Safety Stock & Service Level Calculator

Inputs

• Average demand per day — Units shipped on an average operating day, not a calendar day, if you do not ship every day.
• Standard deviation of daily demand — How much a typical day swings around that average. About two thirds of days land within one of these either side. If you have a forecast, use its RMSE instead.
• Average lead time — Order placed to stock on the shelf and sellable, including receiving and inspection.
• Standard deviation of lead time — How much that lead time actually varies. Set it to 0 only if deliveries land to the day, and check the last ten receipts before you do.
• Cycle service level — The chance of NOT running out during a single replenishment cycle. Between 50 and 99.99.
• Order quantity — How many units one replenishment brings in: your EOQ, a pallet, a container. It sets how often you are exposed, and the fill rate.
• Operating days per year — 365 if you ship every day, about 250 for a weekday operation.
• Cost of one unit — What the unit costs you, not what it sells for.
• Annual holding cost — Storage, insurance, shrink, obsolescence, and the cost of the money tied up, as a share of unit cost per year. 20 to 30% is the usual range.

How to use this calculator

1. Enter your average demand per operating day and how much a typical day swings around it. If you already have a forecast, put its RMSE in the variability field instead: safety stock covers the part of demand you did NOT predict, not the part you did.
2. Enter the average lead time and, more importantly, how much it varies. Pull the last ten receipts from this supplier and look at the spread of the actual days. Almost nobody's lead time is as reliable as the purchase order says.
3. Choose a cycle service level. It is the chance of not running out during ONE replenishment cycle, so 95% with eighteen cycles a year means expecting to run short about once a year, not once in twenty years.
4. Enter the order quantity, because it decides how often you are exposed. The fill rate, which is the share of units supplied on time, depends on it and is the number your customer actually experiences.
5. Read the split between demand variability and lead-time variability. When the supplier's share is the larger one, a reliable delivery date is a cheaper fix than more inventory or a better forecast, and the tool prices that conversation for you.
6. Compare the safety stock ladder at the bottom before you set a policy. The jump from 95% to 99.9% typically costs more than everything below it, so the last points of service are a deliberate purchase rather than a rounding-up.

What each term means

Safety stock

Inventory held purely to absorb being wrong: demand higher than forecast, or a delivery later than promised. Everything else in your warehouse is cycle stock, which is there to be sold.

Cycle service level

The probability of getting through one replenishment cycle without running out. Per cycle, not per year, which is why it sounds so much safer than it is.

Fill rate

The share of demanded units you actually supplied on time. Nearly always higher than the cycle service level, because most stockouts are short and shallow, and it is the figure customers experience.

z (the service factor)

How many standard deviations above average demand you are stocking to. 1.65 buys 95%, 2.33 buys 99%, and 3.09 buys 99.9%, and the jumps get expensive because the tail gets thin.

Sigma over lead time

The combined variability of demand and lead time across the whole exposure window. Variances add, so the two sources combine through a square root, not by adding the two buffers together.

Reorder point

The stock level that triggers the next order: expected demand during the lead time plus the safety stock. The simpler version, which buffers in days of cover rather than in measured variability, is the Reorder Point calculator in this group.

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