What Is Lot Sizing?
Also known as: order policy, lot size rule, order quantity rule
Definition
Lot sizing is the ERP rule that determines how much to order or produce in each replenishment, converting net requirements into order quantities by balancing setup or ordering cost against inventory carrying cost.
Lot Sizing Explained
After MRP nets requirements, it must decide how to group them into orders. Lot-for-lot creates one order exactly matching each period requirement, minimizing inventory but maximizing setups. Fixed order quantity always orders the same amount, which suits parts with a hard supplier pack or minimum. Period order quantity, sometimes called days of supply or fixed period requirements, sums demand across a defined number of periods into one order. Economic order quantity computes a mathematically optimal size from ordering cost, annual demand, and carrying cost.
The classic economic order quantity is the square root of two times annual demand times order cost, divided by unit carrying cost. Its weakness is that it assumes steady demand and constant costs, neither of which holds in discrete manufacturing with engineering changes and seasonal orders. In practice most shops use it as a sanity check on order magnitude rather than as a directly implemented policy, and use period order quantity or lot-for-lot with a minimum for actual planning.
Modifiers layer on top of the base rule. Minimum order quantity enforces a supplier or press minimum. Maximum order quantity caps a single order, often for shelf-life or capacity reasons. Multiple or increment rounds up to a pack, pallet, or bar length. Order these correctly in your head: the base rule computes a raw quantity, then minimum, maximum, and multiple adjust it. Misconfigured modifiers are a leading cause of MRP suggestions that look absurd, such as a 5,000-unit order for a part needing 12.
Lot sizing choices ripple into capacity. Larger lots reduce setup frequency, which increases effective capacity at a bottleneck, but they inflate work in process, lengthen queue time, and delay problem detection because defects are discovered a full lot later. This is why lean environments push toward lot-for-lot and setup reduction rather than optimizing lot size against a fixed setup cost. The setup cost in the economic order quantity formula is treated as a given, when in many plants it is the thing most worth attacking.
Why It Matters
- Lot sizing directly sets average inventory investment, typically about half the lot size plus safety stock for every item.
- Setup frequency driven by lot size consumes real bottleneck capacity, so the rule affects throughput as much as inventory.
- Order minimums and multiples that are stale relative to supplier terms create excess that is invisible until it ages.
- Larger lots delay defect discovery, increasing scrap and rework exposure in quality-critical aerospace and defense work.
In Practice
A frequent gotcha: an order multiple of 100 is entered for a part that MRP needs 12 of, and the buyer never questions the 100-unit suggestion because it comes from the system. Review order modifiers against current supplier terms annually, and put an exception report on any planned order more than three times the netted requirement.
Frequently Asked Questions
What is lot-for-lot ordering?
Lot-for-lot creates a planned order that exactly matches each period net requirement, so no excess inventory is carried between periods. It minimizes holding cost and is the default for expensive, low-volume, or engineered-to-order items. The trade-off is maximum order and setup frequency, which is only acceptable when setup or ordering cost is genuinely low.
Is EOQ still useful in modern ERP?
As a directional check, yes. Economic order quantity tells you the rough scale at which ordering cost and carrying cost balance, which is useful when someone proposes a lot size ten times larger. As a live planning parameter it is fragile, because it assumes steady demand and static costs that discrete manufacturing rarely delivers.
Related Terms
MRP (Material Requirements Planning)
MRP (Material Requirements Planning) is the ERP calculation that explodes demand through bills of materials, nets it against on-hand and on-order inventory, and offsets by lead time to produce timed purchase and work order suggestions.
Reorder Point
A reorder point is the inventory level at which a replenishment order is triggered. It equals expected demand during the replenishment lead time plus safety stock, so stock on hand covers usage until the new supply arrives.
Capacity Planning
Capacity planning is the process of comparing the labor and machine hours a production plan requires against the hours each work center actually has available, identifying overloads early enough to reschedule, add shifts, or outsource.
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