Net Flow Equation

The equation is defined as On-Hand Inventory + On-Order Inventory - Qualified Sales Order Demand. This calculation represents the net available stock position after accounting for all committed future supply and demand. It is not a forecast; it is a snapshot of reality. On-Hand is physical stock. On-Order is stock that has been released but not yet received. Qualified Demand includes sales orders that are due today, past due, or within a defined demand spike horizon.

The result of the Net Flow Equation directly maps to the DDMRP buffer zones:

• Green Zone: Net Flow is high. No action required; the system is stable.
• Yellow Zone: Net Flow has dropped into the rebuild zone. A supply order is typically generated to restore the buffer to green.
• Red Zone: Net Flow is critically low. The system prioritizes this item for immediate replenishment to prevent a stockout. The red zone is further split into a safety base and a safety zone.

Unlike traditional MRP which relies on forecasted demand, the Net Flow Equation uses only qualified actual demand. This decouples the execution signal from forecast inaccuracy. By only subtracting real, qualified sales orders, the equation prevents the nervousness and bullwhip effect caused by constantly changing forecasts. The buffer absorbs variability, while the net flow calculation provides a stable, trustworthy signal for execution.

The Net Flow Equation is the input for calculating replenishment priority. The percentage of the buffer consumed is derived by comparing the Net Flow position against the total buffer level. A lower percentage indicates higher urgency. This allows planners to focus on items where Net Flow is deepest in the red zone, providing an objective, mathematically sound method for prioritizing work orders and purchase orders across thousands of SKUs.

A critical component is the Qualified Sales Order Demand term. To prevent a single large order from instantly crashing the Net Flow and triggering a false emergency, orders are qualified against a defined spike threshold. Demand exceeding this threshold is not fully subtracted from the equation immediately. Instead, it is managed separately, ensuring the Net Flow reflects normal operational demand and prevents overreaction to known, planned large events.

The Net Flow Equation is designed for continuous recalculation. Every inventory transaction, order release, or sales order commitment triggers an update. This provides a real-time, always-accurate view of buffer health. The dynamic nature allows the system to instantly detect emerging problems, such as a supplier delay that prevents on-order stock from arriving, and immediately escalate the priority as the Net Flow position deteriorates.

The Net Flow Equation directly positions inventory within the buffer's color-coded zones. The equation's result is compared against the Green, Yellow, and Red zone thresholds to determine replenishment priority.

• Green Zone: Net Flow is high; no action required.
• Yellow Zone: Net Flow is moderate; normal replenishment is triggered.
• Red Zone: Net Flow is critically low; urgent supply order generation is required to avoid a stockout.

A critical subtrahend in the Net Flow Equation. This represents actual demand that is due today, past due, or within a defined demand spike horizon, not just a raw forecast. It filters out future demand that is not yet actionable to prevent premature buffer erosion.

• Past due sales orders are always included.
• Future demand is only qualified if it falls within a specific spike threshold to prevent a single large order from distorting the buffer status.

The physical stock currently residing in the warehouse, representing the most liquid component of the Net Flow Equation. This is a non-negotiable, deterministic value.

• Cycle Count Accuracy: The Net Flow Equation is useless if on-hand records are inaccurate.
• Quarantine Exclusion: Stock that is physically present but blocked for quality reasons must be excluded from the on-hand component to avoid a false sense of security.

The open supply order quantity added to on-hand stock. This represents inventory that is in-transit or currently being manufactured and is a legitimate claim on future availability.

• Includes purchase orders not yet received.
• Includes manufacturing orders not yet completed.
• The equation assumes these orders will arrive on time; a spike in predictive lead time risk can invalidate this assumption.

The Net Flow Equation is the heartbeat of DDMRP execution. It is calculated daily (or in real-time) to provide a single, actionable percentage. Buffer Status % = (Net Flow / Green Zone Top) * 100 This percentage directly drives the visual prioritization in a Supply Chain Control Tower, allowing planners to ignore green items and focus exclusively on red and yellow exceptions.

The Net Flow Equation is the primary execution signal at a strategic decoupling point. It absorbs demand variability by translating actual consumption into a replenishment signal, protecting the longer-lead-time upstream operations from the Bullwhip Effect.

• It replaces the traditional MRP logic of exploding a forecast, which amplifies variability.
• It enables a pull-based execution model driven by real stock status rather than a push-based forecast.