Purchasing and inventory management exist to solve exactly this problem. One function decides what to buy and from whom; the other decides how much to hold and when to reorder. Get them working together, and cash flow, customer satisfaction, and margins all improve at once.
This article breaks down the full process, the 7 steps of purchasing, the main types of purchasing and inventory, the formulas that make it measurable (EOQ, ROP), and how purchases actually hit your books.
Key Takeaways
- Treat purchasing and inventory as one loop: what to buy, how much, when, and how you record it.
- Run every buy through a 7-step cycle, from need identification to closing the books.
- Match the method to the problem—EOQ and ROP for quantity/timing, FIFO/LIFO for valuation, JIT for lean stock.
- Keep journal entries and valuation methods consistent so financials stay audit-ready.
What Is Purchasing and Inventory Management?
Purchasing management is the function of sourcing, negotiating, and buying the goods and materials a business needs to operate. Inventory management is the process of ordering, storing, tracking, and controlling that stock so you can meet demand without tying up excess cash.
These two functions are not separate silos. They form a feedback loop. Purchasing decisions set your inventory levels—order too much and stock balloons. Inventory data (turnover rates, current stock, sales velocity) tells purchasing what and when to buy next.
When that loop breaks down, the cost shows up fast. One manufacturing case study using ABC/XYZ classification and reorder-point modeling on 13 purchased components cut total inventory-related costs by 33%, according to research published in MDPI's supply chain journal.
Decisions made together free cash that isolated purchasing or inventory calls leave trapped on the shelf.
The 5 R's of Purchasing
Procurement professionals use a simple checklist to keep every purchase decision grounded:
- Right quality: meets your specifications, no more, no less
- Right quantity: matches actual demand
- Right time: arrives when you need it, not before or after
- Right source: a supplier you can rely on
- Right price: reflects true total cost, not just sticker price
The 7 Steps of the Purchasing Process
Every purchase, whether it's raw material for a production run or office supplies, follows the same basic cycle.
- Identify the need: Triggered by demand forecasts, a low-stock alert, or a new project requirement.
- Specify requirements: Define exact quality, quantity, and technical specifications.
- Identify and evaluate suppliers: Compare pricing, minimum order quantities, and reliability track records.
- Create and send the purchase order: Lock in quantities, price, and delivery date in writing.
- Confirm and follow up: Verify the supplier accepted the terms and is on track.
- Receive and inspect goods: Check delivered items against the PO to catch shortages or damage early.
- Match invoices, pay, and update records: Reconcile the invoice against the PO and receipt, then update inventory records.
Skip step 6 or 7 regularly and inventory records drift from reality. Purchasing then starts making decisions on bad data.
Types of Purchasing and Inventory Management
Four Main Types of Purchasing
- Personal/consumer purchasing: Individual buying for personal use; rarely relevant to B2B operations.
- Mercantile (resale) purchasing: Buying goods specifically to resell, common among distributors.
- Industrial/producer purchasing: Buying raw materials or components that become part of a manufactured product.
- Institutional/government purchasing: Purchasing bound by formal bidding and compliance requirements.
A component supplier buying steel coil for resale to fabricators is mercantile purchasing. A manufacturer buying that same coil to stamp into parts is industrial purchasing.
Four Main Types of Inventory
- Raw materials: Unprocessed inputs, like sheet metal before it's cut.
- Work-in-progress (WIP): Partially completed items sitting mid-production.
- Finished goods: Completed products ready to ship.
- MRO (maintenance, repair, operations): Supplies that keep operations running — machine lubricants, spare parts — without becoming part of the final product.
Four Main Types of Inventory Systems
- Periodic system: Inventory counted and updated at set intervals; cheaper to run, but leaves blind spots between counts.
- Perpetual system: Inventory updated in real time with every purchase or sale, so stock levels stay current.
- Manual/spreadsheet systems: Common for very small operations, but error-prone as volume grows.
- Automated/cloud-based PO software: Tracks stock, generates purchase orders, and syncs across warehouses automatically.
Perpetual systems give more current data but cost more to maintain and require more training, according to Penn State's accounting materials. Most growing manufacturers and distributors eventually outgrow spreadsheets and periodic counts simply because the coordination cost of stale data outweighs the software investment.
Integration matters as much as the system itself. Warehouse platforms that handle receiving, put-away, picking, and cycle counting only add value when they connect to your ERP, accounting, and supplier systems. Without that link, you're just moving the spreadsheet problem into a new tool.
Gushwork builds these integrations for industrial distributors and manufacturers, syncing stock quantities, orders, and fulfillment statuses across procurement, warehouse, and accounting platforms. Purchasing decisions then rest on live numbers, not last week's count.
Key Formulas and Techniques: EOQ, ROP, JIT, FIFO, and LIFO
Economic Order Quantity (EOQ)
EOQ answers one question: how much you should order at a time to minimize total ordering and holding costs?
EOQ = √(2DS / CI)
Where D is annual demand, S is cost per order, C is cost per unit, and I is annual carrying-cost rate. The formula assumes fairly steady demand and stable lead times, according to CIPS's operations management guidance.
Reorder Point (ROP)
ROP tells you exactly when to place the next order, before you run out.
ROP = (Average Daily Usage × Lead Time) + Safety Stock
Safety stock is your buffer against demand spikes or a late shipment. Skip it, and a single delayed delivery turns into a stockout.
Just-in-Time (JIT)
JIT purchasing times supplier deliveries to arrive right as production needs them, minimizing holding costs. The tradeoff: it depends entirely on accurate forecasting and dependable suppliers. A single disruption upstream can halt your production line with no buffer stock to absorb it.
FIFO vs. LIFO
EOQ, ROP, and JIT decide when and how much to buy. FIFO and LIFO decide how you value inventory you already hold—and what that does to COGS and taxable income.
| Method | Assumption | Effect |
|---|---|---|
| FIFO | Oldest inventory sold first | Lower COGS in inflationary periods, higher reported profit |
| LIFO | Newest inventory sold first | Higher COGS in inflationary periods, lower taxable income |
Per IRS Publication 538, businesses generally need IRS approval to switch between these methods once adopted, so pick deliberately.
Which technique fits depends on your situation:
- Perishable or fast-moving goods → JIT and FIFO
- Stable-demand durables → EOQ-driven bulk ordering
- Tight cash flow → ROP with conservative safety stock
Accounting for Purchased Inventory
Purchased inventory is a current asset. When you buy on credit, the entry is straightforward:
- Debit: Inventory (or Raw Materials) to increase the asset
- Credit: Accounts Payable to increase the liability
Example: You purchase $10,000 of raw materials on credit.
Debit: Raw Materials Inventory $10,000
Credit: Accounts Payable $10,000
For businesses using a periodic inventory system, the purchases formula connects these figures:
Purchases = COGS + Ending Inventory − Beginning Inventory
Worked example: Beginning inventory is $50,000, ending inventory is $45,000, and COGS for the period is $120,000.
Purchases = $120,000 + $45,000 − $50,000 = $115,000
How you value that inventory (FIFO, LIFO, or weighted average) directly changes reported COGS and taxable income. Pick one method and stick with it: US GAAP requires consistent application and disclosure of the method used.
Frequently Asked Questions
What are the 7 steps of the purchasing process?
Identify the need, specify requirements, select suppliers, create the purchase order, confirm and follow up, receive and inspect goods, then match invoices and update records.
What are the four main types of inventory?
Raw materials (unprocessed inputs), work-in-progress (partially finished items), finished goods (ready to sell), and MRO supplies (maintenance and repair items that support operations).
What is FIFO, LIFO, and JIT?
FIFO and LIFO are inventory costing methods that determine which costs hit your books first. JIT is a purchasing strategy that times deliveries to arrive right as production needs them, minimizing stock on hand.
What is EOQ and ROP?
EOQ calculates the order quantity that minimizes total ordering and holding costs. ROP tells you the stock level at which you need to place your next order to avoid running out.
What are the 5 R's of purchasing?
Right quality, right quantity, right time, right source, and right price: a checklist procurement teams use to evaluate every purchase decision.
What is the journal entry when purchasing inventory?
For a credit purchase, debit Inventory and credit Accounts Payable. For a cash purchase, debit Inventory and credit Cash instead.
