When This Material Runs Out, How Long Can the Production Line Hold Out? — A Seven-Field Inventory Check and Red Alert Line for Supply Disruption Risks
A second-tier supplier's workshop was shut down due to power restrictions, but the first-tier supplier continued to deliver on schedule, accepted the delivery, and reported "production normal." Two weeks later, the main factory had to halt production. A post-incident review found that the problem wasn't a lack of oversight over the supplier, but rather that the inventory check only included the first-tier supplier, and the small factory deep in the BOM was never listed in any table.
Supply disruptions are not about "suppliers not following instructions"; they are a quantifiable measure in days. If you can't calculate it, you can only wait for the production halt notice.
1. Calculate a Number: Net Coverage Days
The core metric for supply continuity is only one, and the supply chain planner must be able to calculate it themselves:
Net Coverage Days = (Current Inventory + Confirmed In-Transit + Available for Reallocation) ÷ Average Daily Consumption
Shortfall Days = Supplier Replenishment Lead Time (LT) − Net Coverage Days
If the shortfall days are positive, it indicates a supply disruption risk; if negative or zero, it indicates a buffer.
For example, consider a sealing component: the average daily consumption is 800 units, the current inventory is 4,200 units, and 2,400 units are in transit (arriving in 10 days), with no associated factories available for reallocation.
- Net Coverage Days = (4,200 + 2,400) ÷ 800 = 8.25 days
- Supplier Replenishment LT = 21 days
- Shortfall Days = 21 − 8.25 = 12.75 days
In other words, if this supplier stops production today, the production line will start to slow down on the 9th day and may only resume supply on the 21st day, with no material available for 12.75 days in between. Assuming the production line generates 600,000 yuan per day, the exposure is approximately 7.65 million yuan—this figure is the basis for deciding whether to invest in buffer inventory, not "this supplier has a good relationship."
2. Seven Fields in the Inventory Check
Fixed fields, used repeatedly, allow for horizontal comparison. Missing any field will degrade the inventory check into a supplier contact list.
| Field | Filling Requirements | Data Responsible Person |
|---|---|---|
| Material Number and Level | Marked to BOM level, fully expanded within three levels | Planner |
| Single-Point Dependency | Whether the material is supplied by only one source, and if there are any production-verified alternatives | Purchasing |
| Concentration | Number of suppliers, whether they are in the same region/group/and the same upstream | Purchasing |
| Replenishment LT | 90th percentile of actual delivery days, not contract days | Purchasing |
| Net Coverage Days | Calculated using the formula in the previous section, including confirmed in-transit quantities | Planner |
| Alternative Path | Whether the alternative material has completed DV/PV and whether the customer has given written approval | SQE |
| Line Stoppage Loss | The daily production value or daily claim amount corresponding to a supply disruption | Finance |
The two data sources most prone to falsification must be clearly stated in the table header: LT uses the 90th percentile actual value (suppliers with a contract of 15 days but actual delivery of 26 days are common); in-transit only counts confirmed shipments, not supplier's verbal production plans.
3. Three-Tier Scoring and Alert Lines
Each dimension is scored from 1 to 5, and the total risk score (3 to 15) is the sum of the three scores:
Impact Score S: 5 = Direct line stoppage with no alternative, involving customer claims; 4 = Single line stoppage or multiple lines slowing down; 3 = Slowing down, rework, expedited freight; 2 = Delayed delivery; 1 = Adequate alternatives and inventory.
Exposure Score E (based on shortfall days): ≥ 20 days 5 points; 10 to 19 days 4 points; 5 to 9 days 3 points; 1 to 4 days 2 points; ≤ 0 days 1 point.
Concentration Score C: Single source 5 points; Two sources in the same region or same upstream 4 points; Two sources in different regions 3 points; Three or more sources with dispersed locations 2 points; Internal backup available 1 point.
For the example sealing component: S = 4 (single line stoppage), E = 4 (shortfall of 12.75 days), C = 5 (single source), total 13 points → Red.
| Risk Score | Tier | Mandatory Actions | Re-evaluation Frequency |
|---|---|---|---|
| ≥ 11 | Red | Develop a dual-source or alternative verification plan within 30 days; Increase safety stock to cover ≥ LT×1.2; Notify the customer | Monthly |
| 7 to 10 | Yellow | Initiate dual-source development, follow up quarterly; Ensure inventory covers ≥ LT | Quarterly |
| ≤ 6 | Green | Include in routine monitoring, track changes in LT and concentration | Semi-annually |
The target safety stock quantity is directly derived from the formula: Average Daily Consumption × (LT × 1.2 − In-Transit Days Converted to Quantity). For the example, to cover 21×1.2 = 25.2 days, 800 × 25.2 ≈ 20,160 units are needed, minus the 2,400 units in transit, leaving a shortfall of approximately 17,760 units—this is the answer to "how much inventory should be prepared."
4. The Three Most Common Mistakes
Only checking the first-tier supplier. The first-tier supplier may have sufficient capacity, but its critical components come from a small factory, which is invisible in the inventory check. For any red material, you must trace two levels up and ask, "Where do the raw materials/critical components for this batch come from, and how many sources are there?"
Sorting by purchase amount instead of risk. Managing high-value items first sounds reasonable, but line stoppages are determined by "whether there are alternatives." A sealing ring that accounts for 0.3% of the purchase amount can still halt the entire production line if it is out of stock. The sorting criterion can only be the risk score, not the amount.
Assuming "having an alternative material" means "it can be used immediately." If the alternative material hasn't been verified, the drawings haven't been changed, and the customer hasn't approved it, it cannot be used on the production line. The alternative path field can only have three states: production-verified and customer-approved, verified but not approved, and not verified. The second state is equivalent to zero on the day of supply disruption.
There is also an unavoidable cross-functional conflict: Purchasing wants to consolidate purchases to get better prices, Planning wants to reduce inventory turnover, and SQE wants to verify dual sources—these three directions naturally conflict. The decision rule should be clearly defined in the process: Red-tier materials are not subject to inventory reduction targets, and the costs for dual-source verification samples and tests are covered by the purchasing cost reduction benefits. Without this rule, red-tier materials will always be at the bottom of the cost reduction list.
5. Five Steps to Make Inventory Checks Routine
- Define the List: Expand the BOM to three levels, remove generic standard parts, and focus on dedicated, custom, and single-source parts, which typically account for 10% to 20% of total material numbers. Start with this batch.
- Collect Data: Use the 90th percentile of actual delivery days over the past 12 months for LT; take a snapshot of inventory, in-transit, and consumption on the same day, and include the date in the table header to avoid different people using different time points.
- Calculate Exposure: Calculate the net coverage days and shortfall days for each material, and only score positive numbers.
- Score and Tier: Sum the S, E, and C scores to determine the tier (red, yellow, green), and immediately assign a responsible person and completion date for red-tier materials.
- Set Re-evaluation Frequency: Monthly for red, quarterly for yellow, and semi-annually for green; if the LT changes by more than 30% or if the supplier experiences equity changes, relocation, or power restrictions, re-calculate immediately without waiting for the re-evaluation date.
After completing the inventory check, you should have three things: a list of red-tier materials, a buffer measures table with responsible persons and dates for each item, and a set of safety stock target values. Without these three, the inventory check is just a form-filling exercise.
Supply disruptions are not sudden accidents; they are the result of a long-term positive shortfall days that no one has calculated—first put the coverage days and LT into a formula, and the buffer quantity will naturally have an answer.
Knowledge code: 4.2.3
Version: v20261025
Author: QTank QTank is dedicated to providing systematic professional knowledge, methodologies, and practical tools for quality management practitioners, helping companies continuously improve their quality capabilities.