Work‑in‑Process (WIP) – In‑Flight Inventory

This topic is part of the SG Systems Global regulatory & operations glossary.

Updated October 2025 • Flow, Inventory & Scheduling • Operations, Manufacturing, Warehouse

Work‑in‑Process (WIP) is inventory that has started value‑adding operations but is not yet finished goods. It spans parts, subassemblies, batches, and intermediates physically or systemically located between operations in the routing. Controlled WIP shortens lead time, stabilizes schedules, exposes bottlenecks, and reduces risk of mix‑ups; uncontrolled WIP hides waste, clogs capacity, and degrades inventory accuracy and traceability.

“You don’t manage lead time by expediting at the end—you manage WIP at every step.”

1) What WIP Covers—and What It Does Not

Covers: items that have entered execution under an open production order or batch ticket, including queued lots at a workcenter, parts mid‑operation, intermediates pending test/approval, and subassemblies staged to downstream steps.

Does not cover: raw materials awaiting release (Component Release) or finished goods post‑completion (those sit under warehouse status until Lot Release). Personal stockpiles, rework without documented routing, and paper “parking” without transactions are not valid WIP.

2) Systems & Data Integrity Anchors

WIP exists in systems, not just on benches. Execution occurs in MES with controlled job travelers and transactions; locations and statuses are enforced by WMS. Records are governed by Document Control, with attributable users and immutable audit trails per Data Integrity and, when electronic signatures apply, Part 11/Annex 11.

3) The WIP Evidence Pack

Audit‑ready WIP shows: the open order and routing, operation start/stop with user IDs and timestamps, consumption of lots/serials (scanned), current physical location (bin/workcenter), quantity good/scrap, in‑process tests (IPC), holds (Quarantine), and any linked NCMR/NCR/Rework actions—all traceable in the system.

4) From Job Release to Finished Goods—A Standard Path

1) Release & kit. Planner issues the order (Job Release), materials are picked (Directed Picking) and verified into a kit (Kitting).
2) Execute. Operators transact start/complete at each step; IPC samples are taken per sampling plan.
3) Move & stage. WIP moves between workcenters with scanned identity and bin updates; any anomalies go to MRB.
4) Complete & QA. Order closes; product enters warehouse as finished but remains blocked until Lot Release / QA Disposition triggers Finished Goods Release.

5) Flow Math—Using Little’s Law

Little’s Law states WIP = Throughput × Lead Time. For a stable line producing 20 units/hour with a 2‑hour lead time, expect ~40 units of WIP. To reduce lead time, cap WIP (kanban), remove bottlenecks, and improve availability (OEE), not just expedite jobs.

6) WIP Limits, Kanban & Balancing

Set explicit WIP caps by step or cell, sized to cycle time and variability. Use pull signals with one‑piece flow, line balancing, and heijunka to smooth release and avoid starving/overloading downstream operations.

7) WIP Status Model & Locations

Model states such as Released → Kitted → In Queue → In Process → Pending QA → On Hold/MRB → Complete. Tie each state to allowed physical locations and system permissions; prevent picks/shipments from WIP or held stock via WMS interlocks (Hold/Release).

8) Identity, Genealogy & Mix‑up Prevention

Maintain item/lot/serial identity on every move with scan checks and printed or electronic travelers. Link consumed components to parents for end‑to‑end genealogy and batch genealogy. Use positive identification at transfer and line clearance to avoid cross‑lot contamination.

9) In‑Process Quality, SPC & Rework

Embed SPC at critical steps with appropriate control limits and capability targets (Cp/Cpk). When defects arise, create NCR/NCMR and route through controlled rework with explicit routing and retest criteria.

10) Scheduling, Queues & Priority

Use finite‑capacity scheduling and visible queues to prevent over‑release (Scheduling, Job Queue). Overuse of expedite priorities increases variability and WIP; reserve for true exceptions with management approval.

11) Costing, Yield & Variances

Accurate WIP transactions drive labor and material capture, yield variance, and scrap accounting. Measure first‑pass flow, rework loops, and queue time to understand cost of delay; connect to OEE and delivery performance (OTIF).

12) Metrics That Demonstrate Control

• WIP turns / Days of WIP and WIP age distribution by step.
• Flow efficiency: value‑add time ÷ total lead time.
• Queue ratio: queue time ÷ operation time (target reduction).
• Release discipline: % orders within WIP caps; expedite rate.
• Traceability completeness: % moves with scanned identity and bin.

Together these show whether WIP is purposeful buffer or unmanaged congestion.

13) Common Pitfalls & How to Avoid Them

• Phantom WIP: items on benches but not in system—enforce scan‑to‑move and e‑travelers.
• Over‑release: jobs launched to keep people “busy”—go pull‑based and cap WIP.
• Mix‑ups: parallel lots without positive ID—use barcode/label checks and line clearance.
• Hidden rework loops: capture rework as routed steps, not “extra touches.”
• Stale queues: no aging alerts—surface WIP age and auto‑escalate stuck orders.

14) Physical Control—Bins, Zones & Moves

Define WIP zones and dedicated bins along the line; transact every move in WMS with directed moves. Use dynamic lot allocation only within defined WIP buffers to avoid starvation and double‑booking.

15) What Belongs in the WIP Record

Order ID and traveler; routing step and resource; operator/time stamps; quantities good/scrap; consumed component lots/serials and remaining kit; current bin/location; test/inspection results and SPC charts; holds and MRB links; rework approvals and outcomes; and final completion posting to inventory. All governed under Document Control with searchable audit trails.

16) How This Fits with V5 by SG Systems Global

Real‑time WIP boards. The V5 platform visualizes WIP by workcenter with live queues, age badges, and bottleneck highlighting. Finite release from the Job Queue enforces step‑level WIP caps.

Scan‑to‑move interlocks. V5 ties MES operations to WMS locations: moves require barcode scans that validate item/lot/serial and permissible bins, maintaining batch‑to‑bin traceability automatically.

Quality in the flow. In‑process checks, SPC, and IPC are embedded; holds create Quarantine status and route to MRB without leaving the line.

Flow analytics & Little’s Law. V5 calculates WIP, throughput, and lead time by step, projecting the impact of WIP cap changes on cycle time and OTIF.

Completion & release. On order close, V5 posts to finished inventory but blocks picks until QA Disposition flips status, feeding compliant labels and paperwork to Pack & Ship.

Bottom line: V5 turns WIP from a hidden pile into a governed, visible flow—every move, test, and hold is scan‑verified, timestamped, and ready for audit.

17) FAQ

Q1. How do I choose WIP caps?
Start from Little’s Law and historical variability; size buffers to absorb normal fluctuations and protect bottlenecks, then adjust based on queue ratios and aging alerts.

Q2. Is WIP the same as queue?
Queue is waiting WIP at a step. Total WIP includes queued, in‑process, and pending‑QA items.

Q3. How is rework counted?
As explicit routed steps with separate yield and time; uncontrolled “extra touches” are not allowed.

Q4. What causes negative WIP in reports?
Out‑of‑sequence moves or missing scans. Enforce scan‑to‑start/complete and reconcile with cycle counting.

Q5. Can I expedite without raising WIP?
Yes—by reallocating capacity or removing constraints. Blanket expedites typically increase variability and WIP.

Q6. How does WIP tie to genealogy?
Every consume and move links parents and children, maintaining end‑to‑end traceability through completion and release.

Related Reading
• Flow & Lean: Kanban | One‑Piece Flow | Line Balancing | Heijunka
• Execution & Routing: MES | Routing | Job Release | Job Queue
• Inventory & Status: WMS | Bin Locations | Quarantine/Hold | Inventory Accuracy
• Quality & Disposition: IPC | SPC | MRB | Lot Release