Dynamics AX 2012 Safety Stock Journal: Minimum Inventory Proposal Calculator
This calculator reproduces the two Calculate proposal methods of the Dynamics AX 2012 safety stock journal: Use service level and Use avg. issue during lead time. Use it to sanity-check the proposed minimum before you post it to item coverage.
Your numbers
Average monthly issue quantity over the history period.
Added to the item lead time to give total lead time.
Your results
Mirrors the formulas Microsoft documents for the AX 2012 Calculate proposal form. AX computes the average and standard deviation from your issue transactions; enter values from the same history period to compare.
Proposed minimum vs item lead time
Sweeps Item lead time from 1 to 90, holding your other inputs fixed.
Want proposals checked across all items?
Send an export of item issues and current minimums, and we will flag items whose AX minimum is far from what the formulas suggest.
Optional. The tool and every export work without this. No spam.
How to use it
Take the average monthly issue and its standard deviation for the item over the history period you use in the journal, enter the item lead time and the lead time margin you will enter in the proposal form, and pick the service level. The calculator shows both proposals. Move the lead time slider or read the chart to see how strongly each method reacts to lead time: the service level method grows with the square root of lead time, the average issue method grows linearly.
- Total lead time = item lead time + lead time margin
- 50% service level gives a minimum of 0
- Use the multiplying factor for seasonal swings
- Compare against the current minimum on item coverage
Where this lives in AX 2012
Microsoft documents the path as Master planning > Journals > Safety stock. Create a journal, add item lines (optionally with a history period), open Journal lines and click Calculate proposal. Choose Use avg. issue during lead time (with Factor used for multiplying) or Use service level (with Service level), set Lead time margin, and tick Proposal = new minimum to copy the result into the New minimum field. Posting the journal updates the minimum on the item coverage record for that item and dimension.
The formulas and a worked example
Microsoft documents the service level method as: minimum = safety factor x standard deviation per month x square root(total lead time / 30 days), with safety factors 0 (50%), 0.84 (80%), 1.28 (90%), 1.65 (95%), 2.05 (98%), 3.00 (99.86%) and 4.00 (99.99%). The average issue method covers average issues during total lead time times the factor; Microsoft does not publish the day conversion, so this tool lets you choose 30 calendar days or 21 working days. Example: 600 per month, deviation 180, lead time 21 plus 3 days, 95%: 1.65 x 180 x sqrt(24/30) = about 266 units, versus 600 x 24 / 30 = 480 by average issue.
Gotchas
Both methods assume issue quantity equals demand and that monthly demand follows a normal distribution, which Microsoft lists as prerequisites. Lumpy or intermittent demand breaks the service level method; a few large issues inflate the deviation. The average issue method is really a reorder cover, not a safety stock, so it usually proposes more. Check whether your lead times are calendar or working days before comparing with AX. A proposal is only stored after you post the journal, so review the New minimum column first.
Frequently Asked Questions
How does the AX 2012 safety stock journal calculate the minimum?
With Use service level it multiplies the safety factor for the chosen service level by the monthly standard deviation of issues and by the square root of total lead time divided by 30 days. With Use avg. issue during lead time it takes the average issue over total lead time times a multiplying factor.
What safety factor does AX 2012 use for 95% service level?
1.65, per Microsoft's table for the Calculate proposal form. The other values are 0 for 50%, 0.84 for 80%, 1.28 for 90%, 2.05 for 98%, 3.00 for 99.86% and 4.00 for 99.99%. A 50% service level always produces a minimum of zero.
What is the lead time margin in the safety stock journal?
Extra days, for example internal order administration, that AX adds to the item's lead time before calculating. Total lead time is lead time plus margin, and both proposal methods use that total. A margin of 3 days on a 21-day lead time gives 24 days.
Does Calculate proposal change the item coverage minimum?
Not directly. It fills the proposal on the journal line, and with Proposal = new minimum ticked it copies the value to New minimum. The item coverage minimum changes only when you post the journal, so you can review and edit proposals first.
If you want planners to ask which AX 2012 items have minimums out of line with their real demand, answered from your own data on your servers, we can set that up.
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