Free tool · Stock planning
Safety Stock Calculator
Work out how much buffer stock to hold and when to reorder, from your own demand and supplier lead times
- Free
- No signup
- Indian rules
Lead time also varies
Adds supplier delays with the combined formula
Good to know
- Demand and lead time must use the same time unit. This tool works in days; weekly figures are converted.
- A 95% service level means stock should last through about 19 of every 20 order cycles. It is not the share of units supplied (fill rate).
- Higher levels cost more: going from 95% to 99% raises Z from 1.645 to 2.326, about 41% more safety stock.
- The formula assumes demand and lead time follow a normal curve and do not affect each other. It is weak for slow movers with many zero days.
Results
Safety stock
139 units
About 3.5 days of average demand at a 95% service level
Reorder point
539 units
Lead-time demand 400 + safety stock 139
Working
SS = Z × √(L̄ × σd² + d̄² × σL²)
= 1.645 × √(10 × 8² + 40² × 2²)
= 1.645 × √(640 + 6,400)
= 1.645 × 83.9 = 138.02
→ 139 units, rounded up to whole units
ROP = d̄ × L̄ + SS = 400 + 139 = 539
Estimate only. Safety stock is only as good as the demand and lead-time history behind it. Review it when sales, suppliers or seasons change, and round to your pack or order size.
How it works
What this tool does.
Calculate safety stock with the service level formula (Z × σ × √lead time, with or without lead-time variation) or the simple max minus average method. Paste past demand to get the standard deviation, and see the reorder point and the value of the buffer.
Pick a method. The service level method needs your average daily demand, how much it swings (the standard deviation), the average lead time in days and the service level you want, such as 95%. If you do not know the standard deviation, paste your recent daily or weekly demand and the tool works out the mean and the sample standard deviation (dividing by n − 1). Weekly figures are turned into daily ones, because demand and lead time must use the same time unit. Switch on lead-time variation if your supplier is not always on time, and the combined formula is used. The simple method needs only your average and highest daily usage and your average and longest lead time. Both show the safety stock, the reorder point, the working with your figures and, if you give a unit cost, the money tied up in the buffer.
Rules and dates last reviewed: October 2026
Official sources
- Peter L. King, "Understanding safety stock and mastering its equations", APICS magazine, July/August 2011 (MIT course 2.810 reading)
- NIST/SEMATECH e-Handbook of Statistical Methods: cumulative distribution table for the standard normal distribution
- ICAI Board of Studies: Practice questions and solutions, Cost and Management Accounting, Material Costing (re-order and minimum stock levels)
- NC State University Supply Chain Resource Cooperative: Reorder point formula, inventory management models (cycle service level and fill rate)
Safety Stock Calculator formula
Service level method, demand varies: Safety stock = Z × σd × √L̄. Demand and lead time both vary (independently): Safety stock = Z × √(L̄ × σd² + d̄² × σL²). Here Z comes from the service level (90% = 1.282, 95% = 1.645, 97.5% = 1.960, 98% = 2.054, 99% = 2.326, 99.9% = 3.090), σd is the standard deviation of daily demand, d̄ is average daily demand, L̄ is average lead time in days and σL is the standard deviation of lead time in days. Simple method: Safety stock = (maximum daily usage × maximum lead time) − (average daily usage × average lead time). Reorder point = d̄ × L̄ + safety stock.
Worked example
A Pune distributor sells two-wheeler brake pad sets. Sales average 40 sets a day with a standard deviation of 8 sets. The supplier takes 10 days on average, with a standard deviation of 2 days. Each set costs ₹350 and the stores manager wants a 95% service level.
- Z for a 95% service level is 1.645
- Demand variation over the lead time: L̄ × σd² = 10 × 8² = 640
- Lead-time variation: d̄² × σL² = 40² × 2² = 6,400
- Safety stock = 1.645 × √(640 + 6,400) = 1.645 × 83.90 = 138.02, rounded up to 139 sets
- Reorder point = 40 × 10 + 139 = 400 + 139 = 539 sets
- Money in the buffer = 139 × ₹350 = ₹48,650
Hold 139 sets as safety stock and place the next order when stock falls to 539 sets. The buffer ties up ₹48,650. If the supplier were always on time, 1.645 × 8 × √10 = 41.62, so 42 sets would do: most of this buffer covers late deliveries.
Sleet ERP
Want this done on every order?
Sleet ERP alerts you when an item falls below its minimum stock and drafts a purchase order with the reorder quantity and default vendor.
- Orders that matchPurchase orders are matched to the goods received and the supplier’s bill.
- Dispatch with papersDelivery challans and e-way bills for stock moving between godowns and branches.
- Labels from your itemsBarcodes print from the item master and are scanned at billing and stock counts.
Questions
Safety Stock Calculator, answered.
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What is the safety stock formula?
The standard formula is Safety stock = Z × σd × √L, where Z comes from your service level, σd is the standard deviation of daily demand and L is the lead time in days. If lead time also varies, use Z × √(L × σd² + d² × σL²), where d is average daily demand and σL is the standard deviation of lead time. A simpler rule of thumb is (maximum daily usage × maximum lead time) − (average daily usage × average lead time).
How do I calculate safety stock from my past sales?
Take the daily quantity sold or issued for the last few weeks. Work out the average and the standard deviation. Choose a service level and look up its Z (1.645 for 95%). Multiply Z by the standard deviation and by the square root of the lead time in days. For example, with σd = 8 units, a 10-day lead time and 95%: 1.645 × 8 × √10 = 41.6, so keep 42 units. You can paste the figures into this calculator to skip the arithmetic.
What is the difference between safety stock and reorder point?
Safety stock is the extra stock kept for demand above average or a late delivery. The reorder point is the stock level at which you place the next order. Reorder point = average daily demand × average lead time + safety stock. If you sell 40 a day, the supplier takes 10 days and safety stock is 139, you reorder at 400 + 139 = 539.
What service level should I use?
Most businesses aim for 90% to 98%. A 95% level means stock should last through about 19 of every 20 order cycles. Higher levels cost more stock: going from 95% to 99% raises Z from 1.645 to 2.326, which is about 41% more safety stock. Set a higher level for items that stop production or lose a key customer, and a lower one for cheap or slow items. A 100% service level is not possible with this formula.
Why must demand and lead time be in the same unit?
The formula multiplies demand variation by the square root of the lead time, so both must count the same periods. If your standard deviation is per week and lead time is in days, convert first: daily standard deviation = weekly standard deviation ÷ √7, and daily average = weekly average ÷ 7. Mixing weekly demand with a lead time in days gives a buffer that is far too large.
Is the max minus average method good enough?
It is quick and needs no statistics. Its reorder point, maximum usage × maximum lead time, is the re-order level in ICAI study material, and its safety stock equals the ICAI minimum stock level. But it has no service level behind it, and one unusual day or one very late delivery in your records can make the buffer much bigger than you need. With 40 a day average, 55 maximum, and lead times of 10 and 14 days, it gives 55 × 14 − 40 × 10 = 370 units, against 139 from the 95% service level method.
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