Free Tool by VNDLY
Safety Stock Calculator
Calculate safety stock with the real statistical Z-score formula or the quick min–max method. Get your reorder point and EOQ instantly. Free, no signup.
The safety stock formula
The statistical safety stock formula accounts for variability in both demand and lead time:
σ_LT = √(L × σ_d² + d̄² × σ_L²)
Safety stock = Z × σ_LT
Reorder point = d̄ × L + safety stock
d̄ = average daily demand (units/day)
σ_d = standard deviation of daily demand
L = average lead time (days)
σ_L = standard deviation of lead time
Z = service-level Z-score (see table below)
σ_LT = demand variability over the lead time
Worked example (defaults above): d̄=50 units/day, σ_d=10, L=7 days, σ_L=2, Z=1.65 (95% service level) → σ_LT = √(7×100 + 2500×4) = √10,700 ≈ 103.4 → safety stock = ⌈1.65 × 103.4⌉ = 171 units, reorder point = 50×7 + 171 = 521 units.
When lead time is perfectly reliable (σ_L = 0), the formula simplifies to Z × σ_d × √L.
Z-score table for common service levels
| Service level | Z-score | Meaning |
|---|---|---|
| 90% | 1.28 | No stockout in 9 out of 10 replenishment cycles |
| 95% | 1.65 | No stockout in 19 out of 20 cycles — typical default |
| 97% | 1.88 | No stockout in ~97 out of 100 cycles |
| 99% | 2.33 | No stockout in 99 out of 100 cycles — for critical SKUs |
| 99.9% | 3.09 | Near-zero stockout risk — very high holding cost |
How to estimate demand standard deviation
Standard deviation measures how much your daily sales fluctuate around the average.
Quick estimate
(max daily demand − min daily demand) ÷ 4. This rule of thumb assumes roughly normal demand and works well for a fast first estimate.
Excel / Sheets
Export daily sales for 90–180 days and use STDEV(). More data = more accurate. Exclude outliers (promotions, one-off bulk orders) if they aren't repeatable.
Automatic with VNDLY
VNDLY computes demand standard deviation per SKU from your live sales history and recalculates safety stock automatically — no spreadsheet exports needed.
Statistical vs min–max method
Use the statistical method when…
- • You have real sales history (3+ months)
- • You want to hit a specific service level
- • Lead times vary between suppliers or seasons
- • You're making decisions for high-value SKUs
Use min–max when…
- • You only know rough usage highs and lows
- • You're doing a quick back-of-envelope sizing
- • You're new and building your first inventory policy
- • You have very low SKU count and predictable demand
How to use this calculator
1. Choose your method
Statistical (recommended) uses the Z-score formula. Min–max is the classic simpler method.
2. Enter your demand data
Average daily demand and its standard deviation (how much it fluctuates day to day).
3. Enter your lead time
Average days from ordering to receipt, plus standard deviation (set to 0 if lead time is fixed).
4. Pick a service level
95% is typical. Higher = more safety stock but fewer stockouts. Lower = leaner but riskier.
From a static buffer to dynamic safety stock
This calculator gives you a one-time answer. Real operations need safety stock that updates as demand patterns shift, suppliers change, or you add new SKUs. VNDLY calculates safety stock and reorder points per SKU from your live sales history — automatically, across every product and warehouse. Starting at $49/mo.
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Frequently asked questions
What is safety stock?
Safety stock is buffer inventory held to protect against demand spikes and supplier delivery delays. It sits on top of your regular cycle stock so you don't run out while waiting for a replenishment order to arrive.
What is the safety stock formula?
The statistical formula is: safety stock = Z × σ_LT, where σ_LT = √(L × σ_d² + d̄² × σ_L²). Z is the service-level Z-score, σ_d is the standard deviation of daily demand, L is average lead time, and σ_L is the standard deviation of lead time. The simpler min–max formula is: (max daily usage × max lead time) − (avg daily usage × avg lead time).
What Z-score should I use for 95% service level?
Use Z = 1.65 for a 95% service level. This means you'll have enough stock to avoid a stockout in 95% of replenishment cycles. For 99% use Z = 2.33; for 90% use Z = 1.28.
What is a good service level?
95% is typical for most products. Use 99% or higher only for critical items (e.g. medical supplies, top-selling SKUs) — safety stock grows non-linearly at high service levels, so the holding cost jumps sharply above 99%. For slow-moving or low-margin items, 90% may be adequate.
What's the difference between safety stock and reorder point?
Safety stock is how much extra to hold as a buffer. The reorder point (ROP) is the trigger level: when your stock hits this number, you place a new order. Reorder point = average daily demand × lead time + safety stock.
Statistical vs min–max: which should I use?
Use the statistical method when you have real sales data and want to hit a specific service level. Use min–max when you only know rough highs and lows and want a quick estimate. The statistical method is more precise and accounts for both demand variability and lead-time variability.
How do I estimate demand standard deviation?
Quick estimate: (max daily demand − min daily demand) ÷ 4. For a more accurate number, export daily sales for the past 90–180 days and calculate the standard deviation (STDEV in Excel/Sheets). VNDLY computes this per SKU from your live sales history automatically.
How much safety stock is too much?
Safety stock has a real cost: storage, cash tied up, risk of obsolescence. The right level balances stockout risk against holding cost. Start with 95% service level and review SKUs that rarely stock out — they may be over-buffered.