We live and die by inventory in supply chain.
Success depends on how we manage inventory.
Failure depends on inventory too.
Of course, there are factors that you cannot control. Supplier delays. Sudden market shifts. Competitors’ moves.
Don’t waste energy there.
Focus on what you can control.
Start with the basics. Master the simple calculations that rule inventory planning.
In this article, we will do the following:
- The concept behind each formula
- Common challenges and how to address them
- Action to take on the job
1. Gross Requirements
Formula: Forecast (for MTS) + Customer Orders (for MTO)
What it calculates: The total demand you must plan for. It’s the “big picture” before you account for what’s already in stock or on the way. Net requirements (formula 2) comes after this, once inventory and receipts are deducted.
Example: Forecast = 1,000 cookie packs, confirmed orders = 200 → gross requirement = 1,200.
Challenge: Forecasts may be inflated or orders duplicated.
Fix: Cross-check demand forecasts with confirmed orders and clean duplicates regularly.
Action:
- Demand Planner: Validate demand signals from sales and POS data.
- Supply Planner: Don’t plan production until forecast + orders align.
2. Net Requirements
Formula: Gross Requirements – (Available Inventory + Scheduled Receipts)
What it calculates: The real demand gap you still need to cover after using what’s already available. It’s like the “refined version” of gross requirements. Net drives your production and purchase orders.
Example: Need 1,200 chocolate bars. Already have 400 in stock, 300 arriving. Net = 500.
Challenge: Inventory records may be outdated or receipts delayed.
Fix: Reconcile stock regularly and confirm inbound dates with suppliers.
Action:
- Supply Planner: Double-check available-to-promise numbers before scheduling runs.
- Materials Planner: Confirm open POs with suppliers to avoid last-minute surprises.
3. Cycle Stock
Formula: Order Quantity ÷ 2
What it calculates: The average inventory sitting in between replenishments. Unlike gross/net requirements (formulas 1 & 2) that tell you how much you need, cycle stock shows you how much you’re holding in steady state. It connects later to projected ending inventory (formula 5), since both describe stock positions.
Example: Order 1,000 soda cans each month → average stock = 500 cans.
Challenge: Large orders tie up cash, while small ones create frequent ordering headaches.
Fix: Balance order sizes by reviewing carrying vs ordering cost.
Action:
- Materials Planner: Run test simulations with smaller lot sizes to see the impact.
- Supply Planner: Match batch sizes to production runs to avoid excess.
4. Fill Rate
Formula: (Units Shipped ÷ Units Ordered) × 100
What it calculates: The percentage of demand you actually served. It’s a service-level metric that reveals how well you converted gross/net requirements into real deliveries. Unlike throughput rate (formula 6), which looks inside the factory, fill rate looks outside at the customer.
Example: Customer orders 1,000 chip packs. You ship 950. Fill rate = 95%.
Challenge: Low fill rates hurt service. High fill rates may hide overstocks.
Fix: Pair fill rate with root cause analysis (forecast miss? supply issue? stockout?).
Action:
- Demand Planner: Compare forecast error vs fill rate to find demand-driven misses.
- Supply Planner: Trace missed shipments back to downtime or material gaps.
5. Projected Ending Inventory
Formula: Beginning Inventory + Production – Requirements
What it calculates: The future balance of stock after producing and fulfilling requirements. It’s forward-looking, while net requirements (formula 2) is “what to cover right now.” Projected ending inventory is key to see if you’ll stay above safety stock after all plans are executed.
Example: Start with 500 milk cartons. Produce 300 more. Demand = 600. Ending = 200.
Challenge: Ignoring scrap, spoilage, or delays gives false optimism.
Fix: Build in adjustment factors for waste and reliability.
Action:
- Supply Planner: Share realistic projections with sales so promises are credible.
- Materials Planner: Adjust purchases based on real consumption.
6. Throughput Rate
Formula: Total Output ÷ Time Period
What it calculates: The speed of production. Unlike gross/net requirements (formulas 1 & 2), which say how much is needed, throughput shows if your factory can actually deliver at that pace. It directly affects whether fill rate (formula 4) is met.
Example: Candy line makes 10,000 lollipops in 5 hours → 2,000/hour.
Challenge: Averages mask downtime or bottlenecks.
Fix: Track throughput at SKU/line level daily, not just monthly averages.
Action:
- Supply Planner: Flag when throughput underperforms vs plan.
- Materials Planner: Monitor usage vs throughput to catch mismatches early.
7. Lead Time
Formula: Order Time + Production Time + Shipping Time
What it calculates: The total wait time from request to delivery. It connects to net requirements (formula 2). If your lead time is too long, you’ll need to plan earlier. It also connects to projected ending inventory (formula 5), as long lead times mean higher safety stock.
Example: Cheese = 1 day, dough = 2 days, shipping = 1 day → 4 days lead time.
Challenge: Variability (late suppliers, transport delays) makes plans unreliable.
Fix: Measure average vs worst-case lead times and adjust safety stock accordingly.
Action:
- Demand Planner: Forecast further ahead than the longest lead time.
- Supply Planner: Reduce lead time via local suppliers or faster logistics.
- Materials Planner: Hold buffer stock only for items with volatile lead times.
Takeaway: Each formula connects with the others. Gross tells you total demand. Net shows the gap. Cycle stock tells you what’s held. Fill rate measures service. Projected ending inventory predicts your balance. Throughput checks production speed. Lead time shows how fast the chain moves. Together, they give you the full inventory story.
