Prophet 21 Inventory Replenishment Calculator
Distributors using Prophet 21 live and die by the order point and max level on each item and location. This calculator gives you a defensible starting number from demand, lead time and service target, so you can check what the system is suggesting or sanity check a manual override.
Your numbers
Trailing twelve months of net sales units for the item and location.
Order release to usable on-hand, including receiving and putaway.
Standard deviation of daily sales, not of monthly sales.
Capital, storage, shrink and obsolescence as a percent of inventory value.
Round order quantities up to a multiple of this value.
Your results
Planning estimate using the normal-demand safety stock formula. Compare it with the replenishment method and parameters configured in your own Prophet 21 setup before changing item or location records.
Order point vs lead time
Sweeps Replenishment lead time from 1 to 60, holding your other inputs fixed.
Replenishment parameter review
Optional: send us an item and location export and we will outline where order points and max levels look out of line.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter trailing twelve month net sales units for one item at one location, then your realistic lead time and the standard deviation of daily demand. Pick a service level, enter the cost of placing a purchase order and the unit cost, and set the vendor buy multiple. The order point tells you when to reorder, the buy quantity tells you how much, and the max level is the order point plus one buy. Run it for your top movers first, since a 2 to 3 day error in lead time on a fast item changes the order point more than any other input.
- Use net sales, not gross, so returns and cancelled lines do not inflate demand
- Measure lead time from PO release to usable stock
- Use daily standard deviation, not monthly
- Round to the vendor pack or buy multiple
The formulas behind it
Safety stock is Z times the standard deviation of daily demand times the square root of lead time days. Order point is average daily demand times lead time, plus safety stock. EOQ is the square root of two times annual demand times order cost, divided by unit cost times the carrying rate. These are the standard textbook formulas used across distribution. They assume reasonably stable demand and a fixed lead time. Lumpy, project driven or seasonal items break those assumptions, and a forecast driven or manual method fits them better than a statistical order point.
Reading the result against P21
Prophet 21 supports several ways of driving replenishment, including min and max style levels and demand based approaches, configured per item and location. Compare this calculator's order point with the value your configuration produces. If the system number is far higher, check for an inflated lead time, a stale demand history, or a safety stock setting that was never revisited. If it is far lower, look for one large past order skewing history. Use the chart to see how sensitive the order point is to lead time before you negotiate with a vendor.
Common mistakes
The most frequent error is using the quoted vendor lead time instead of the measured one. The second is treating a one time project sale as normal demand. The third is applying the same service level to every item, which ties up cash in slow, low margin lines. Segment by ABC class and set service targets accordingly, perhaps 98 to 99 percent for A items and 90 to 95 percent for C items. Finally, remember that order point should be compared against available plus on-order stock, not on-hand alone.
Frequently Asked Questions
What service level should I use for a distributor?
It depends on item class and margin. Many distributors use roughly 97 to 99 percent for fast, critical A items and 90 to 95 percent for slow C items. Higher targets raise safety stock sharply, so test the cost with the chart before committing a whole catalog to 99 percent.
Does this match what Prophet 21 calculates?
Not necessarily. P21 has its own replenishment methods and parameters, and results depend on your configuration, history periods and lead time settings. Use this as an independent cross check, then adjust the inputs until you understand any difference.
How do I handle intermittent demand?
The normal-demand formula overstates reliability for items that sell a few times a year. For those, use a fixed min and max based on a judgment call, or stock to a specific order count rather than a statistical level, and review them quarterly.
Why does lead time matter so much?
Lead time drives both the demand you must cover and, through the square root term, the safety stock. Doubling lead time roughly doubles lead time demand and raises safety stock by about 41 percent, so shaving days off vendor lead time pays directly.
If you want replenishment parameters reviewed across thousands of items and locations, we can build a read-only analysis over your ERP data that flags outliers for your buyers to approve.
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