ABC analysis: ranking inventory by value and setting policy from it

ABC analysis exists because attention is the scarcest thing in an inventory team. You cannot review 4,000 items weekly, forecast them individually, or count them every month — and you do not need to, because value is never spread evenly across a catalogue. ABC analysis ranks items by how much money moves through them each year and splits that ranking into three classes, so effort follows value.

ABC analysis

A Pareto classification of inventory items by annual usage value. Items are sorted from highest value to lowest, a running cumulative percentage of total value is taken, and classes are cut at chosen thresholds — commonly 80% for A and 95% for B, with everything after that class C. Each class then gets a different management policy.

The one input people get wrong

Annual usage value = annual quantity consumed × unit cost. Three wrong inputs get used in its place, and each produces a plausible-looking classification that sends effort to the wrong place:

• Unit cost alone. This makes a $9,000 spare that ships once every three years a class A item, and a $0.42 bolt consumed 26,000 times a year a class C item. The bolt stops a production line; the spare sits in a cage.
• On-hand balance. Ranking by what you are holding answers a different and also useful question — where is the cash parked — but it rewards items you over-bought and hides fast movers you keep lean.
• Sales revenue. Fine for a commercial ABC, but it mixes margin into an inventory decision. Use cost when the output is a stocking policy, and never mix bases in one list.

A worked ABC analysis, all twelve items

Twelve items from one warehouse, ranked by annual usage value. Unit cost and annual usage are both shown so every value column is checkable. Total annual usage value is $1,290,480.

Check the boundaries by hand. Cumulative value at rank 3 is $384,800 + $296,000 + $198,400 = $879,200, which is 68.1% of $1,290,480 — still under 80%, so rank 4 joins class A and takes the cumulative to $1,034,600, or 80.2%. Class A is therefore the shortest set of items whose combined value reaches the threshold. The same rule at 95% puts rank 7 in class B, ending it at $1,235,840, or 95.8%.

Class policies: what each class actually earns

A classification that changes nothing is a spreadsheet column. The output of ABC analysis is a policy table, and it should be short enough that a planner can hold it in their head.

The counting arithmetic makes the case on its own. Under this policy the twelve items need 4 × 12 + 3 × 4 + 5 × 1 = 65 counts a year, with 80.2% of value passing under a monthly count. Counting all twelve monthly would be 144 counts — the ABC schedule does the same job for 55% fewer. Counting all twelve quarterly is 48 counts, less work, but leaves four fifths of your value checked four times a year. Scale sample sizes with the cycle count sample size calculator.

The XYZ overlay: value is not predictability

ABC tells you which items are worth money. It says nothing about whether their demand is forecastable, and buffer size is driven almost entirely by that second question. XYZ classification fills the gap by ranking items on the coefficient of variation of period demand — standard deviation divided by the mean.

Run it on the three top items above. VLV-300 has a monthly mean of 2,600 ÷ 12 = 217 units and a monthly deviation of 38, so its coefficient of variation is 38 ÷ 217 = 0.18 — class X. PMP-120 averages 267 a month with a deviation of 165, so 165 ÷ 267 = 0.62 — class Y. MTR-075 averages 53 a month with a deviation of 62, giving 62 ÷ 53 = 1.16 — class Z, because it ships in project lots rather than steadily.

All three are class A, and all three need different treatment. AX items are where formula-driven replenishment works best: tight buffers, high service level, low effort. AY items need seasonal parameters and a planner who looks at the shape. AZ items are the ones that break planning systems — a large fixed buffer, a make-to-order arrangement or a customer commitment is usually cheaper than trying to forecast them. Size the buffers per cell with the safety stock calculator, and set the trigger with the reorder point guide.

How to run an ABC analysis

1. 01

   Pull 12 months of issues by item

   Quantity consumed, not quantity purchased and not the current balance. Exclude intercompany transfers, returns and scrap adjustments, or you will double-count movement. Twelve months smooths seasonality; six months is acceptable if the assortment changed materially.

2. 02

   Attach a consistent unit cost, and fix part-year items

   Standard cost or a 12-month average cost, applied the same way to every line — mixing last purchase price for some items and standard cost for others quietly re-ranks the list. Then annualise anything live for only part of the window: an item three months old shows a quarter of its true value and lands in class C. Drop discontinued lines rather than letting run-out volume earn them an A class they will never need again.

3. 03

   Multiply, sort descending, and take a running cumulative percentage

   Annual value = usage × unit cost. Sort highest to lowest, then compute each item's share of the grand total and the running cumulative. The cumulative column is the whole analysis; the item shares are only for reading the shape of the curve.

4. 04

   Cut the classes at your thresholds

   80% and 95% are the conventional starting points. Keep the item that crosses each threshold in the lower class, so class A is the shortest list reaching 80%. If your curve is very steep, a 70% A threshold may produce a more workable A list — the thresholds are policy, not formula. Pick and document them.

5. 05

   Attach policies, and change something

   Write count frequency, review cycle, service level and order rule per class, then push them into the item master fields your ERP actually uses for replenishment. If nothing on any item record changes, the analysis has not happened yet.

6. 06

   Overlay demand variability before setting buffers

   Compute the coefficient of variation of monthly demand per item and assign X, Y or Z. Set service levels by ABC and buffer method by XYZ. This is the step that separates a classification that works from one that produces stockouts on your most important items.

7. 07

   Schedule a quarterly refresh and diff the classes

   Re-run every quarter and produce the list of items that changed class. A C item that became A means demand shifted and nobody re-planned it — that report is frequently more valuable than the classification. Refreshing monthly makes items flip on noise and destroys the stability the policies depend on.

Where ABC analysis is the wrong tool

• Criticality is not value. A $12 sensor that stops a $4,000-an-hour line belongs in class A whatever the arithmetic says, and a contractual availability commitment overrides class outright. Keep a manual override list, and keep it short and reviewed.
• Very short life cycles. If items are replaced every two quarters, a 12-month usage history describes a catalogue you no longer sell. Classify by planned volume instead.
• Single-site logic on a multi-site business. Value distribution differs by warehouse. A national ABC assigns branch policies from a distribution none of the branches has.

Getting the ranked list out of your ERP

The classification takes a minute once the list exists. Building the list is the job: twelve months of issue quantities by item and location, on a consistent cost basis, net of intercompany and returns, with part-year items annualised. That is a saved search, an export, a cost lookup, a pivot and a manual review of new items — every quarter, forever, which is why so many ABC classifications in production are three years old.

Ask ERPray for "annual usage value by item for the last 12 months, excluding intercompany transfers, ranked descending" and the answer is computed live from your own account with the query shown underneath, so you can confirm the cost basis rather than assume it. Then sort within class by margin using the inventory turnover calculator — the ranking in inventory turnover: how to read it is the natural second pass on the same list.