Inventory Planning Calculator

1. Enter Average demand and choose units / day, units / week, or units / month.
2. Add Demand standard deviation (optional), Average lead time (days), and Lead time standard deviation (days, optional).
3. Choose Target service level, or select Custom... and enter Custom service level (%).
4. Enter both Ordering cost per order and Holding cost per unit per year for EOQ, or leave both blank.
5. Enter both Annual cost of goods sold (COGS) and Average inventory value for turnover, then choose a Currency symbol.
6. Click Calculate to show the result. Click Reset to clear the entered values.

How the inventory planning calculator reads your inputs

Average demand accepts units / day, units / week, or units / month. Week input uses 7 days, month input uses 30 days, and daily demand is multiplied by 365 for annual demand. Demand standard deviation uses the selected demand period; Lead time standard deviation is always in days.

Average demand and Average lead time (days) must be greater than zero. Blank optional deviation fields become zero. Custom service level (%) must be at least 50.01% and below 100%. A blank or nonnumeric required field produces an error when you click Calculate.

Input:
Average demand: 100 units / day
Demand standard deviation (optional): blank
Average lead time (days): 7
Lead time standard deviation (days, optional): blank
Target service level: 95%

Output:
Reorder point (ROP): 700 units
Safety stock: 0 units

How to calculate safety stock, reorder point, and EOQ

The Target service level is converted to a Z-score. Safety stock combines demand standard deviation with lead-time standard deviation. Reorder point adds that safety stock to average daily demand multiplied by lead time:

Safety stock = Z x sqrt(lead time x demand SD^2 + average demand^2 x lead-time SD^2)
Reorder point = (average demand per day x lead time in days) + safety stock
EOQ = sqrt(2 x annual demand x ordering cost / holding cost per unit per year)

EOQ appears only when both Ordering cost per order and Holding cost per unit per year are positive. The demand sensitivity table tests -20%, -10%, the base values, +10%, and +20%. The lead-time table changes lead time and its standard deviation while demand stays constant.

When inventory turnover is available

Enter Annual cost of goods sold (COGS) and Average inventory value together. Leave both blank to skip turnover. Entering only one produces an error, and a zero Average inventory value also produces an error. With two positive values, the result uses Inventory turnover = COGS / Average inventory value and Days of inventory on hand = 365 / Inventory turnover; otherwise it shows N/A.

The calculator uses the figures you enter. It does not forecast demand, read live stock levels, connect to an inventory or ERP system, or include variation outside the model.

Where this comes up

• Enter weekly demand and supplier lead time to set a reorder point before reviewing a purchase order.
• Test a 20% demand increase in the sensitivity table while lead time stays fixed.

Choosing a related business calculator

Open Business Calculators for profit, pricing, marketing, SaaS revenue, and cash runway calculations.

Use Safety Stock Calculator for safety stock from demand variability, lead-time variability, and service level. Use Economic Order Quantity (EOQ) Calculator for EOQ from demand, ordering cost, and holding cost. Use Inventory Turnover Calculator for turnover and days of inventory on hand from COGS and average inventory.

Frequently asked questions