Safety Stock Calculator
Calculate safety stock for a target service level. Enter the service-level factor, demand variability, and lead time to size your buffer, with the formula explained.
Safety stock is the buffer that protects against running out when demand or supply varies. The service-level method sizes it from how much daily demand swings and how long replenishment takes.
How it works
Pick a service-level factor (z) for your target service level, multiply by the standard deviation of daily demand, and scale by the square root of the lead time.
The formula
Safety stock = z x standard deviation of daily demand x sqrt(lead time). The square root reflects that variability accumulates over the lead time, not linearly.
Worked example
For a 95% service level (z = 1.64), demand standard deviation of 3 units/day, and a 15-day lead time: 1.64 x 3 x sqrt(15) = about 19 units.
Frequently asked questions
What service-level factor should I use?
Common values: 90% = 1.28, 95% = 1.65, 98% = 2.05, 99% = 2.33. Higher service levels need more buffer.
What if lead time also varies?
Use the combined form that adds lead-time variance: sigma over lead time = sqrt(LT x sigmaD^2 + D^2 x sigmaLT^2). This tool assumes constant lead time.
How do I use the result?
Feed it into the reorder point calculator so your trigger level covers both expected demand and this buffer.
Related tools
This is a planning estimate. Results depend on your inputs and assumptions; confirm against your own data before ordering.
- Lead-time demand is approximately normally distributed.
- Lead time is constant; only demand varies.
- Daily demand observations are independent (so variance scales with LT).