How to Automate Amazon FBA Restock Decisions

Every stockout costs you ranking. Every overstock costs you storage fees. The sweet spot is a system that tells you exactly what to reorder, how much, and when. Most sellers check inventory "when they remember" — which is usually too late. A single 3-day stockout on a product selling 10 units per day costs you $500+ in lost revenue, plus weeks of degraded organic ranking that requires increased PPC spend to recover.

This guide covers the complete restocking framework: the manual calculations, the formulas, the seasonal adjustments, and how to automate the entire process so you never think about restocking again until the alert tells you to act.

The Cost of Getting Restocking Wrong

Restocking mistakes cost money in both directions, and most sellers underestimate the true cost of each:

Stockouts (too little inventory): The direct cost is lost sales revenue — every day out of stock is a day of zero revenue on that ASIN. But the indirect costs are far more damaging. Amazon's algorithm demotes products that go out of stock, dropping your organic ranking. Rebuilding that ranking takes 2-4 weeks of above-average sales velocity, typically requiring a 30-50% increase in PPC spend. A 5-day stockout on a $30 product selling 8 units per day costs $1,200 in direct lost sales, plus $500-1,500 in PPC costs to recover ranking. Total damage: $1,700-2,700 from one preventable stockout.

Overstocking (too much inventory): Amazon charges monthly storage fees ($0.87-2.40 per cubic foot depending on time of year) and aged inventory surcharges for stock sitting longer than 181 days. A pallet of slow-moving inventory can cost $200-500 per month in storage alone. Worse, that capital is tied up in inventory instead of being invested in PPC, new products, or other growth activities. The opportunity cost of overstocking is often larger than the storage fees themselves.

The compounding effect: Stockouts and overstocking create a vicious cycle. A stockout forces you to rush-order replacement inventory (often at higher shipping costs), which arrives in a large batch that then sits in FBA warehouses longer than optimal, incurring storage fees. Breaking this cycle requires a systematic approach to inventory management — not "checking when you remember."

For related inventory management strategies, see our inventory SOP guide and our IPI score explainer.

The Manual Approach

Step 1: Calculate days of supply per ASIN. Current FBA units ÷ average daily velocity = days of supply. Use 30-day average velocity for stable products, 14-day average for trending products (up or down). Check this in Seller Central under Inventory → Inventory Planning → Restock Inventory, or calculate it from your Business Reports data.

Step 2: Determine your total lead time. This isn't just manufacturing time. Total lead time = manufacturing time + quality inspection + shipping to prep center + prep/labeling time + shipping to Amazon + Amazon receiving time. For overseas suppliers (China → US), typical total lead time is 60-90 days. For domestic suppliers, 14-30 days. Map this out for each supplier and update it quarterly — lead times change with factory capacity, shipping conditions, and Amazon's receiving speed.

Step 3: Set reorder points. Reorder point = (total lead time in days × daily velocity) + safety stock buffer. If your supplier takes 60 days total and you sell 5 units per day, your base reorder point is 300 units. Add safety stock (typically 14-21 days of supply) and your reorder point becomes 370-405 units. When your FBA inventory hits this number, it's time to place a new order.

Step 4: Calculate order quantities. Order quantity = (target days of supply × daily velocity) - current inventory + units already in transit. Target days of supply is typically 60-90 days after the new shipment arrives. Don't order based on gut feel — run the math for each ASIN.

Step 5: Check weekly at minimum. Velocity changes. Seasonal trends shift demand. A viral TikTok can triple your sales overnight. Weekly checks are the minimum cadence; daily monitoring is better for high-velocity products. Miss one week during a sales spike and you're stocking out.

This manual approach works if you're disciplined. But with 10+ ASINs, each with different velocities, lead times, and seasonal patterns, the spreadsheet becomes a full-time job. And humans are bad at consistency — we check diligently for two weeks, then get busy with other things and skip a week, which is exactly when something goes wrong.

The Reorder Point Formula

When your FBA inventory reaches 370 units, place your next order. The order will arrive in approximately 60 days, during which you'll sell approximately 300 units, leaving you with the 70-unit safety buffer intact.

Order quantity formula:

These formulas seem simple, but they require accurate inputs. If your velocity estimate is wrong by 20%, your reorder timing will be off by days or weeks. Use rolling averages (not single-day snapshots) and adjust for known upcoming events (Prime Day, BFCM, seasonal shifts).

Factoring in Seasonality

Seasonality is where most manual restocking systems fail. Your daily velocity in July is different from October, which is different from December. If your reorder formula uses a flat velocity number, you'll either stock out during peak season or overstock during slow periods.

How to adjust: Pull last year's monthly sales data for each ASIN. Calculate the velocity multiplier for each month: (month's daily velocity ÷ annual average daily velocity). Apply this multiplier to your reorder formula.

The critical planning window is August-September. Q4 orders for overseas suppliers need to be placed by late August to arrive before the November rush. If you wait until October to order, your inventory won't arrive until December — after the peak has passed. For complete Q4 planning, see our Q4 calendar and BFCM preparation guide.

IPI Score and Storage Limits

Your Inventory Performance Index (IPI) score determines how much storage space Amazon gives you. Below 400 IPI triggers storage limits that cap how many units you can have in FBA warehouses. This creates a restocking constraint that the basic formula doesn't account for.

If your IPI is healthy (500+): You have essentially unlimited storage. Focus on optimal reorder quantities without worrying about space constraints.

If your IPI is moderate (400-500): Monitor your storage utilization. Prioritize stocking your highest-velocity, highest-margin products. Consider reducing order quantities on slow-moving ASINs.

If your IPI is low (below 400): Storage limits are your biggest constraint. You may not be able to stock 90 days of supply for all products. Prioritize ruthlessly: stock your top 20% revenue-generating ASINs fully, reduce stock on the bottom 80%, and consider removing slow-moving inventory to free up space for winners.

IPI is calculated from four factors: excess inventory, sell-through rate, stranded inventory, and in-stock rate. Improving any of these improves your IPI, which improves your storage limits, which makes restocking easier. See our IPI guide for detailed improvement strategies and our stranded inventory guide to fix one of the easiest IPI improvement levers.

Calculating Safety Stock

Safety stock is your buffer against uncertainty — the extra inventory that protects you when velocity spikes unexpectedly, suppliers ship late, or Amazon's receiving takes longer than planned. Too little safety stock and you risk stockouts. Too much and you're paying unnecessary storage fees.

Simple method: Safety stock = maximum daily velocity × maximum lead time − average daily velocity × average lead time. If your max velocity is 8 units/day, max lead time is 70 days, average velocity is 5 units/day, and average lead time is 60 days: Safety stock = (8 × 70) − (5 × 60) = 560 − 300 = 260 units. This is conservative but safe.

Practical method: Most sellers use 14-21 days of average velocity as safety stock. For a product selling 5 units/day, that's 70-105 units. This is less mathematically precise but easier to maintain and sufficient for most products.

Adjust for product characteristics: High-velocity products (10+ units/day) need proportionally less safety stock because small velocity fluctuations have less impact. Low-velocity products (1-2 units/day) need proportionally more because a single day's spike can represent a 100%+ velocity increase. New products without reliable velocity data need larger buffers until you have 60+ days of sales history.

Common Restocking Mistakes

Using selling price instead of velocity for decisions. A $50 product selling 2 units per day needs different restocking than a $10 product selling 20 units per day, even though both generate $100/day in revenue. Restock based on unit velocity, not revenue.

Ignoring Amazon's receiving time. Amazon's fulfillment centers can take 1-14 days to receive and process inbound shipments, depending on the time of year and FC capacity. During Q4 peak, receiving can take 2-3 weeks. Factor this into your lead time calculation — your inventory isn't sellable until Amazon finishes processing it.

Not accounting for units in transit. If you have 200 units in FBA and 500 units on a ship headed to Amazon, you have 700 units of supply (approximately). Forgetting in-transit inventory leads to double-ordering.

Setting and forgetting reorder points. Velocity changes over time. A product selling 10 units/day in March might sell 6 in August and 15 in November. If your reorder point doesn't change, you'll overstock in slow periods and stockout in peak periods. Review and adjust reorder points monthly at minimum.

Ordering round numbers instead of calculated quantities. "Let's just order 1,000 units" feels clean but rarely aligns with actual demand. Order the calculated quantity, even if it's an awkward number like 347 units. The math exists for a reason.

The Automated Approach

Here's what automated inventory monitoring looks like — real-time analysis across your entire catalog with seasonal adjustments, urgency-based alerts, and recommended order quantities calculated automatically: