Perpetual Inventory System

Ecommerce depends on perpetual inventory because storefronts and marketplaces need a live, accurate count to avoid overselling. Every receipt, sale, transfer, and adjustment updates the balance, but records still drift from physical stock through mis-picks, damage, and theft. For example, if 4 of 400 daily picks go wrong without being caught, records can drift by about 360 units over a 90-day quarter. Cycle counts and reconciliation catch that drift. Compare system and counted quantities by SKU every month.

Perpetual inventory became practical for retailers after the first UPC scan in 1974, as point-of-sale scanning updated stock counts with each sale instead of waiting for periodic counts.

Do I still need physical counts with a perpetual inventory system?

Yes. Mis-picks, receiving errors, damage, and theft make records drift, so perpetual systems are verified with cycle counts or periodic full physical counts.

What is the difference between perpetual and periodic inventory systems?

Perpetual systems update inventory and cost of goods sold with each transaction, so you always have a current balance. Periodic systems rely on physical counts at set intervals and calculate cost of goods sold only after each count.

How does a perpetual system calculate cost of goods sold?

It records COGS at the time of each sale, using a costing method such as FIFO, weighted average, or standard cost. That gives a running gross margin instead of one calculated only after a count.

• Periodic Inventory SystemStock records updated by periodic counts
• Cycle CountCounting a little stock on a rotating schedule
• Inventory AccuracyHow well system counts match the shelf