Amazon Inventory Forecasting: The Framework We Use

Running out of stock on Amazon doesn't just cost you today's sales. It costs you the organic ranking you spent months building, the ad efficiency you dialed in, and the velocity that feeds the algorithm. We've watched brands lose $200K+ in a single month from a preventable stockout on one hero ASIN.

Overstocking is the quieter killer. Your cash is locked up in FBA warehouses, you're eating long-term storage fees, and your IPI score tanks — which limits your future storage capacity. Both problems come from the same root cause: bad amazon inventory forecasting.

The 4-Layer Forecasting Framework

After managing inventory across 100+ brands, we've settled on a four-layer approach. No single data source is reliable enough on its own.

Layer 1: Baseline Velocity

Start with your trailing 30-day and 90-day average daily units sold. The 30-day number captures recent momentum. The 90-day number smooths out spikes. We weight them 60/40 in favor of the 30-day figure for most products.

Exception: if you're in a highly seasonal category (outdoor, holiday decor, cold/flu), flip the weighting or switch to year-over-year comparisons.

Layer 2: Seasonal Multipliers

Pull your month-over-month sales data for the past two years. Calculate the percentage change for each month relative to your annual average. These become your seasonal multipliers.

Example: if your average monthly units are 3,000 and October consistently hits 4,500, your October multiplier is 1.5x. Apply this to your baseline velocity when forecasting October replenishment.

Layer 3: Promotional Lift

Every Lightning Deal, coupon, or Prime Day event creates a demand spike. Track the lift from past promotions and bake it into your forecast. A typical Lightning Deal drives 3-8x normal daily volume for the deal window. Coupons with 15%+ discounts usually add 30-60% to daily units.

If you're planning a major promotion, increase your inbound shipment 4-6 weeks before the event — not 2 weeks before when everyone else is flooding FBA.

Layer 4: Advertising Spend Changes

This is the layer most sellers miss entirely. If you're planning to increase PPC spend by 40% next month, your units sold will increase — and you need inventory to support that. We map our advertising budget plans directly into our inventory forecasts. A 40% spend increase on a well-optimized campaign typically translates to a 25-35% unit volume increase.

Lead Time: The Variable That Wrecks Everything

Your forecast is only as good as your lead time accuracy. We track four separate lead time components:

• Manufacturing time: Order placed to goods ready for shipment.
• Shipping time: Factory to port to your 3PL or Amazon warehouse.
• FBA check-in time: The 5-14 day black hole between delivery and items being available for sale. This one fluctuates wildly — plan for the worst case.
• Buffer stock: We add 14 days of safety stock on top of everything for hero ASINs and 7 days for long-tail products.

Amazon Inventory Forecasting Tools We Actually Use

Amazon's Restock Inventory report in Seller Central is a decent starting point but not sufficient alone. We combine it with SoStocked or Inventory Planner for multi-SKU brands, and validate everything against our own spreadsheet models. The key is reconciling Amazon's sell-through recommendations with your actual lead times and promotional calendar.

FBA Inventory Age and Inventory Health reports are non-negotiable weekly checks. If any SKU crosses 180 days of age, you need a liquidation or removal plan — the long-term storage fees at 271+ days will eat your margin alive.

The Reorder Formula

Here's the simplified version of what we calculate for every SKU, every week:

Reorder Point = (Average Daily Units x Total Lead Time Days) + Safety Stock

Reorder Quantity = (Average Daily Units x Days of Cover Target) - Current FBA Inventory - Inbound Units

We target 60 days of cover for most SKUs and 90 days for slow-moving or long-lead-time products.

Good amazon inventory forecasting isn't magic — it's discipline. If you want help building this system for your catalog, our Amazon operations team sets this up and manages it week over week.