Size the buffer inventory that protects you against demand and lead-time variability. Enter your average daily demand, its variability, the replenishment lead time, and a target service level — the tool converts the service level to a safety factor (Z) and computes safety stock and the reorder point live in your browser.
Safety stock is the extra inventory you hold to absorb the uncertainty in demand and supply so you do not stock out before a replenishment arrives. Hold too little and you risk lost sales and stockouts; hold too much and you tie up cash and warehouse space. This calculator turns a target service level into a statistical safety factor (Z) and combines it with your demand and lead-time variability to compute the right safety stock and the reorder point — all live in your browser.
The cycle service level is the probability of not running out during a replenishment cycle. Assuming demand over lead time is approximately normal, the safety factor Z is the standard-normal quantile of that probability: a 95% service level gives Z ≈ 1.645, 97.5% gives Z ≈ 1.960, and 99% gives Z ≈ 2.326. This tool computes Z directly from your entered service level using a high-accuracy inverse-normal approximation (Acklam's algorithm), so you can use any value, not just the textbook ones. Higher service levels demand exponentially more safety stock — going from 95% to 99.9% can more than double it.
When both demand and lead time vary, the safety stock formula is SS = Z · √(LT·σd² + d²·σLT²), where LT is the average lead time, σd is the standard deviation of demand per period, d is the average demand per period, and σLT is the standard deviation of lead time. The first term under the root captures demand uncertainty over the lead time; the second captures the effect of lead-time uncertainty multiplied by the demand rate. If lead time is effectively constant (σLT = 0), this reduces to the familiar SS = Z · σd · √LT. This tool shows both so you can see how much lead-time variability is costing you in buffer stock.
The reorder point (ROP) is the inventory level at which you place a new order. It equals the expected demand during lead time (the cycle stock, d × LT) plus the safety stock: ROP = d·LT + SS. When on-hand inventory drops to the ROP, you order enough to cover the next cycle (often an EOQ-sized order). The cycle stock covers average demand while you wait for the order; the safety stock covers the times demand or lead time run higher than average.
The single most common mistake is mixing time units. Average demand, demand standard deviation, and lead time must all use the same period. If demand is in units per day, lead time must be in days and σd in units per day; if you forecast weekly, convert everything to weeks. Also make sure σd is a true standard deviation of period demand, not a forecast error or a coefficient of variation. Finally, remember that this normal-distribution model is an approximation — for intermittent or lumpy demand, very long lead times, or strong seasonality, validate the result against historical stockout rates and adjust.
Safety stock is buffer inventory held to protect against variability in demand and replenishment lead time. It is the amount above the expected lead-time demand that lets you maintain your target service level — the probability of not stocking out before the next delivery arrives.
Z is the standard-normal quantile corresponding to your cycle service level. For 90% service Z ≈ 1.282, 95% ≈ 1.645, 97.5% ≈ 1.960, 99% ≈ 2.326, and 99.9% ≈ 3.090. This calculator computes Z from whatever service level you enter using an inverse-normal approximation, so it is not limited to tabulated values.
Include it whenever your supplier lead time is not reliably constant — most real supply chains. If lead time swings significantly, the d²·σLT² term can dominate the safety stock, meaning supply reliability matters more than demand variability. If lead time is genuinely fixed (e.g., a scheduled internal transfer), set σLT to 0 and the formula reduces to Z·σd·√LT.
Safety stock is the buffer for variability. The reorder point (ROP) is the trigger level: ROP = average demand during lead time (d × LT) + safety stock. You order when inventory falls to the ROP; the safety stock is the portion of the ROP that protects you against running short before replenishment.
Not exactly. A 95% cycle service level means there is a 5% chance of a stockout occurring during any given replenishment cycle — it is a per-cycle probability, not a fraction of total time or units. A related but different metric is the fill rate (fraction of demand met from stock), which is usually higher than the cycle service level for the same safety stock. Pick the metric that matches how your business measures availability.