Reorder Point Calculator

Reorder point calculation for Australian wholesalers and distributors

The reorder point is the inventory level at which you should place a new purchase order so that stock arrives before you run out. Get it right and you maintain service levels without excess inventory. Get it wrong and you either stockout (losing sales and damaging customer relationships) or overstock (tying up working capital and paying unnecessary holding costs).

For Australian distributors with overseas suppliers, where lead times can be 30-60 days for sea freight from China or 5-10 days from New Zealand, reorder point calculation is particularly critical. The longer and more variable the lead time, the higher your reorder point needs to be.

Basic vs advanced reorder point formulas

The basic formula is: Reorder Point = (Average Daily Sales x Lead Time) + Safety Stock. Safety stock is expressed as extra days of cover. This works when demand and lead times are relatively stable.

The advanced formula accounts for variability: Safety Stock = Z x sqrt(L x sigmaD^2 + D^2 x sigmaL^2), where Z is the service level factor, L is lead time, sigmaD is demand standard deviation, and sigmaL is lead time standard deviation. This produces more precise safety stock levels that reflect both demand uncertainty and supplier reliability.

Worked example: fastener distributor importing from China

A Melbourne-based industrial fastener distributor sells 200 units per day of their highest-volume SKU (M12 hex bolts, grade 8.8). The supplier is in Ningbo, China, with a quoted lead time of 42 days (sea freight including port handling). But actual lead times over the past 12 months ranged from 38 to 55 days.

Basic formula (7 days safety stock):

• Average daily demand: 200 units
• Lead time: 42 days
• Safety stock: 7 days x 200 = 1,400 units
• Reorder point: (200 x 42) + 1,400 = 9,800 units

Advanced formula (95% service level):

• Average daily demand: 200, standard deviation: 35
• Average lead time: 42 days, standard deviation: 5 days
• Z-score at 95%: 1.645
• Safety stock: 1.645 x sqrt(42 x 35^2 + 200^2 x 5^2) = 1.645 x sqrt(51,450 + 1,000,000) = 1.645 x 1,026 = 1,688 units
• Reorder point: (200 x 42) + 1,688 = 10,088 units

The advanced formula recommends 288 more units of safety stock than the basic approach. At AU$0.85 per unit, that is AU$245 in additional inventory holding cost. But a single stockout on this SKU costs the distributor AU$3,000-5,000 in lost margin and expedited air freight to fill back orders. The extra safety stock pays for itself if it prevents even one stockout per year.

Choosing your service level

Service level is the probability of not stocking out during a replenishment cycle. A 95% service level means you expect to meet demand from stock 95% of the time. The table below shows how safety stock scales with service level targets:

Going from 95% to 99% adds 703 units and AU$597 in holding cost. Whether that is worthwhile depends on the cost of a stockout. For a critical fastener that stops a customer's production line, 99% may be justified. For a slow-moving accessory item, 90% is usually sufficient.

Common mistakes in reorder point calculation

Using quoted lead times instead of actual. A supplier may quote 30 days, but if actual delivery averages 38 days with occasional 50-day outliers, using 30 days guarantees stockouts. Track actual receipt dates against purchase order dates to calculate real lead times. When AP automation matches invoices to POs, you get this data automatically.

Ignoring demand seasonality. Average daily sales across 12 months smooth out seasonal peaks. If demand doubles in September-November (pre-construction season for many industrial products), the reorder point should increase for those months. A single annual reorder point underestimates demand in peak periods and overestimates it in quiet months.

Setting safety stock by gut feel. "Two weeks of extra stock" sounds reasonable but has no mathematical basis. The advanced formula exists precisely because it accounts for the two variables that actually cause stockouts: demand variability and lead time variability. If both are low, you need less safety stock than intuition suggests. If either is high, you need more.

Frequently asked questions

What is the difference between the basic and advanced formula?

The basic formula uses a fixed number of days as safety stock. The advanced formula calculates safety stock statistically, using the standard deviation of both demand and lead time plus a service level target. The advanced formula is more accurate when demand or lead times are variable, which is the case for most Australian distributors importing from overseas.

How do I choose the right service level?

Compare the cost of a stockout against the cost of carrying extra inventory. For critical items where a stockout stops production or loses a customer, target 99%. For commodity items with multiple substitutes and low switching costs, 90-95% is usually sufficient. Most distributors use 95% as a default and adjust up or down by SKU category.

How does AP automation improve reorder point accuracy?

When purchase orders and supplier invoices are matched automatically, you get data on actual delivery lead times for every order. This replaces the quoted lead time with real performance data, which feeds directly into more accurate safety stock calculations. The result is fewer stockouts and less excess inventory.