Demand and lead-time safety stock: practical guide

What to prepare

• Gather order history, item dimensions, weights and supplier lead times.
• Identify the same units and planning period across stock, demand and capacity inputs.
• Check which freight, storage, import or production costs the model includes.

Understand the inputs

Cycle-service normal Z value

Approximate Z: 90% = 1.282, 95% = 1.645, 99% = 2.326.

The calculation method

Safety stock = Z × square root of (lead time × daily demand variance + daily demand² × lead-time variance).

Worked example

Illustrative inputs and their result.

Example inputs

Average daily demand units

40

Daily demand standard deviation

8

Average lead time (days)

10

Lead-time standard deviation (days)

2

Cycle-service normal Z value

1.645

Calculated example result

Safety Stock

139

Reorder Point

539

How to interpret the result

A capacity, inventory or landed-cost estimate is not a guarantee of service availability. Variability in demand, lead time, packing and customs treatment can change the operating result.

Keep the service requirement fixed while testing demand, lead time or shipment size. Check practical constraints such as handling, access and supplier minimum quantities separately.

Common mistakes to avoid

• Do not mix usable capacity with nominal capacity.
• Keep net goods weight separate from packaged or shipping weight.
• Do not treat a supplier's usual lead time as a guaranteed maximum.

Assumptions and sources

Assumes independent demand and lead-time variation and a normal approximation. Cycle service is not fill rate.

Related logistics & production guides

• Economic order quantity: practical guide
• Inventory reorder point: practical guide
• Stock cover and incoming supply: practical guide
• Imported goods landed cost: practical guide
• Pallet box capacity: practical guide
• Container box capacity: practical guide