Retail math · By category
Inventory Formulas for Apparel Brands
Inventory formulas — OTB, stock-to-sales, WOS, FWOS, receipt plan, BOP/EOP, safety stock, and fill rate — that govern what to buy and when.
17 inventory formulas
Planner
Open-to-Buy (OTB)
OTB = Planned Sales + Planned EOP + Planned Markdowns − Planned BOP − Receipts on Order
Planner
Weeks of Supply (WOS)
WOS = On-Hand Units ÷ Average Weekly Unit Sales
Planner
Forward Weeks of Supply (FWOS)
FWOS = On-Hand Units ÷ Forecasted Average Weekly Unit Sales
Allocator
Fill Rate
Fill Rate % = Units Shipped ÷ Units Ordered × 100
Designer / Product Dev
Total Line Count (Styles × Colors)
Total SKUs = Styles × Colors × Sizes
Designer / Product Dev
Style-Color Depth
Units per Style-Color = Total Buy Units ÷ (Styles × Colors)
Planner
Stock-to-Sales Ratio (SSR)
SSR = Beginning-of-Period Inventory ÷ Sales for Period
Planner
Receipt Plan
Receipts = Sales + EOP + Markdowns − BOP
Planner
Beginning-of-Period (BOP) Inventory
BOP (Period N) = EOP (Period N−1)
Planner
End-of-Period (EOP) Inventory
EOP = BOP + Receipts − Sales − Markdowns
Buyer
Buy Quantity (Units)
Buy Units = (Sales Forecast Units + EOP Target − BOP) ÷ (1 − Expected Markdown %)
Buyer
PO Coverage %
PO Coverage % = Units on Order ÷ Units Needed × 100
Buyer
Size Curve Allocation
Units per Size = Total Buy × Size % from Curve
Allocator
Fair-Share Allocation
Door Units = Total Allocation × (Door Demand ÷ Chain Demand)
Allocator
Replenishment Trigger Point
Trigger = (Lead Time in Weeks × Average Weekly Sales) + Safety Stock
Allocator
Safety Stock
Safety Stock = Average Weekly Demand × Safety-Days Coverage ÷ 7
Supply Chain / Ops
Inventory Accuracy %
Accuracy % = 1 − |System Count − Physical Count| ÷ System Count
Other categories
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RetailNorthstar runs these formulas live inside a connected apparel planning workflow — OTB through allocation, by department, class, and style.