Safety-Stock & Reorder-Point Calculator: Set Reorder Rules with Math, Not a Gut Feel

The problem this kills

Most reorder points and min/max levels are set by feel. Someone eyeballs an item, picks a round number that "feels safe," and it sits frozen in the item master for years — long after demand grew, the supplier got slower, or the part went seasonal. The result is the worst of both worlds: you stock out on the items that matter and you sit on mountains of cash in the items that don't.

The math to do this properly has existed for decades, but it lives in dense textbooks and intimidating spreadsheets full of Z-scores and standard deviations. So planners avoid it. They keep guessing, the guesses go stale, and nobody can explain why a given item's reorder point is what it is — which means nobody trusts it enough to change it.

A safety-stock and reorder-point calculation should be a real, governed tool that shows its work: here's your demand variability, here's your lead-time variability, here's the service level you asked for, here's the safety factor that implies, and here's the number that falls out. Set the rule with a method, see exactly why, and update it the moment the inputs change.

What you'll build

A simple internal web app for your planning team. You import your demand history per item (so the tool can measure average demand and how much it swings), your lead-time data (average lead time and how much it swings), and your target service level — and you can let that service level vary by item criticality, so an A-item that can never stock out gets a higher target than a cheap C-item.

The tool computes, for every item: a safety stock that buffers against demand and lead-time variability at your chosen service level, and a reorder point (expected demand over the lead time plus that safety stock). For each item it shows the formula and the exact inputs that produced it, and it compares the recommendation against your current reorder settings so you can see what would go up, what would come down, and by how much. It flags items with too little demand history as low-confidence so you don't blindly trust a number built on three data points. The planner reviews the recommendations, adjusts what looks wrong, and approves — and only then are the approved new levels exported as a clean CSV to update the item master. Nothing auto-applies.

What's inside the Implementation Plan

The downloadable plan is a step-by-step file you paste into an AI coding agent. It opens by interviewing you about your business — how you set reorder levels today, which ERP or spreadsheet holds your item master, exactly how your demand and lead-time data are shaped and named (SKU conventions, units, time buckets), your typical and peak SKU volumes, your service-level policy by item class, and the messy edge cases (new items with no history, lumpy or seasonal demand, suppliers with wildly variable lead times). It reflects a short tailored spec back to you and waits for your thumbs-up before it builds anything, so the calculation matches your data and your policy — not a generic template.

From there it walks the agent through the data model, the demand and lead-time imports, the statistics (average and standard deviation of demand and of lead time), the service-level-to-safety-factor mapping explained in plain language, the safety-stock and reorder-point engine that shows its formula, the comparison against current settings, the low-history flagging, the planner approval gate, the change log, and the CSV export in the exact columns your item master expects. Every step ends with a ready-to-copy prompt. There's a full "No API yet?" path that uses Google Sheet / CSV imports as the data source and produces a clean CSV export — so you can build and run the whole thing this weekend regardless of what ERP you're on.

The governance it includes (this is the point)

This tool changes the numbers your buyers reorder against, so the controls aren't optional. The plan builds in login so only your planning team can use it, row-level security so you only ever see your own organization's items, a complete audit trail of who changed which assumption and who approved which recommendation, a hard human-approval gate so no new level ever leaves the tool until a planner reviews the math and signs off, and duplicate guards keyed on SKU so the same item can't get two recommendations in one run and a re-import can't double-count demand.

Who it's for

Inventory planners, demand planners, buyers, and supply-chain leads — especially anyone who currently sets min/max levels by feel and lives between overstock and stockout. If you can export your demand history and lead times and tell the tool what service level each item class deserves, you can build this.

You've got this — start with the plan, paste the first prompt, and answer the interview. You'll have method-driven reorder points on screen before the weekend's out.