How much safety stock do I actually need?

Q: How much safety stock do I actually need?

Enough to cover the variability in demand AND the variability in your supplier's lead time. The commonly used formula covers only the first, which understates the buffer badly whenever the supplier is unreliable. Use the full form that includes both terms.

Enough to cover two different things going wrong, and the formula most people use only covers one of them.

Demand varies. So does your supplier.

Demand over a lead time moves for two independent reasons: customers order unpredictably, and the lead time itself is unpredictable. The full expression accounts for both:

σ_LT = √( L × σ_d² + d² × σ_L² )

The version in common use is z × σ_d × √L. Put the two side by side and you will see it: that is the full expression with the second term deleted. It is correct only when your supplier never varies.

How much does that matter?

On the sample inside the calculator, the supplier term is 86% of the total variance, and the common formula understates the true variability by 2.7 times. In buffer terms that is 567,379 against 154,233 in the same units: over 400,000 of cover simply unaccounted for, on 5 of 8 parts where the supplier is the bigger risk.

Which is why "we need better forecasting" is often the wrong project. If most of your variance is the supplier, a perfect forecast barely moves the buffer.

Cycle service level is not fill rate

Two numbers get used interchangeably and should not be. Cycle service level is the chance of surviving a replenishment cycle without a stockout. Fill rate is the share of demand you actually meet, and it is usually much higher. Quoting one while meaning the other is how a 95% promise turns into an argument.

What this will not do, deliberately. Tell you what your service level should be. That depends on what a stockout costs you, and no calculator can see that. What it will do is price every level in cash so you can make the trade with the real number in front of you.

Supply Chain Management Plimsoll SCM

Splits your buffer into demand variability and lead time variability, and prices every service level in working capital. Free, runs in your browser.

Free, no signup, nothing you type is uploaded. Written by Chris Maras, who spent 21 years in operations before building these. · All ten tools