Lot Size
Lot size is the quantity of units planned, ordered, produced, or received as one replenishment or production batch in an inventory or manufacturing system. In ERP and supply planning, it is a master-data parameter that tells the system how to calculate replenishment quantities, cost per unit, or rounding rules for MRP and supply planning.
On the shop floor, lot size governs how many units are released per production run, purchase order, or replenishment receipt. Larger lots cut setup frequency and stabilize throughput but increase work-in-process, finished-goods exposure, and storage demand; smaller lots trim inventory but add changeover time and can starve bottleneck lines. In warehousing, lot size interacts with lot-controlled inventory: received or produced material is grouped for traceability, quality release, and issue-to-production decisions, and the replenishment quantity often must align with pallet, tank, or case-pack capacities to avoid partial pallets and split receipts. Planners set minimum, maximum, and rounding values in the item master so MRP or supply planning generates feasible quantities that respect supplier pack sizes, transport economics, and production campaign constraints. When lot sizes are deliberately used as buffers, they absorb variability in demand, supply, and service levels across multistage supply chains, though that buffering comes at the cost of extra cycle stock.
- Chronic stockouts at the line: When lot size sits below real consumption, the system creates too-small, too-frequent orders, demand outruns supply, and a bottleneck shortage halts the full downstream schedule.
- Excess cycle stock and hidden carrying costs: Oversized lots make plants receive or produce more than needed, tying up cash, floor space, and warehouse capacity while raising obsolescence risk for shelf-life-sensitive and slow-moving SKUs because batches sit in inventory longer.
- Receiving and changeover mismatches: When lot size ignores pallet quantities, supplier case packs, or campaign sizes, operations get partial pallets, split receipts, extra handling, and avoidable changeovers, which can bottleneck staging on the warehouse floor.
Is lot size the same as lot number?
No. Lot size is the quantity per batch or replenishment rule, while a lot number is the traceability identifier assigned to a specific material batch. They are managed together in ERP and WMS but are distinct concepts.
Does lot size always control MRP?
No. Some systems use lot size for MRP planning, but others use the field only for costing or setup calculations. Infor CSI, for example, states its lot size field is not used for MRP or APS.
What is the difference between fixed lot size and lot-for-lot?
Lot-for-lot orders exactly the net requirement, while fixed lot size orders the same predefined quantity each time, even if demand is smaller or larger. Both are common lot-sizing procedures in supply planning.