Inventory Turnover in Manufacturing: Calculate and Improve

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Estimated reading time: 10 minutes

Your inventory value has grown, but the factory still runs short of the parts it needs. Some material sits untouched, unfinished orders wait between machines, and completed products occupy space while customers chase different items. Cutting every stock balance would lower the inventory value, but it would not necessarily make production better.

Inventory turnover measures how many times a business sells and replaces its average inventory during a period. Calculate it by dividing cost of goods sold by average inventory value. For a manufacturer, that inventory includes raw materials, work in process and finished goods.

The useful question comes after the calculation: which part of that inventory is unnecessary, and what is keeping it there? When early starts and disconnected machine queues are building stock, SkyPlanner APS helps you coordinate materials, capacity and order priorities in one production schedule. You can reduce unnecessary work waiting in production instead of spending each day reconciling separate queues.

How to calculate inventory turnover

Use the same period and a consistent cost basis for both inputs:

Inventory turnover = cost of goods sold ÷ average inventory value

The result is expressed as turns during the period, not a percentage. Annual cost of goods sold gives annual turns when paired with average inventory for that year.

Use cost of goods sold, not sales revenue

Cost of goods sold, or COGS, is the cost assigned to the products sold during the period. In manufacturing, it includes the materials, direct labor and allocated production overhead carried by those products. Take the figure from your agreed financial records.

Sales revenue includes your selling margin, so dividing revenue by inventory at cost produces a different measure. Purchases are not a substitute either: some purchased material may still be on the shelf. Nor is the cost of goods manufactured the same as COGS; completed products can remain unsold in finished inventory.

Calculate a representative average

A simple starting point is:

Average inventory = (beginning inventory + ending inventory) ÷ 2

For a seasonal factory or one receiving large deliveries, two snapshots may miss most of the year. Average regular month-end balances instead, or use more frequent observations when stock fluctuates sharply. Keep the observation method consistent when comparing periods.

Include the value of all three manufacturing stages, counting each item once at its current stage. WIP carries the costs accumulated so far, not automatically the full cost of a finished product. Reconcile the balances with finance so changes in valuation, missing receipts or unrecorded completions do not look like changes in physical flow.

Keep the time period visible

Compare a year’s COGS with that year’s average inventory, or a quarter’s COGS with the quarter’s average. A quarterly ratio is turns in that quarter, not annual turns. Multiplying a seasonal quarter by four can give a misleading annual picture.

Inventory transactions are different again. Moving a pallet from stores to machining, then to finishing, creates stock movements. It does not mean the business has sold and replaced that inventory three times.

A worked manufacturing example

Imagine a manufacturer with annual COGS of $3,600,000. Its regular inventory observations give these average balances for the same year:

Inventory stage	Average value
Raw materials and purchased components	$300,000
Work in process	$420,000
Finished goods	$180,000
Total inventory	$900,000

The calculation is:

$3,600,000 ÷ $900,000 = 4 inventory turns per year

At that year’s cost-of-sales rate, the business sold the equivalent of four times its average inventory value. This does not mean every component or product turned four times. A frequently used fastener and an obsolete assembly can sit inside the same company average.

You can express the same relationship in days:

Inventory days = days in the period ÷ inventory turnover

Using a 365-day year:

365 ÷ 4 = 91.25 days

This is an aggregate, backward-looking cost-based measure. It is not a promise that each order takes 91.25 days, or that today’s available stock will cover the next 91.25 days of demand. Actual order lead time and forward stock coverage answer different questions.

What is a good inventory turnover ratio?

There is no single good number for every manufacturer. Product mix, supplier lead times, production routes, seasonality and the service you promise all affect how much inventory the business needs. A long-route equipment manufacturer and a producer of short-life standard goods should not inherit the same target.

Start with your own comparable periods and product families. Then use relevant industry peers as context, checking whether their inventory scope and calculation match yours. A ratio based only on finished goods is not directly comparable with one that includes raw materials and WIP.

Read the trend alongside delivery reliability, shortages, output and inventory age:

• Lower turns with rising stock and unchanged demand: investigate excess buying, early production, blocked work and products that no longer sell.
• Higher turns with stable deliveries and less average inventory: the business may be supporting demand with less capital tied up.
• Higher turns with more shortages or emergency freight: stock may have been cut below what the process can reliably support.
• A ratio change after a major cost or valuation change: check how much came from accounting values rather than faster movement.

A high total can still hide dead stock in one family and shortages in another. The target is reliable customer supply with less unnecessary inventory, not the largest possible ratio.

Find the cause in raw materials, WIP or finished goods

The total ratio tells you where to start asking questions. The stage breakdown helps you decide who needs to act.

Where inventory is accumulating	What to investigate	A useful first action
Raw materials	Purchase quantities, arrival timing, changed demand, unusable stock or a justified supply buffer	Compare upcoming requirements with usable balances and open purchase orders before buying again
Work in process	Early starts, overloaded downstream resources, missing components or quality holds	Follow an aging order’s remaining route and identify what prevents its next operation
Finished goods	Production ahead of demand, canceled requirements, dispatch holds or slow-selling products	Match completed stock to real demand and review further replenishment

Raw materials: separate protection from excess

A slow-moving component may still protect an important customer commitment. Before reducing it, check the consequence of running out and how reliably you can replenish it. A justified safety stock allowance is different from material ordered twice because incoming supply was invisible.

For a stage-specific turnover measure, match the flow to that stage. In the illustrative factory, annual raw-material consumption of $1,800,000 divided by average raw-material inventory of $300,000 gives 6 raw-material turns. That measures consumption into production, not sales of the whole factory’s inventory. It does not replace the total ratio of 4.

Do not divide total COGS by each stage balance and treat the results as directly comparable stage turnover rates. The material, conversion and finished-product cost boundaries differ. Use matched stage flows where available, and inspect age and waiting reasons to diagnose the operational problem.

WIP: find what the next operation is waiting for

WIP is the largest average balance in the example, so it deserves attention. The next step is not to stop all new work. Identify the orders that have waited longest or accumulated substantial cost, then check their remaining operations, material needs and delivery commitments.

Imagine machining finishes a batch on Monday, but coating cannot take it until Friday. Another order needs a purchased component that is not due until next week. Starting both orders early keeps the first machine busy, yet creates unfinished stock that cannot continue.

That is a coordination problem. You need a plan for the whole route, not a separate instruction to make every machine look busy. Some ready work may be needed to protect a constrained resource; the unnecessary part is work released without a useful path forward.

Finished goods: completion is not the same as sale

Moving an order out of WIP can make production look better while leaving the same goods in the warehouse. Before producing the next replenishment batch, check what is already complete and whether customer demand still supports it.

If finished stock is old because demand disappeared, the remedy involves sales, product decisions or an agreed disposal route. Rescheduling more production cannot create that missing demand. If it is waiting on a dispatch or documentation hold, resolve that hold rather than treating the problem as a shortage of manufacturing capacity.

Use SkyPlanner to connect material arrivals with the work that needs them

When the investigation points to premature starts or conflicting queues, use SkyPlanner to turn the remaining work into a shared production schedule. Arcturus, its scheduling AI, calculates the sequence from workstations, shifts, operators, operation durations, dependencies and your priorities. The Gantt view shows how the work fits across resources.

With material-aware scheduling enabled, inventory availability also affects the calculation. Purchase-order delivery dates represent expected material availability. When the material arrives and the purchase is confirmed as delivered, the stock balance is updated. An incoming purchase is therefore useful planning information, without being mistaken for material already on the shelf.

If a supplier changes a date, update it and recalculate the schedule. You can review the affected operations alongside the capacity and other orders competing for the same resources, rather than correcting a purchasing list and several machine queues separately.

For the waiting machining batch, the useful decision is whether its early operation should run now or whether another order can progress more effectively. You set the priorities; Arcturus handles the scheduling calculation within your rules. This makes it easier to avoid adding material and conversion effort to the wrong work too early, while protecting the orders that need to ship.

Delaying a start does not make already purchased material disappear. The financial improvement comes from sustaining demand with less unnecessary inventory across the whole flow, not simply relabeling raw material as WIP. A connected schedule gives you a practical way to coordinate that change and see the delivery implications before you act.

SkyPlanner integrates with any ERP or MES through its open REST API, so you can connect existing order and production data with that scheduling work instead of maintaining another isolated plan.

What changed at Piristeel

Piristeel, a manufacturer of roof safety products and rainwater systems, previously relied on large batches and buffer stocks. Production could appear smooth, but high inventory values and poor visibility between departments made changes difficult.

In its SkyPlanner review, Piristeel describes detailed scheduling across three production departments, a shared Gantt view and integration with Visma Nova. The company gained better coordination of smaller runs and improved inventory accuracy, with stock events transferred to its ERP in real time. Large buffers were no longer a prerequisite for smooth operations.

The relevance to turnover is the operating change: better visibility and coordination reduced the need to use stock as a substitute for a manageable plan. For a factory carrying excess inventory to compensate for uncertain production timing, that is a practical reason to bring its stock information and scheduling together.

Check that the improvement reaches the total

Return to the illustrative manufacturer. Suppose it supports the same annual COGS of $3,600,000 with average WIP of $240,000 instead of $420,000, while the other stage averages remain unchanged. Average total inventory becomes $720,000.

$3,600,000 ÷ $720,000 = 5 turns per year

365 ÷ 5 = 73 inventory days

That is $180,000 less average inventory supporting the same cost of sales. It is not $180,000 of profit or an automatic cash receipt. Check that the reduction is real, rather than a transfer into another stock category or a valuation adjustment, and that deliveries remain dependable.

For your first improvement, choose one slow product family. Record its inventory by stage, the age and reason for waiting, incoming material dates and the remaining production route. Agree on the change with the people responsible for buying, production and customer commitments, then review turns and service together over comparable periods.

Book a SkyPlanner demo with that route and a recent example of work started too early. You can explore how the material dates, competing orders and available capacity come together in one schedule. If you prefer to explore the system first, try SkyPlanner with demo data. The calculation shows where capital is tied up; the connected plan helps you change the work that keeps it there.