Lot sizing is the process of determining the quantity of an item to order or produce in a single batch. In Material Requirements Planning (MRP), the lot-sizing rule tells the system how to group net requirements into planned orders — whether to order exactly what is needed, combine multiple periods of demand, or use a formula-based quantity.
At User Solutions we see lot sizing as one of the most impactful yet underappreciated planning decisions. The wrong lot-sizing rule can inflate inventory by 30% or create unnecessary capacity bottlenecks. The right rule balances cost, capacity, and delivery performance.
How Lot Sizing Works
After MRP calculates net requirements by period, it applies the lot-sizing rule to determine order quantities. Different rules produce different results:
Common Lot-Sizing Methods
Lot-for-Lot (LFL) — Orders exactly the net requirement each period. Zero excess inventory but maximum ordering/setup frequency.
Economic Order Quantity (EOQ) — Uses the classic formula to balance ordering and holding costs. Works best for stable, predictable demand.
Fixed Order Quantity (FOQ) — Always orders the same predetermined quantity. Simple to manage; may result in excess inventory or multiple orders per period.
Period Order Quantity (POQ) — Combines net requirements over a fixed number of periods into one order. Reduces ordering frequency while adapting to demand changes.
Least Unit Cost (LUC) — Evaluates different lot sizes and selects the one with the lowest cost per unit including setup and carrying costs.
Least Total Cost (LTC) — Selects the lot size where ordering cost most closely equals carrying cost over the planning horizon.
Lot Sizing Example
Component Y-200 has these net requirements over 8 weeks, with an ordering cost of $50 and a carrying cost of $2 per unit per week:
| Week | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Net Req | 30 | 0 | 45 | 20 | 0 | 35 | 25 | 0 |
Lot-for-lot: 5 orders × $50 = $250 ordering cost + $0 carrying = $250 total
POQ (3 periods):
- Week 1: order 75 (covers Weeks 1-3) → carry 45 one week + carry 0 = $90 carrying
- Week 4: order 55 (covers Weeks 4-6) → carry 35 one week + carry 0 = $70 carrying
- Week 7: order 25 (covers Weeks 7-8) → $0 carrying
- 3 orders × $50 = $150 ordering + $160 carrying = $310 total
FOQ at 80 units:
- Week 1: order 80 → excess covers into Week 3
- Week 3: order 80 → excess covers into Week 6
- Week 6: order 80 → excess covers through Week 7+
- 3 orders × $50 = $150 ordering +
$250 carrying = **$400 total**
In this case, lot-for-lot wins because the demand is lumpy and carrying costs accumulate quickly. But if ordering costs were $200 instead of $50, consolidating orders would become more attractive.
Why Lot Sizing Matters for Scheduling
Directly impacts setup time and capacity. Larger lots mean fewer setups — freeing capacity for production. Smaller lots mean more setups but shorter lead times and lower WIP. Scheduling software like Resource Manager DB must account for lot-size-driven setup frequency when loading work centers.
Determines inventory investment. Lot sizing is one of the primary levers for controlling inventory levels. Switching from FOQ to lot-for-lot for high-value items can free significant working capital.
Affects lead times and delivery flexibility. Smaller lot sizes flow through the shop faster, reducing manufacturing lead times. This improves responsiveness to customer changes — a key advantage in make-to-order environments.
Interacts with safety stock policy. Lot-sizing decisions should be made in conjunction with safety stock levels. Increasing lot sizes inherently provides a buffer; reducing lot sizes may require higher safety stock to maintain service levels.
Related Terms
- Lot-for-Lot — The lot-sizing method that orders exactly what is needed each period, minimizing carrying cost.
- Economic Order Quantity (EOQ) — The formula-based method that calculates the cost-optimal order quantity.
- Net Requirements — The per-period demand figures that lot-sizing rules act upon to generate planned orders.
FAQ
The most common methods are lot-for-lot (order exactly what is needed each period), EOQ (Economic Order Quantity based on the classic cost-balancing formula), fixed order quantity (always order the same amount), period order quantity (combine requirements over a fixed number of periods), and least unit cost / least total cost (dynamic methods that minimize cost per unit or total cost over the planning horizon).
Consider the item's value, demand pattern, and ordering costs. High-value items with irregular demand favor lot-for-lot. Low-value items with steady demand favor EOQ or fixed order quantity. Items with significant setup costs benefit from methods that consolidate orders. Most ERP systems let you assign different lot-sizing rules to different items.
Yes, significantly. Larger lot sizes mean fewer setups but longer production runs, concentrating capacity load. Smaller lot sizes spread production more evenly but consume more setup time. The lot-sizing decision directly impacts machine utilization, lead times, and scheduling flexibility.
This term is part of the Manufacturing Glossary. For a deep dive into material planning, see our MRP Guide.
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