Inventory Reorder Calculator

What this tool does

CM8-07 answers the two questions every storeroom faces for every SKU: when should I reorder? and how much should I order? It uses the standard continuous-review inventory model: a reorder point tells you when, an economic order quantity (adjusted for supplier MOQ and pack size) tells you how much, and a safety stock buffer absorbs demand and lead-time surprises. Enter your current stock and open orders and the tool flags each item OK, REORDER NOW or CRITICAL, then builds a printable purchase-suggestion sheet.

Everything runs inside this single HTML file. No installation, no account, no network requests — your part numbers, costs and stock levels never leave your computer.

Safety stock — your insurance policy

If usage were perfectly steady and suppliers always on time, you could reorder exactly when remaining stock equalled the demand during lead time. Reality is noisy: some days you use more, and deliveries slip. Safety stock is extra inventory held to cover those surprises.

The square-root term is the combined uncertainty of demand during the lead time: the first part covers day-to-day demand scatter, the second covers late (or early) deliveries. The z factor converts your service-level target into a number of standard deviations:

• Service level target — z factor — Roughly means
• 90 % — 1.282 — 1 stock-out per 10 replenishment cycles
• 95 % — 1.645 — 1 per 20 cycles — a common default
• 97.5 % — 1.960 — 1 per 40 cycles
• 99 % — 2.326 — 1 per 100 cycles — for critical spares

Don't know your std dev? Pick “High–low estimate” and enter a typical busy day and a typical slow day; the tool uses σ ≈ (busy − slow) ÷ 4, a standard range-based approximation.

Reorder point — when to order

Expected usage during the lead time, plus the safety buffer. When your inventory position (on hand + already on order) falls to or below the ROP, place an order — what's left should just carry you through until the delivery arrives.

EOQ — how much to order

Every order costs money to place and receive; every unit on the shelf costs money to hold. Order rarely in big lots and holding cost dominates; order often in small lots and ordering cost dominates. The economic order quantity minimises the sum:

The tool then rounds the EOQ up to a multiple of your pack size and raises it to the supplier MOQ. Derived figures shown per item:

• Max stock = SS + order qty — the level just after an on-time delivery.
• Average inventory = SS + order qty ÷ 2, valued at unit cost.
• Orders per year = D ÷ order qty.
• Annual holding + ordering cost = avg inventory × H + orders/yr × S.

On the reorder list, the suggested qty to order is one EOQ plus whatever extra is needed to lift a deeply depleted item's position back to its reorder point — again rounded to pack and MOQ.

Status flags

• Flag — Condition — Meaning
• OK — position > ROP — No action needed this cycle.
• REORDER NOW — position ≤ ROP — Place the suggested order; remaining stock should cover the lead time.
• CRITICAL — position < SS — You are already into the safety buffer — order immediately and consider expediting.

Field guide

• Field — How to fill it in
• Avg daily usage — Total usage over a recent period ÷ calendar days in the period. Use calendar days, not working days, so it matches lead times quoted in calendar days.
• Usage std dev — Std dev of daily usage from your records, or the high–low estimate. 0 = perfectly steady demand (rare).
• Lead time — Days from placing the order until goods are usable on the shelf — include approval, transit and receiving/inspection time.
• Lead-time variability — Std dev of lead time in days. Quick estimate: (latest delivery − earliest delivery) ÷ 4.
• Cost per order — What one purchase order really costs: buyer time, approvals, freight minimums, goods-in handling, invoice processing. Typical range 10–75 in most currencies.
• Holding rate — Annual cost of keeping one currency-unit of stock: cost of capital + warehouse space + insurance + obsolescence/shrinkage. 18–25 %/yr is a common planning range.
• MOQ / pack size — Supplier constraints. Suggested quantities are raised to the MOQ and rounded up to full packs.
• On hand / on order — Physical shelf count and open PO quantity not yet received. Together they form the inventory position used for the status flag.

Worked example

Assumptions & limits

• Annual demand uses 365 calendar days. If your usage figures are per working day, convert them to calendar-day averages first.
• The safety-stock formula assumes demand and lead time are roughly normally distributed and independent. For very slow movers (a few units per month) it over- or under-shoots — treat those items with min/max judgment instead.
• EOQ assumes a constant unit price (no quantity-break discounts) and steady demand through the year. Strong seasonality deserves a seasonal review of the daily-usage figure.
• Results are planning suggestions, not purchase orders. Review quantities against shelf life, storage space, cash flow and upcoming known demand before buying.
• Currency is a display label only — no exchange-rate conversion is performed.

Saving your work

• Autosave: your item list, currency, sort order and report details are kept in this browser's local storage (when the browser allows it) and restored next time you open the file. The indicator in the top bar shows the last autosave time.
• Save: the toolbar Save button writes the current state to this browser straight away and flashes Saved ✓ HH:MM — useful before closing the tab.
• Report details: set a report title, company, preparer and closing notes in the toolbar; they head the printed report and travel with the saved project.
• Save project (.json): a complete portable snapshot — move it between machines or keep dated copies alongside your stocktake records.
• Export CSV: the full item table (inputs + all computed columns) or just the reorder list, ready for a spreadsheet or ERP import.
• Print Report: builds a dedicated report — executive summary tiles, the purchase-suggestion list, the three charts, the full item table, method notes and your closing notes. The screen furniture is dropped, so the browser print dialog → “Save as PDF” gives a clean document.