A backorder is an order accepted by a supplier for an item they do not currently have in stock — the item is on order from their own supplier or in production and will ship when available, extending your effective lead time by an unpredictable duration.
Quick answers
What is a backorder? A backorder means the supplier accepts your purchase order but cannot fulfill it immediately. The item will ship when the supplier's own stock is replenished. Your stated lead time of 5 days might become 15–30 days. Unlike a stockout on your end — where the customer gets nothing — a backorder is a deferred fulfillment. The supplier owes you the goods; you just don't know exactly when they'll arrive.
How is a backorder different from a stockout? A stockout is your problem — you are out of an item and cannot fulfill customer demand. A backorder is your supplier's problem — they are out and cannot fulfill your order. But your supplier's backorder rapidly becomes your stockout. The distinction matters operationally: a stockout requires you to find an alternative or lose the sale; a backorder requires you to decide whether to wait, substitute, or source elsewhere.
How do backorders affect lead time? Backorders inject variance into lead time that your standard safety stock formula does not account for. If your lead time is normally 5 days with σ = 1 day, a backorder can push delivery to 20+ days — a 15-day deviation that no reasonable z-score would buffer against. Backorder risk is better handled by supplier diversification and visibility than by carrying more safety stock.
What is a partial shipment? When a supplier ships the portion of your order they have in stock and backorders the remainder. You receive 40 of 60 cases now, 20 cases in two weeks. This creates split receiving, split invoicing, and freight cost inefficiency — the overhead of two deliveries for one order.