Where inventory lives determines how efficiently every picker in the building works. Putaway strategy is a long-lived decision - poor choices compound into ghost inventory, failed cycle counts, and irreversible cube waste.
Directed vs. Undirected Putaway
Undirected putaway: Worker decides where to put product based on personal memory. Works briefly in tiny operations. Destroys accuracy and cube utilization over time. Common symptom: same SKU in 6 locations across 3 zones, none of which are the designated slot.
Directed putaway: WMS assigns every location. The sequence:
- SKU Profile Lookup - velocity class, dimensions, weight, temperature, hazmat, lot/expiration
- Slot Availability Check - real-time: open, correctly sized, not at capacity, storage-compliant
- Business Rule Application:
- ABC zone assignment (A-items near outbound, C-items in far aisles)
- FIFO/FEFO for dated product (food, pharma)
- Family grouping (co-picked SKUs stored adjacent)
- Hazmat segregation
- Task Assignment - operator receives: “Put pallet 001234 in location A-01-03-02.” RF scan or voice confirmation. Instant WMS update.
Result: receiving accuracy above 99.9% because every put is confirmed, not assumed.
Task Interleaving: Free Productivity
The problem: Without interleaving, a forklift completes putaway and drives back to staging empty. Deadhead travel - moving but producing nothing.
The fix: WMS routes operator directly from putaway completion to the nearest open pick or replenishment task before returning to staging.
Result: 15-25% lift truck productivity gain. On a 50-truck fleet, that’s 7-12 trucks of additional productive capacity - no new equipment, no new hires. It’s a WMS configuration decision.
Task interleaving is free productivity. The truck moves either way - the WMS decides whether it moves productively or empty.
Four Storage Strategies
| Strategy | Selectivity | Space Efficiency | WMS Dependency | Best For |
|---|---|---|---|---|
| Fixed Location | 100% | Low (reserved slots sit empty) | None | <3,000 SKUs, manual systems, stable assortments |
| Random (Dynamic) | 100% | Maximum | Complete | High-SKU 3PLs, constantly changing product mix |
| Class-Based (ABC) | 100% within zone | High | Moderate | Most modern DCs - best balance |
| Combination | Variable | Variable | Moderate-high | Mixed operations with distinct channels |
Class-Based storage is the standard in modern DCs:
- A-items in Zone 1 (near outbound/packing), B-items in Zone 2, C-items in far aisles
- Within each zone: random location assignment for cube efficiency
- Gets travel efficiency of fixed-zone + cube efficiency of random
Random storage warning: Complete WMS dependency. If WMS goes down, picking and putaway stop. Plan for system redundancy.
The 85% Cube Rule
Warehouse cube utilization should not exceed 85% of theoretical capacity under normal operating conditions. Above 85%:
- Pick face replenishment becomes congested
- Overflow inventory blocks travel paths
- Put-away errors spike (no good location available, operators improvise)
- Cycle count accuracy degrades
The 15% buffer absorbs seasonal peak inventory without operational breakdown. When designing storage capacity, always size to peak inventory (not average) and maintain the 85% ceiling at that peak.
Slotting Optimization
Moving fast-moving SKUs closer to outbound/packing is the highest-ROI non-capital improvement in most DCs.
ABC classification inputs:
- Basic velocity: units sold / days (default for most operations)
- Weighted velocity: (units sold × order frequency) / days (for sporadic demand)
- Volume-based velocity: (units sold × unit volume) / days (space-constrained ops)
Standard ABC split:
- A items: ~20% of SKUs, ~80% of picks → prime pick locations, golden zone
- B items: ~30% of SKUs, ~15% of picks → secondary locations
- C items: ~50% of SKUs, ~5% of picks → high-density storage, less-accessible locations
Re-slot frequency: Monthly at minimum for fast-changing SKU velocity profiles (e-commerce, fashion). Quarterly for stable operations.