Inventory Reorder Point & Safety Stock Calculator

Plan inventory with confidence set reorder points and safety stock using demand and lead time variability. Choose service levels, run sensitivity checks and review periodic targets.

Inputs

Please enter a valid average demand.
Demand unit scales calculations automatically.
Enter lead time in days.
Please enter a valid demand standard deviation.
Set to 0 if negligible.
For periodic review; set 0 for continuous review.

Results

Enter values and click Calculate to see results.
Demand during lead time
—
Std Dev during lead time (σLT)
—
Safety Stock
—
Reorder Point (ROP)
—
Target Level (Periodic Review)
—
Suggested Order Qty to Target
—
SKU d/day L (days) σd/day σL (days) Svc Lvl z DLT σLT SS ROP Review (days) Target On‑Hand Open PO Order Qty

Example & Saved Scenarios

Three example rows are provided. Add your scenarios using the “Add to Table” button. Edit inputs directly and click Recompute to refresh outputs.

SKU Avg Demand Unit Lead Time (d) σd σd Unit σL (d) Svc % Use LT Var Review (d) z DLT σLT SS ROP Target
A-Widget 120 week 10 35 week 2 95 yes 0 ——————
B-Gadget 30 day 7 8 day 0 99 no 14 ——————
C-Spare 400 month 21 120 month 4 90 yes 0 ——————

Formulas Used

Notation — d: average demand per day; L: lead time (days); σd: std dev of demand per day; σL: std dev of lead time (days); z: service level factor.

Demand during lead time: DLT = d × L

Std dev during lead time (when demand & lead time vary independently):
σLT = √( L × σd2 + d2 × σL2 )

Std dev during lead time (only demand varies):
σLT = σd × √L

Safety stock: SS = z × σLT

Reorder point: ROP = DLT + SS

Periodic review target level: Target = d × (L + R) + SS, where R is review period (days).

z corresponds to the cycle service level (probability of no stockout during lead time). Typical values: 90%→1.2816, 95%→1.6449, 97.5%→1.96, 99%→2.3263, 99.9%→3.0902.

How to Use This Calculator

  1. Enter your average demand and unit (per day, week, or month). The tool converts to a daily basis automatically.
  2. Enter lead time (days) and the standard deviation of demand using the same unit you selected for average demand.
  3. Optionally enter the standard deviation of lead time and keep “Include lead time variability” checked to use the combined-variance formula.
  4. Choose the desired service level or switch to custom z. Click Calculate to see ROP and safety stock.
  5. For periodic review, set a review period (days) to compute a target order-up-to level.
  6. Use Add to Table to save a scenario, then export results as CSV or PDF.

FAQs

Common targets are 90–99% depending on item criticality and cost of stockouts. Higher service levels raise safety stock. Align with business policy and customer expectations.

Use historical data at the same cadence as your input (daily, weekly, or monthly). For demand, compute the sample standard deviation of periods. For lead time, use variability between completed orders.

Consider segmenting by season, smoothing with moving averages, or using demand forecasting to derive period‑level means and standard deviations that reflect the upcoming season.

Periodic review must cover both the lead time and the time until your next review. Therefore the target includes demand for L + R plus safety stock.

Yes. Enter demand and its standard deviation per day, week, or month. The calculator converts to a daily basis consistently for accurate lead‑time statistics.

Disclaimer: results are for planning support only. Validate with your data quality, business rules, and supplier constraints.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.