to translate between the two depending on your audience.
Days sales of inventory formula
Example: a $2.1M private label seller
A private label seller doing $2.1M in annual revenue with 18 SKUs (average selling price $34, average landed cost per unit $11).
- Annual COGS: ~61,765 units sold × $11 = $679,412
- Average inventory value: $104,500 (average of monthly snapshots: ~9,500 units × $11)
DSI = ($104,500 ÷ $679,412) × 365 = 56.2 days
Cross-check: Turnover = $679,412 ÷ $104,500 = 6.5 turns. DSI = 365 ÷ 6.5 = 56.2 days. Both methods agree.
This means every dollar invested in inventory takes roughly 56 days to convert into a sale. For a seller with 72-day lead times, this is tight but healthy: inventory sells before the next batch arrives, minimizing overlap and storage fees.
| DSI Range | Cash impact | FBA storage risk |
|---|---|---|
| < 30 | Fast cash cycle | Low fees, possible stockout risk |
| 30 to 60 | Healthy | Normal monthly storage fees |
| 60 to 90 | Moderate | Approaching excess thresholds |
| > 90 | Slow | Aged inventory surcharges likely |
FBA-specific considerations
DSI in an FBA context differs from textbook DSI in several important ways:
FBA storage fees create a hard cost floor for high DSI. In a self-managed warehouse, holding inventory longer costs you rent (fixed). In FBA, holding inventory longer costs you per-cubic-foot monthly fees plus aged inventory surcharges that kick in at 181 days and 271 days. A DSI above 180 means you are paying premium storage rates on that inventory.
DSI and days of supply can diverge significantly. DOS uses unit velocity; DSI uses COGS. If your catalog has a mix of high-cost and low-cost SKUs, a few expensive slow movers can inflate your DSI even while your unit-based DOS looks healthy. Always check both metrics.
Multi-channel sellers need to choose a scope. If you sell the same SKU on FBA and your own website (FBM), decide whether DSI covers all channels or just FBA. Mixing channels inflates your COGS without reflecting the FBA-specific inventory carrying cost, which gives you a misleading DSI for FBA planning.
Common mistakes
- Confusing DSI with days of supply. DSI uses financial values (COGS and inventory at cost). Days of supply uses units. They answer related but different questions. A SKU with a $2 COGS and a $80 COGS SKU can have identical days of supply but very different DSI values because DSI weights by cost.
- Using retail value instead of cost for inventory. Valuing inventory at selling price inflates your average inventory value and overstates DSI. Always use landed cost (what you paid for the inventory, including freight and duties).
- Calculating DSI over inconsistent time periods. If you use quarterly COGS, multiply by the days in that quarter (90 or 91), not 365. Mixing annual COGS with monthly inventory snapshots without adjusting the period produces nonsensical results.