How to Find a Z Value with NORM.S.INV in Excel

Use NORM.S.INV to get the z value for a service level, then size safety stock and control limits.

You will learn to

• Find the z value for a one-sided service level
• Turn a z value into safety stock
• Find the two-sided z for a confidence level

How it works

NORM.S.INV returns the z value that has a given share of the standard normal curve below it. =NORM.S.INV(0.95) returns about 1.645, the number of standard deviations you must stay above average to cover 95% of outcomes.

Inventory planners multiply that z by the standard deviation of demand to get safety stock. For two-sided intervals, such as a 99% confidence level, split the leftover 1% between both tails: =NORM.S.INV(1-(1-99%)/2) returns about 2.576.

=NORM.S.INV(0.95)