Inventory Reorder Point, Safety Stock and Pack Order Calculator

When should you reorder, and how many packs should you buy?

Looking only at shelf stock can miss demand during delivery time, while ignoring outstanding purchase orders can lead to duplicate buying.
This calculator uses one SKU’s daily sales to estimate a reorder point and a safety buffer, then applies minimum order quantity and pack size to a replenishment quantity.
It helps a shop or online seller prepare a supplier discussion with quantities, assumptions and warnings in one report.

The intended setting is repeated sales with a reasonably stable, fixed lead time and continuous monitoring of inventory position.
The initial data are an illustrative example, not recommended stock levels.
The model is country independent and applies no tax rates, statutory shelf lives or industry price assumptions.
It does not forecast a launch, optimize economic order quantity, connect to an ERP or place a purchase order.

Safety stock, reorder point and order quantity

Prepare daily sales before calculating

1. Use one SKU and one whole-unit measure.
   Do not combine variants that cannot substitute for each other, or locations whose stock cannot be transferred.
   Separate returned stock from normal sales instead of entering negative daily demand.
   
2. Enter 2–366 daily observations, separated by newlines, spaces, tabs or semicolons.
   Each observation must be an integer from 0 to 1,000,000.
   Do not use commas: write one thousand as 1000, because 1,000 is rejected to avoid confusing a thousands separator with multiple days.
   
3. Keep zero-sales days in the record.
   If the record excludes closed days, use the same operating-day basis for lead time and additional coverage.
   Do not mix calendar-day delivery times with operating-day sales statistics.
   
4. Identify stockout days, promotions, price changes, seasonal shifts and day-of-week patterns.
   Observed sales cannot reveal all demand lost when stock was unavailable.
   If sales are not representative, uncheck the confirmation and review the data before using an order quantity.
   

The calculator uses sample standard deviation with denominator n − 1.
A short-sample message appears below 30 observations, but 30 is a display reminder rather than a statistical guarantee.
More observations do not automatically make intermittent or changing demand suitable for the model.

Separate physical stock, incoming units and commitments

Inventory position is on hand plus incoming units minus unshipped commitments.
It may be negative when promises exceed secured supply.
A positive position does not prove that an order can ship today: the goods may still be in transit.
The calculator flags low physical availability, but it does not simulate each scheduled arrival.

Formulas and rounding order

The safety-stock card rounds up to whole units.
Both the reorder point and target use the unrounded buffer inside their own totals, so rounding is not compounded.
Equality at the reorder point triggers order review.
Above the trigger, the current order is zero even if inventory position remains below the target; MOQ is not applied to a zero order.

Additional coverage C sets the target under this continuous-review policy.
It is not permission to stop reviewing stock for C days.
A business that checks inventory only once a week needs a periodic-review model that covers the time until the next review as well.
The target formula and shelf-life screening are explicit operating choices of this tool, not optimized values prescribed by the source materials.

Cycle service level is different from fill rate

The selected service level represents a target probability of avoiding a stockout during lead time under the model.
It is not the fraction of all requested units shipped immediately.
Selecting 95% does not guarantee that 95% of orders, units or revenue will be fulfilled.
The initial 95% is an example, and the calculator makes no industry-specific recommendation.

Available targets are 90%, 95%, 97.5% and 99%, using one-sided standard-normal z values.
Constant observations produce zero standard deviation and zero safety stock, which do not imply zero future risk.
If every observation is zero, the calculator withholds the order quantity because there is no observed sales rate for replenishment planning.

Worked example: from 77 required units to 7 packs

For transparent arithmetic, use daily sales of 8, 10 and 12 units.
The mean is 10 and the sample standard deviation is 2.
This three-day illustration is not an adequate operating history by itself.
Set lead time to 4 days, additional coverage to 7 days, service level to 95%, on hand to 35, incoming to 10, commitments to 5, MOQ to 80 and pack size to 12.

Expected net stock at arrival is 40 + 84 − 40 = 84 units.
At 10 units per day, that is 8.4 days of coverage.
If only 7 saleable days remain after arrival, the mean-sales screen identifies 14 units beyond that horizon.
Discuss a smaller MOQ or split deliveries instead of assuming the supplier will accept fewer than the required packs.

Interpret pack rounding and saleable-life warnings

Expected net arrival stock is max(0, IP + Q − μL).
Coverage divides that stock by μ, and excess is max(0, arrival stock − μ × saleable days).
This simplified screen treats stock as sharing the entered remaining life and assumes incoming units arrive within L.
Older lots may expire sooner, receipts may arrive on different days, and actual demand will vary.
Zero excess does not guarantee no waste, and positive excess is not a prediction of discarded units.
Review expiry by lot and your first-expiry-first-out or first-in-first-out process separately.

Use the result in a purchasing workflow

• For regularly sold consumables, reconcile recent sales with observed supplier lead time and holiday schedules.
  Keep operating days and calendar days consistent.
  
• For shared online-channel stock, combine the same SKU’s sales but count reservations once.
  Do not include inventory that cannot physically serve those customers.
  
• For seasonal goods or new products, assess whether previous sales describe upcoming demand.
  Missing history should lead to review and a supplier discussion, not invented daily observations.
  
• Save the TXT report with the inputs, formulas, output and warnings.
  Confirm arrival dates, outstanding orders, MOQ, units per pack and life by lot with the supplier, then recalculate if conditions change.
  

Frequently asked questions