By Volkan Haslak, Founder & CEO of ShipHouse. Volkan works with brands that split their inventory between Amazon, Walmart, their own store, and a U.S. 3PL.
Ecommerce inventory management is how an online brand decides how much stock to buy, where to keep it, and when to reorder, so it can fill every order without tying up cash in products that don't sell. For brands that sell on more than one channel, it also means deciding how much inventory each channel gets.
Getting it wrong is expensive. IHL Group estimates that inventory distortion, meaning out-of-stocks and overstocks combined, cost retailers $1.73 trillion worldwide in 2025, including $415 billion in North America.
This guide covers how to structure inventory across channels, the formulas that drive reorder decisions (with a worked example), the metrics worth tracking, Amazon FBA and Walmart WFS stock rules, and the warehouse practices that keep your numbers accurate.
Start with the multichannel inventory model
Most growing brands now keep stock in several places at once:
- Amazon FBA: inventory in Amazon's network for Amazon orders.
- Walmart Fulfillment Services (WFS): inventory in Walmart's network for Walmart orders.
- A 3PL warehouse: inventory for Shopify, eBay, TikTok Shop, seller-fulfilled marketplace orders, and wholesale.
The simplest model that works treats the 3PL as the main pool and the marketplace networks as forward pools:
Pool | Ships | Replenished from | Target stock |
|---|---|---|---|
3PL warehouse (main pool) | Shopify, eBay, TikTok Shop, seller-fulfilled orders, B2B | Your supplier (containers, LTL) | Everything not needed in forward pools, plus the reorder buffer |
Amazon FBA (forward pool) | Amazon Prime orders | The 3PL, in regular smaller shipments | Enough to stay above Amazon's low-inventory threshold without paying for aged stock |
Walmart WFS (forward pool) | Walmart orders | The 3PL | A few weeks of supply, well under Walmart's long-term storage limit |
This model has three advantages:
- You buy once. Containers land at one U.S. warehouse instead of being split at the factory.
- You keep flexibility. Units stay in the main pool until a channel needs them, so a slow channel doesn't trap stock that another channel could sell.
- You control marketplace fees. Forward pools stay lean, which keeps storage fees and aged-inventory charges down.
Shipping only what each marketplace needs also means prep and labeling happen in one place. See our FNSKU labels guide and FBA vs FBM.
The formulas behind reorder decisions
Reorder point
The reorder point is the stock level at which you place your next purchase order:
Reorder point = (average daily sales × lead time in days) + safety stock
Lead time means the full time from placing the order to having sellable stock: production, ocean or air freight, customs, drayage, and receiving.
Safety stock
Safety stock is the extra inventory that covers demand that's higher than average while you wait for the next order. The common statistical formula is:
Safety stock = Z × σ(daily demand) × √(lead time in days)
- Z is the service-level factor: 1.28 for 90%, 1.65 for 95%, 2.05 for 98%, and 2.33 for 99%.
- σ (sigma) is the standard deviation of daily unit sales.
This version accounts for demand variability only. If your lead times also vary a lot (common with ocean freight), a longer formula includes lead-time variability too. A simpler rule of thumb works when you don't have clean data:
Safety stock = (max daily sales × max lead time) − (average daily sales × average lead time)
A worked example
A brand sells 40 units a day across all channels, with a standard deviation of 12 units a day. Its lead time from the factory to sellable stock at the 3PL is 21 days. It wants a 95% service level (Z = 1.65).
Step | Calculation | Result |
|---|---|---|
Lead-time demand | 40 × 21 | 840 units |
Safety stock | 1.65 × 12 × √21 (4.58) | 91 units |
Reorder point | 840 + 91 | 931 units |
When total stock across all pools drops to about 931 units, it's time to place the next purchase order.
Splitting stock across channels
Say 20 of those 40 daily units sell on Amazon, 5 on Walmart, and 15 through Shopify, eBay, and TikTok Shop:
Pool | Daily sales | Target | Units to hold |
|---|---|---|---|
Amazon FBA | 20 | 5–8 weeks of supply | 700–1,120 |
Walmart WFS | 5 | 4–6 weeks of supply | 140–210 |
3PL main pool | 15 direct + replenishment | Everything else | Remaining stock |
These targets are illustrative. Set your own based on how long it takes to get a replenishment shipment checked in at each marketplace.
Economic order quantity (EOQ)
EOQ estimates the order size that minimizes total ordering and holding costs:
EOQ = √(2 × annual demand × cost per order ÷ annual holding cost per unit)
For our example, with 14,600 units a year, $150 per order, and $2 per unit per year to hold: √(2 × 14,600 × 150 ÷ 2) ≈ 1,480 units. In practice, supplier minimums and container sizes often matter more than EOQ, but it's a useful check on whether you're ordering too often or too much.
Inventory metrics to track
Metric | Formula | What it tells you |
|---|---|---|
Inventory turnover | Cost of goods sold ÷ average inventory | How many times you sell through your stock in a year |
Days inventory outstanding (DIO) | (Average inventory ÷ cost of goods sold) × 365 | How many days of sales your inventory represents, in cost terms |
Weeks of supply | Units on hand ÷ average weekly unit sales | How long current stock will last, by SKU and by pool |
Sell-through rate | Units sold ÷ units received × 100 | How quickly a shipment or season's stock is selling |
GMROI | Gross margin ÷ average inventory cost | How much margin each dollar of inventory earns |
Stockout rate | Days out of stock ÷ total days | How often you're losing sales |
Dead stock | Units with no sales in a set period (for example, 180 days) | Cash tied up in products that aren't moving |
A quick example: with $600,000 in annual cost of goods sold and $150,000 in average inventory, turnover is 4 and DIO is about 91 days. If gross margin is $400,000, GMROI is 2.67, meaning each dollar in inventory earns $2.67 in margin.
Use ABC analysis to focus your effort
ABC analysis ranks SKUs by their share of sales value:
- A items: a small share of SKUs that drive most of the revenue. Watch them closely, count them often, and never let them stock out.
- B items: the middle group.
- C items: the long tail of SKUs that drive little revenue. Order less often, and consider whether they're worth keeping.
There's no fixed split, but a common rule of thumb is that about 20% of SKUs generate 70% to 80% of sales value. Use the ranking to set safety stock levels, cycle count frequency, and which SKUs go into forward pools.
Amazon FBA and Walmart WFS inventory rules
Marketplace fees reward lean, fast-moving inventory and penalize both too little and too much stock.
Amazon FBA
- Low-inventory-level fee. Amazon charges this fee when an ASIN's stock is too low relative to its sales. Amazon announced on November 5, 2026 that from February 1, 2027, the fee will apply only when inventory falls below 28 days of supply, measured on actual shipments in the four weeks before and after the assessment date.
- Storage utilization surcharge. Currently charged when your stored volume is high relative to what you ship. Amazon announced it will be eliminated on February 1, 2027.
- Aged inventory surcharge. Amazon will merge it into monthly storage fees as a single age-based rate from February 1, 2027. Inventory that turns in under six months will see flat or lower fees, and aged inventory will pay more. Amazon hadn't published the new rates as of November 2026.
- Capacity limits. Amazon sets monthly FBA capacity limits based on factors including sales history, forecasts, and the Inventory Performance Index (IPI).
Together, these rules mean the right FBA position is a few weeks of supply, replenished regularly. That's easiest when a U.S. warehouse holds the rest of your stock and can ship to Amazon on short notice.
Walmart WFS
- Monthly storage: $0.75 per cubic foot from January to September. From October to December, items stored longer than 30 days pay an additional $1.50 per cubic foot.
- Long-term storage starts on day 366 after receipt: $2.25 per cubic foot per month from day 366 to 450, and $7.50 after day 450.
See our Walmart Fulfillment Services guide for prep requirements.
Keep multichannel stock in sync
Overselling happens when two channels sell the same last unit before inventory updates. To prevent it:
- Use one source of truth. Connect every store and marketplace to a single inventory system, usually your 3PL's warehouse management system (WMS) or an inventory app connected to it.
- Hold a channel buffer. Show slightly less stock than you have on each channel, especially on marketplaces that penalize cancellations, such as eBay and Walmart.
- Account for kits and bundles. A bundle uses up component stock. Your system should reduce each component's count when a bundle sells.
- Account for sync delays. If inventory syncs every 15 minutes, a fast-selling SKU can oversell between updates. Add that window to your safety stock.
See Shopify fulfillment, eBay fulfillment, and TikTok Shop fulfillment for channel-specific details.
Warehouse practices that keep inventory accurate
Formulas only work if your stock counts are right. Most inventory errors start in the warehouse.
Receiving
Count every inbound shipment against the advance shipping notice (ASN) or packing list, and flag differences before stock goes live. A shortage found at receiving is a supplier claim. A shortage found three months later is a mystery.
Cycle counting
Cycle counting means counting a small set of locations every day or week instead of shutting down for a full physical count once a year. It catches errors while they're still easy to fix. A common approach counts A items monthly or quarterly, B items quarterly or twice a year, and C items once a year.
FIFO and FEFO
- FIFO (first in, first out): ship the oldest stock first. Standard for most products.
- FEFO (first expired, first out): ship the stock with the earliest expiration date first. Essential for supplements, cosmetics, food, and anything with a shelf life. It also matters when sending stock to Amazon and Walmart, since both set minimum remaining shelf life for dated products.
Your 3PL should track lot numbers and expiration dates by location so FEFO happens automatically.
Returns
Returned items change your stock counts. Inspect, grade, and restock them quickly so they're available to sell again, and keep damaged units out of sellable inventory. See our reverse logistics guide.
Dead stock
Review SKUs with no sales in 90 to 180 days. Options include bundling them with best-sellers, discounting, selling through outlet channels, liquidating, or donating. The sooner you act, the more you recover, and the less you pay to store them.
Where a 3PL fits
A 3PL takes on the physical side of inventory management: receiving and counting inbound shipments, storing stock by location, cycle counting, tracking lots and expiration dates, shipping orders from every channel, and sending replenishment to Amazon and Walmart. You keep control of buying and forecasting, with real-time stock levels from the warehouse.
ShipHouse stores and ships inventory for multichannel brands from New Jersey and Delaware. Pallet storage starts at $40 per month, and standard receiving is free for up to 4 labor hours per shipment. See our published starter rates.
Frequently asked questions
What is ecommerce inventory management?
It's the process of planning, storing, tracking, and replenishing the products an online business sells, so it can fulfill orders on every channel without stockouts or excess stock.
How do I calculate a reorder point?
Multiply average daily sales by your lead time in days, then add safety stock. For example, 40 units a day with a 21-day lead time and 91 units of safety stock gives a reorder point of 931 units.
What is the safety stock formula?
A common version is Z × the standard deviation of daily demand × the square root of lead time in days. Z is 1.65 for a 95% service level.
What is a good inventory turnover ratio for ecommerce?
It depends on the category. Fast-moving consumer goods turn more often than furniture or seasonal products. Track your own turnover over time, by SKU, and compare it with your margin through GMROI.
How do I manage inventory across Shopify, Amazon, Walmart, and TikTok Shop without overselling?
Use one inventory system connected to every channel, hold a small buffer on each channel, account for bundles and sync delays, and keep your main stock in one warehouse that replenishes the marketplace networks.
How many weeks of supply should I keep at Amazon FBA?
Enough to stay above Amazon's low-inventory threshold, which is 28 days of supply, without paying for aged inventory. Many brands aim for about 5 to 8 weeks, depending on how long replenishment takes to check in.
What is changing with Amazon's FBA storage fees in 2027?
Amazon announced that from February 1, 2027, it will eliminate the storage utilization surcharge, merge the aged inventory surcharge into an age-based monthly storage rate, and change how it assesses the low-inventory-level fee.
When does Walmart WFS charge long-term storage?
From day 366 after receipt: $2.25 per cubic foot per month from day 366 to 450, and $7.50 after day 450.
What's the difference between cycle counting and a physical inventory count?
A physical count checks all inventory at once, usually requiring a shutdown. Cycle counting checks a small portion continuously, so errors are caught sooner without stopping operations.
What is ABC analysis in inventory management?
It groups SKUs by their share of sales value: A items drive most revenue, C items drive little. It helps decide where to focus counting, safety stock, and buying effort.
Should I use FIFO or FEFO?
Use FIFO for products without an expiration date and FEFO for anything that expires, such as supplements, cosmetics, and food.
How do I deal with dead stock?
Find SKUs with no sales in 90 to 180 days, then bundle, discount, liquidate, or donate them before storage costs grow.
The bottom line
Good inventory management comes down to three things: the right reorder math, the right amount of stock in each channel, and accurate counts in the warehouse. Keep your main stock in one U.S. warehouse, keep marketplace pools lean, and let the formulas, not guesswork, trigger each order.
If you sell on several channels and want one place to manage your stock, tell us about your catalog and channels.
Formulas in this guide follow standard inventory-management definitions. Amazon and Walmart rules are from their official seller announcements and help pages, current as of November 2026. The featured image is AI-assisted and was created from photos of the ShipHouse team.
