Safety Stock Calculator

Demand swings, supplier delays and a target service level in — safety stock and reorder point out

Updated 2026-09-28

Average units sold per day

Daily demand standard deviation

units

Average supplier lead time

days

Lead time standard deviation

days

Target service level

Unit cost (optional)

Z-score for that service level: 1.645
Safety stock: 146 units
Reorder point: 546 units
Safety stock if lead time never varied: 63 units
Cash held in safety stock: $1,168.00

Safety stock = Z × √(lead time × demand SD² + daily demand² × lead time SD²). Service level is the chance of not running out during one replenishment cycle.

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Safety stock is the stock you hold for the days that go wrong: a week when demand runs hot, or a supplier who arrives four days late. Too little and you stock out on your best sellers; too much and cash sits on a shelf. The rule-of-thumb "add a few days of cover" works for steady products, but it cannot tell you how much protection those days buy. This calculator uses the standard service-level formula, so you choose the risk of running out and it sizes the buffer to match.

The safety stock formula

Safety stock = Z × √(L × σd² + d² × σL²). Here d is average daily demand, σd is the standard deviation of daily demand, L is the average lead time in days, σL is the standard deviation of the lead time, and Z is the number of standard deviations that matches your target service level.

The first term under the square root is demand uncertainty over the lead time. The second is lead-time uncertainty: if the truck can be two days late, you need two days of sales in reserve for it, scaled by how often that happens. If your supplier is perfectly reliable, σL is zero and the formula collapses to Z × σd × √L.

Worked example

A shop sells an average of 40 units a day with a daily standard deviation of 12. The supplier takes 10 days on average, give or take 2 days (standard deviation). The target is a 95% service level, so Z = 1.645.

Demand term: 10 × 12² = 1,440. Lead-time term: 40² × 2² = 6,400. Square root of 7,840 = 88.54. Safety stock = 1.645 × 88.54 = 145.6, rounded up to 146 units. Reorder point = 40 × 10 + 146 = 546 units.

Now set the lead-time variability to zero: safety stock falls to 1.645 × 12 × √10 = 62.4, or 63 units. More than half of the buffer in this example exists only because the supplier is unreliable. That is the number to take into your next supplier conversation.

Choosing a service level

Service level here means cycle service level: the probability you do not run out during one replenishment cycle. Each step up costs more stock than the last, because the normal curve flattens in the tail.

Best sellers and items customers will not wait for: 97.5–99%
Ordinary lines with substitutes on the shelf: 90–95%
Slow, expensive or perishable stock: 85–90%, and review by eye

Safety stock for the worked example at different service levels (σ over lead time = 88.54 units)
Service level	Z-score	Safety stock	Stock value at $8/unit
90%	1.282	114 units	$912
95%	1.645	146 units	$1,168
97.5%	1.960	174 units	$1,392
98%	2.054	182 units	$1,456
99%	2.326	206 units	$1,648
99.5%	2.576	229 units	$1,832
99.9%	3.090	274 units	$2,192

Getting the standard deviations from your sales history

Export daily unit sales for the last 60 to 90 normal trading days, leave out one-off bulk orders, and use a spreadsheet's STDEV.S function on the column. If you only have weekly totals, divide the weekly standard deviation by √7 (about 2.65): a weekly standard deviation of 31.7 units is roughly 12 units a day. That shortcut assumes each day's sales are independent of the next, which is close enough for most retail.

For lead time, list the days from order to shelf for your last 8 to 10 purchase orders from that supplier and take the standard deviation of that list. A supplier with no history yet: use a quarter of the gap between their best and worst quoted delivery as a starting σL.

What the buffer costs to hold

Safety stock never sells in a normal cycle — it is permanent inventory. Holding costs (capital, space, insurance, shrinkage and obsolescence) are often estimated at 20–30% of the stock's value a year. At 25%, the 146-unit buffer above worth $1,168 costs about $292 a year to hold. Moving from 95% to 99% adds 60 units, $480 of stock and roughly $120 a year. Whether that is worth it depends on the margin you lose, and the customers you disappoint, in the stockouts it prevents.

Putting the number to work

Safety stock only protects you if something watches it. Enter the reorder point from this calculator as the low-stock threshold on the product in ExpressBizTools; the low-stock alert then becomes the trigger for your purchase order. Recalculate each quarter and before seasonal peaks, when both demand and its variability change.

Keep the numbers in a spreadsheet

Ready-made Excel and Google Sheets templates that pick up where this calculator stops. One-off purchase, instant download.

Inventory Tracker & Stock Control Spreadsheet
150 SKUs, stock in/out log, automatic on-hand counts, reorder alerts, a reorder list and a dashboard.
$9See template

Frequently asked questions

What is the formula for safety stock?

Safety stock = Z × √(lead time × demand SD² + average demand² × lead time SD²). With a perfectly reliable supplier it simplifies to Z × demand SD × √lead time.

What Z-score should I use?

1.645 for a 95% service level, 1.960 for 97.5%, 2.326 for 99%. Enter the service level in the calculator and it works out Z for you.

What is the difference between safety stock and reorder point?

Safety stock is the buffer for surprises. The reorder point is expected demand over the lead time plus that buffer — the stock level at which you place the next order.

Is the "days of cover" method wrong?

No — it is a simpler version for steady products. It just cannot tell you what service level a given number of days buys, or how much of the buffer is caused by an unreliable supplier.

Should safety stock be the same for every product?

No. Set higher service levels for best sellers and items with no substitute, and lower ones for slow, expensive or perishable lines.

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