Your shelves are empty on your busiest weekend of the quarter — and the back room is stuffed with slow movers you ordered three months ago. Both problems come from the same place: guessing instead of calculating. Three simple inventory formulas fix that, and you can run all of them in a spreadsheet this afternoon.
Stockouts are not a rounding error. Out-of-stock shelves cost retailers roughly $1.2 trillion in lost sales a year globally, and nearly half of shoppers who hit a stockout on a product they buy regularly switch to a different retailer. On the other side, every dollar of excess inventory you hold costs you 20 to 30 cents a year in warehousing, insurance, shrinkage, and tied-up capital. Ordering too little bleeds revenue; ordering too much bleeds cash. Here is how to land in between.
1. Economic Order Quantity (EOQ): How Much to Order Each Time
Every purchase order forces a trade-off. Order in huge batches and you place fewer orders (low ordering cost) but sit on piles of stock (high holding cost). Order tiny batches frequently and your shelves stay lean, but you pay ordering, freight, and receiving costs over and over. The economic order quantity is the batch size where the two costs balance and your total cost bottoms out.
The formula:
EOQ = √((2 × D × S) / H)
- D = annual demand in units
- S = cost of placing one order (staff time, freight, receiving — everything except the goods themselves)
- H = holding cost per unit per year (storage, insurance, shrinkage, and the opportunity cost of the cash)
Worked example
Say you sell 12,000 hand-poured candles a year. Each purchase order costs you $50 all-in to place and receive, and holding one candle for a year costs $2.
EOQ = √((2 × 12,000 × 50) / 2) = √600,000 ≈ 775 units per order
That means about 15 orders a year (12,000 ÷ 775) instead of, say, 4 giant ones or 52 weekly dribbles. At 775 units, your combined ordering and holding cost is at its minimum — order more or less per batch and one of the two costs rises faster than the other falls.
Getting S and H right
The formula is only as good as its inputs, and these two are where small businesses usually guess:
- Ordering cost (S): Time your actual process once. If placing and receiving an order takes two hours of a $30/hour employee's time plus a $40 freight charge, S is $100 — not the $10 "processing fee" on the invoice.
- Holding cost (H): The standard benchmark for total carrying cost is 20 to 30 percent of inventory value per year. If a unit costs you $10, start with H = $2.50 (25%) and refine it as you track real storage, insurance, damage, and obsolescence costs.
When EOQ breaks down
EOQ assumes steady demand, stable costs, and no quantity discounts — real life violates all three. Recalculate quarterly, and if a supplier offers a volume discount that beats the EOQ math, take the discount and rerun the numbers. Seasonal businesses should compute a separate EOQ for peak and off-peak demand rather than averaging the year into meaninglessness.
2. Safety Stock: Your Buffer Against Surprises
EOQ tells you how much to order. It says nothing about demand spikes or late trucks. Safety stock is the extra cushion you keep so a good week or a slow supplier does not empty your shelves.
The simple method (no statistics required)
Safety stock = (Maximum daily sales − Average daily sales) × Lead time in days
If you sell 5 units a day on average, 8 on your busiest days, and your supplier takes 14 days to deliver:
Safety stock = (8 − 5) × 14 = 42 units
That cushion covers you when every day of the lead time runs at peak demand. It is conservative, easy to explain to staff, and good enough for most small retailers getting started.
The statistical method (tighter and cheaper)
If you want less cash sitting in the buffer, size it from your actual demand variability and the service level you want:
Safety stock = Z × σ × √L
- Z = service factor: 1.65 for a 95% in-stock rate, 1.28 for 90%, 2.33 for 99%
- σ = standard deviation of daily demand (your spreadsheet's STDEV function on daily sales)
- L = lead time in days
Aiming for 95% availability with daily demand that varies by 3 units (σ = 3) and a 14-day lead time:
Safety stock = 1.65 × 3 × √14 ≈ 19 units
Notice the trade-off the formula makes visible: jumping from 95% to 99% service roughly doubles the buffer (Z goes from 1.65 to 2.33). For most small businesses, 95% on everyday items and 99% only on the handful of SKUs that drive the most margin is the sweet spot. Protecting every SKU at 99% is how cash goes to die on the shelf.
3. Reorder Point: Exactly When to Place the Next Order
The reorder point is the inventory level that triggers a new purchase order — timed so the new stock arrives just as you are about to dip into safety stock.
Reorder point = (Average daily demand × Lead time in days) + Safety stock
Using the earlier example — 5 units a day, 14-day lead time, 42 units of safety stock:
Reorder point = (5 × 14) + 42 = 112 units
When your on-hand count hits 112, you place an order for your EOQ quantity (775 in the candle example). During the 14-day wait you will sell roughly 70 units, landing at 42 — your safety buffer — right as the truck arrives. The three formulas now work as a system: the reorder point says when, EOQ says how much, and safety stock absorbs the surprises in between.
Lead time is the silent killer
Most reorder-point failures are really lead-time failures. Measure the full cycle — the day you send the PO to the day stock is received and sellable — not just the supplier's quoted ship time. If your supplier quotes 10 days but receiving and inspection add 4, your lead time is 14. Track actual lead times per supplier for a quarter; the vendor whose "10 days" is really 18 needs a longer lead time in your formula or a frank conversation.
Putting the System to Work
You do not need expensive software to start. A spreadsheet with one row per SKU and columns for average daily demand, max daily demand, lead time, EOQ, safety stock, and reorder point covers the essentials. Then build two habits:
- Review the inputs monthly, not the outputs. Demand drifts, suppliers slow down, freight costs change. Updating average demand and lead time monthly keeps every formula honest. Set a calendar reminder for the first Monday of the month.
- Rank SKUs with ABC analysis. Your A items (roughly 20% of SKUs driving 80% of sales) deserve the statistical safety-stock treatment and weekly monitoring. C items — the long tail — can run on the simple method with quarterly reviews. Spending equal effort on every SKU is how inventory projects die of exhaustion.
Common mistakes to avoid: setting reorder points once and forgetting them, using supplier-quoted instead of measured lead times, and protecting slow movers with the same service level as bestsellers. Each of those quietly converts cash into shelf decorations.
How This Shows Up in Your Books
Inventory discipline is bookkeeping discipline. Every unit on the shelf is cash parked on your balance sheet instead of in your bank account, and the carrying costs — rent for the space, insurance, shrinkage, write-downs on obsolete stock — leak into your expenses month after month. When you right-size orders with EOQ and stop over-buffering with bloated safety stock, two things happen in your ledger: your inventory asset balance drops toward what you actually need, and your cost of goods sold starts tracking sales tightly instead of swinging with panic orders and clearance markdowns.
Track inventory as its own balance-sheet account, count it on a schedule (cycle-count A items weekly, full counts quarterly), and reconcile the physical count to the books every time. The variance lines — shrinkage, damage, obsolescence — are management information, not embarrassment: they are the holding-cost inputs that make next quarter's EOQ calculation sharper.
Keep Your Inventory and Your Cash in Sight
Right-sizing stock only pays off if you can see the effect. When your inventory, cost of goods sold, and cash accounts all live in one transparent ledger, a glance tells you whether last month's ordering discipline actually freed up cash — or just moved the bloat to a different shelf. Beancount.io provides plain-text accounting that gives you complete transparency and control over your financial data, including inventory accounts you can reconcile against physical counts. Get started for free and see why developers and finance professionals are switching to plain-text accounting.
