What is Safety stock?
Reviewed by CA Apurav GargLast reviewed
In short
Safety stock is extra inventory kept above expected demand to cover sales spikes and supplier delays, so a product does not go out of stock while a replenishment order is on its way.
Why you need it
Sales do not arrive at a steady rate, and suppliers do not always deliver on the promised day. Safety stock absorbs both. Without it, any day busier than average, or any delivery later than average, ends in a stock-out.
Safety stock is the last part of the reorder point formula. A higher safety stock means you reorder earlier and carry more stock at all times.
Two common ways to work it out
1. Maximum and average method
Safety stock = (highest daily sales × longest lead time) − (average daily sales × average lead time)
It needs only your sales history and your supplier's delivery record, so it is easy to explain and check. Because it assumes the worst sales day and the slowest delivery happen together, it usually gives a large buffer.
2. Statistical method
Safety stock = Z × standard deviation of daily sales × √(lead time in days)
Z is a factor for how often you are willing to run out. A Z of about 1.65 aims to avoid a stock-out in about 19 order cycles out of 20, and a Z of about 2.33 in about 99 out of 100. When lead time also varies, an extended version of the formula adds that variation. A spreadsheet can calculate the standard deviation from daily sales data.
How much safety stock is right
Not every SKU deserves the same buffer:
- Fast-moving, high-margin SKUs justify a higher buffer, because each lost sale costs more and the stock turns over quickly.
- Slow-moving or low-margin SKUs usually justify a smaller buffer, because the stock ties up cash for longer.
- Products with a shelf life, such as food, cosmetics and supplements, need a buffer small enough to sell well before expiry. Quick-commerce and modern-trade buyers often refuse stock that has too little shelf life left when it arrives.
- Imported products with long, uneven lead times need a larger buffer than products from a nearby supplier.
What it costs
Safety stock is money sitting on a shelf. It adds to warehouse rent or fulfilment-centre storage fees, including higher long-term storage fees on some marketplaces, and it raises the risk that stock goes out of fashion or expires. Review the buffer at least every quarter, and recalculate after a sale event using normal days only, so one festive spike does not inflate it for the rest of the year.
Safety stock in your books
Safety stock is ordinary inventory: there is no separate accounting for it. Like all inventory, it is valued at the lower of cost and net realisable value, so slow-moving or near-expiry units may need to be written down at the year end.
Example in rupees
The same SKU as in the reorder point example:
| Input | Figure |
|---|---|
| Average daily sales | 40 units |
| Highest daily sales in the period | 60 units |
| Average lead time | 20 days |
| Longest lead time in the period | 25 days |
Maximum and average method: (60 × 25) − (40 × 20) = 1,500 − 800 = 700 units.
At a landed cost of ₹250 a unit, that buffer is ₹1,75,000 of stock held at all times.
Statistical method: if daily sales have a standard deviation of 10 units and the lead time is a steady 20 days, a buffer meant to avoid a stock-out in about 19 order cycles out of 20 is 1.65 × 10 × √20 = about 74 units, or ₹18,500.
The gap between the two answers is normal. The first method assumes your worst sales day and your slowest delivery happen together. The figures are illustrative.
Related terms
Work it out for your business
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General information only, not professional advice. Rules and rates change — check the official source before you act.