Key takeaways
Short answer: Lot-for-lot and fixed order quantity are two lot-sizing rules MRP uses to turn net requirements into planned orders. Lot-for-lot (L4L) orders exactly what each period needs — the net requirement, no more — so it carries almost no cycle inventory but can generate many small, frequent orders. Fixed order quantity (FOQ) always orders the same predetermined amount whenever replenishment is triggered, which simplifies ordering and captures setup or quantity-break economics but leaves leftover inventory between needs. The choice is the classic trade-off: lot-for-lot minimizes holding cost, fixed order quantity minimizes ordering and setup cost.
Lot-for-lot (L4L) is the simplest lot-sizing rule in MRP: for each planning period, order exactly the net requirement — the quantity needed, no more and no less. If period 3 needs 40 units, the planned order is 40; if period 4 needs 25, the planned order is 25. The result is that almost no cycle inventory is carried from period to period, because each order is consumed by the demand that triggered it. Lot-for-lot shines when holding inventory is expensive or undesirable: high-value items where tied-up capital matters, perishable or obsolescence-prone parts, and lean or just-in-time flows where the whole philosophy is to make and move only what is needed. It is also the natural rule when setup or ordering cost is genuinely low, because then there is little penalty for ordering frequently. The cost of lot-for-lot is exactly that frequency: it can generate many small orders, each carrying its own ordering or setup burden, which is wasteful when those costs are high.
Fixed order quantity (FOQ) takes the opposite stance: whenever a replenishment is needed, order the same predetermined quantity every time, regardless of the exact net requirement. If the fixed quantity is 100, then every planned order is 100 — even if this period only needs 30 — with the surplus carried forward to cover future periods until it runs out and another 100 is ordered. The fixed quantity is not arbitrary; it usually reflects a real constraint or economy: a supplier's minimum order quantity, a full pallet, drum, or container size, a quantity-break price threshold, or an economic order quantity (EOQ) calculated to balance ordering and holding cost. FOQ's advantages are simplicity and economics — fewer, larger orders that capture setup efficiencies, volume discounts, and packaging units. Its cost is leftover inventory: because orders rarely match requirements exactly, FOQ carries surplus stock between needs, tying up capital and space in exchange for ordering less often.
Underneath both rules is the same fundamental tension: holding cost versus ordering (or setup) cost. Every time you place an order or set up a machine, you incur a fixed cost that is independent of quantity — paperwork, receiving, a changeover. Every unit you hold in inventory incurs a carrying cost — capital, space, insurance, obsolescence. Lot-for-lot drives ordering cost up (many small orders) to drive holding cost to nearly zero. Fixed order quantity drives holding cost up (leftover stock) to drive ordering cost down (few large orders). The right balance depends on the ratio of those two costs, which is exactly what the economic order quantity formula computes — and a well-chosen FOQ is often just the EOQ. When ordering or setup cost dominates, lean toward larger fixed quantities; when holding cost dominates, lean toward lot-for-lot. The two rules are endpoints of this single trade-off, not unrelated methods.
The two rules produce visibly different inventory patterns over time. Lot-for-lot yields a nearly flat, near-zero cycle-stock line: inventory rises just to meet each period's need and is drawn straight back down, with little or nothing carried between periods. Fixed order quantity yields the classic sawtooth: inventory jumps up by the fixed quantity when an order arrives, then steps down as periods consume it, until it is low enough to trigger the next fixed order and the pattern repeats. The area under that sawtooth — the average inventory carried — is what FOQ pays in holding cost to buy fewer orders. Lot-for-lot's flat line is what it saves in holding cost at the price of more frequent ordering. Seeing the two patterns side by side makes the trade-off concrete: FOQ trades a hump of carried inventory for fewer transactions, while L4L trades more transactions for almost no carried inventory. (Note that both are about cycle stock; safety stock to cover variability is a separate layer — see safety stock vs reorder point.)
Suppose net requirements over five periods are 20, 50, 10, 30, and 40 units — 150 total. Under lot-for-lot, MRP plans five orders of exactly 20, 50, 10, 30, and 40. Cycle inventory carried between periods is essentially zero, but you place five separate orders, paying the ordering or setup cost five times. Under fixed order quantity with a quantity of 60, you order 60 in period 1 (covering the 20 and most of period 2), order another 60 when stock runs low to cover the middle periods, and a third 60 toward the end — three orders of 60 for 180 units, leaving 30 units of leftover inventory carried across various periods. Now put numbers on the costs: if ordering costs 50 per order and holding costs 2 per unit-period, lot-for-lot pays about 250 in ordering and almost nothing in holding, while FOQ pays about 150 in ordering plus the carrying cost of that leftover stock. Whichever total is lower wins — and that depends entirely on the ratio of the 50 to the 2. Change the ordering cost to 200 and FOQ pulls clearly ahead; drop holding cost and lot-for-lot's penalty for frequency shrinks.
Choose by which cost dominates and by physical constraints. Favour lot-for-lot when holding cost is high (expensive, bulky, perishable, or obsolescence-prone items), when setup and ordering costs are low, and when you run lean or just-in-time flow that wants minimal inventory. Favour fixed order quantity when ordering or setup cost is high, when suppliers impose minimum order quantities or sell in fixed pack sizes, when quantity-break pricing rewards larger orders, or when an EOQ analysis shows a clear economic batch. Between the two extremes sit middle-ground rules like period order quantity (order enough to cover a fixed number of periods) that smooth the frequency of lot-for-lot without carrying as much as a flat FOQ. The decision is not ideological — it is arithmetic plus constraints: estimate ordering and holding costs, respect supplier and packaging realities, and pick the rule (or middle option) that minimizes total cost for that specific item. Different parts in the same plant will rightly use different rules.
Lot-sizing reaches the shop floor as changeover load, and changeovers hit the Availability factor of OEE. Lot-for-lot's many small, exact orders mean more frequent production runs and therefore more setups and changeovers — each one a planned stop that costs availability unless setup time is short. Fixed (larger) order quantities mean fewer, longer runs and fewer changeovers, protecting availability but at the cost of more work-in-process inventory. This is the lot-sizing-versus-OEE tension in a nutshell: smaller lots improve flow and reduce inventory but raise changeover frequency, so they only pay off when changeover time is driven down (the SMED logic behind faster changeovers). It also connects to flow strategy — small lots are the heartbeat of pull production. Choosing a lot-sizing rule is therefore not just a planning decision; it sets the changeover burden your OEE has to absorb.
Fabrico makes the OEE cost of your lot-sizing visible by capturing changeover and setup losses against live OEE. When small lot-for-lot runs multiply changeovers, the availability hit shows up directly in the data — telling you whether your lot sizes are flooding the floor with setups, and whether faster changeovers would let you run smaller, leaner lots without losing uptime. That turns an abstract planning trade-off into a measured one. Book a demo to see how your lot sizes affect changeover losses and availability.
Lot-for-lot orders exactly the net requirement for each period, carrying almost no cycle inventory but placing many small orders. Fixed order quantity always orders the same predetermined amount, carrying leftover inventory but ordering less often. The difference is a trade-off between holding cost and ordering cost.
Use lot-for-lot when holding cost is high (expensive, bulky, perishable, or obsolescence-prone items), when setup and ordering costs are low, and in lean or just-in-time flows that want minimal inventory. It minimizes carried stock at the cost of more frequent ordering.
Fixed order quantity is better when ordering or setup cost is high, when suppliers impose minimum order quantities or fixed pack sizes, when quantity-break pricing rewards larger orders, or when an EOQ analysis shows a clear economic batch. It minimizes ordering cost at the price of leftover inventory.
Not exactly. EOQ is a formula that calculates the order quantity minimizing total ordering plus holding cost. A fixed order quantity is any predetermined order size — it is often set equal to the EOQ, but it can also be driven by supplier minimums or packaging units rather than the EOQ calculation.
Lot sizing drives changeover frequency. Lot-for-lot's small, frequent runs create more setups and changeovers, which reduce the Availability factor unless changeover time is short. Larger fixed quantities mean fewer changeovers but more work-in-process inventory. Faster changeovers let you run smaller lots without losing uptime.
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