Amazon FBA Restock Forecast Calculator
Forecast when your inventory runs out, when to reorder, and how much safety stock you need to avoid stockouts.
Units in transit to FBA
Supplier order to FBA check-in
1.0 = normal, 1.5 = 50% uplift
What This Calculator Does
The Restock Forecast Calculator projects your daily inventory level over time, accounting for current stock, inbound shipments, sales velocity, seasonality, and growth trends. It identifies three critical dates: when your inventory hits safety stock levels (warning), when you need to place a reorder (action), and when you'll stock out if you don't reorder (deadline). Unlike a simple reorder point calculator that gives you a single trigger number, this tool shows the full trajectory so you can plan ahead.
Reorder Points vs. Forecast Models
A reorder point is a static number: "when inventory drops to X units, place an order." It's simple and works well for stable products. A forecast model (what this tool provides) projects inventory depletion over time, accounting for changing variables like sales growth and seasonality. Use the reorder point for day-to-day monitoring and the forecast for strategic planning — especially before peak seasons or product launches when sales velocity is expected to change.
How Safety Stock Works
Safety stock is your buffer inventory — extra units beyond what you need to cover normal sales during lead time. It protects against two types of uncertainty: demand variability (days when you sell more than average) and supply variability (supplier or shipping delays). A 14-day safety stock means keeping enough inventory to cover 14 extra days of sales beyond your lead time. When the chart shows your inventory hitting the orange safety stock line, it's an early warning that you're running low.
Why Seasonality and Lead Times Matter
Seasonality and lead times compound each other. If your product sells 2× during Q4 and your supplier lead time is 45 days, you need to order your Q4 stock by mid-August — well before peak season starts. The seasonality multiplier in this calculator adjusts your daily sales projection to reflect these peaks. Set it to 1.5 for a 50% Q4 uplift, 2.0 for doubling, etc. Then add your full lead time (supplier production + freight + Amazon receiving) to find the true reorder-by date.
Example Scenarios
Scenario 1 — Stable product, sea freight: 800 units on hand, 12 units/day, 35-day lead time, 14-day safety. Days of cover = ~67 days. Reorder by day ~32. Plenty of time for sea freight.
Scenario 2 — Fast mover approaching Q4: 500 units on hand, 20 units/day, 1.5× seasonality (30 units/day effective), 40-day lead time. Days of cover = ~17 days. Reorder immediately — you're already past the safe window.
Scenario 3 — New product with growth: 1,000 units, 5 units/day, 10% monthly growth. Days of cover shrinks faster than expected because sales accelerate. Check the chart curve — it's steeper than a straight line.
Formula & Methodology
- Adjusted Daily Sales = Daily Sales × Seasonality Multiplier × (1 + Growth%)^(day/30)
- Daily Inventory = Previous Day Inventory − Adjusted Daily Sales
- Safety Stock (units) = Adjusted Daily Sales × Safety Stock Days
- Stockout Day = First day where inventory ≤ 0
- Reorder-By Day = Stockout Day − Lead Time
- Suggested Reorder Qty = (Lead Time × Adjusted Daily Sales) + Safety Stock Units
Frequently Asked Questions
How do I estimate my daily sales velocity?
In Seller Central, go to Business Reports → Sales and Traffic → Detail Page Sales and Traffic by ASIN. Look at "Units Ordered" over the past 30–90 days and divide by the number of days. Use a 30-day window for fast-changing products and a 90-day window for stable ones. Exclude any anomalous periods (stockouts, viral moments, Prime Day) for a more representative average.
What safety stock level should I use?
For most FBA products, 7–21 days of safety stock is appropriate. Use 7 days for products with reliable domestic suppliers and stable demand. Use 14–21 days for products sourced internationally with longer, less predictable lead times. High-demand seasonal products may need 30+ days of safety stock heading into peak season.
How does seasonality affect my reorder timing?
Seasonality accelerates inventory depletion, which means you need to reorder earlier and in larger quantities. A product that normally lasts 60 days of inventory might last only 40 days at 1.5× seasonality. The critical mistake is using off-season velocity to plan peak-season restocks — always apply the seasonality multiplier when forecasting ahead.
What happens if my supplier lead time changes?
Lead time changes directly shift your reorder-by date. If your supplier needs an extra 10 days, your reorder-by date moves 10 days earlier. This is why safety stock exists — it provides a buffer for supply delays. Monitor supplier performance and use worst-case (not average) lead times for critical restocks.
Should I factor in Amazon's receiving time?
Yes — Amazon's warehouse receiving can take 5–14 days (sometimes longer during peak). Add this to your supplier lead time. Your total lead time should be: supplier production + freight transit + customs/clearance + Amazon receiving. Use the Weeks of Cover Calculator to monitor health while waiting for inbound shipments.
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