Get Ahead of the Game: 10 Inventory Management Formulas for a More Successful Business

By

Brahm Meka

August 15, 2023

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11 min

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It's a common misconception that manufacturing is solely about hammering wood, fusing metal, and shaping plastic. The reality is, it's also a game of numbers, particularly when it comes to optimizing inventory management.

Are you keen on enhancing efficiency and minimizing waste? Then, mastering the art of knowing when to replenish materials, how much to order, or deciphering when your finished goods will deplete, is crucial.

This article serves as a treasure trove of essential inventory management formulas. Consider it your go-to guide whenever you find yourself knee-deep in inventory calculations. Don't forget to bookmark this page for easy access in the future!

1. Lead Time

Commonly referred to as cycle time in the world of logistics, lead time serves as a vital meter. It clocks the duration taken from when a business initiates a purchase order to a supplier until the ordered goods are received.

Wondering how to calculate lead time? Let's simplify it:

Lead Time = Delivery Date - Order Date

Let's take a hypothetical example. Suppose a company places an order for raw materials on the 15th of each month, and the stock consistently arrives on the 23rd. How do we calculate the lead time?

It's simple: 23 (Delivery Date) - 15 (Order Date) = 8 days of lead time.

Why does this matter? This logistics Key Performance Indicator (KPI), quantified in days, provides the logistics manager with a discerning lens into the efficiency of the company’s supply chain. It's like having a pulse on the heartbeat of your supply chain, wouldn't you agree?

2. Economic order quantity (EOQ)

Let's talk about the Economic Order Quantity (EOQ), a crucial formula in inventory management that helps you pinpoint the optimal order quantity to minimize total inventory costs. It's a delicate balance, considering the tradeoff between setup or ordering costs and holding or carrying costs. By applying the EOQ formula, manufacturing businesses can determine approximately how much inventory they should order in a single instance to effectively reduce their overall costs.

Curious about the economic order quantity formula? Here it is:

EOQ = √[(2DK)/H]

Now, you might be asking, "What do these symbols represent?"

D is the Annual demand in units

K stands for Ordering costs per order

H represents Carrying costs per unit per year

Let's see what it would look like with an example.

Picture this - a firm is planning to purchase inventory for a product with a yearly demand of 2,000 units, a procurement cost of $300, and a storage cost of $1 per unit annually. The EOQ formula would be applied in the following manner:

EOQ = √[(2 x 2,000 x 300) / 1] EOQ = √1,200,000 = 1095

So, what's the significance of this? Simply put, the most cost-effective order size is 1095 units. It's now clear how this formula becomes an essential instrument for inventory management, isn't it?

3. Safety stock

Safety stock is a term used in inventory management to describe a reserve of products stored in your facility. Think of it as a safety net, ready to catch you when unforeseen events occur; be it a sudden surge in demand for a specific product, unexpected changes in SKU turnover, or delays from suppliers.

Safety Stock = (Maximum Lead Time - Average Lead Time) × Average Product Demand

Let's consider a real-world scenario to illustrate how to use this formula effectively. Suppose a production center requires 200 units of a product to fulfill existing production orders. If the average lead time to receive this product is 5 days and the maximum lead time is 8 days, the resulting safety stock calculation would be:

(8 - 5) × 200 = 600 units of safety stock

So, why go through all this trouble to calculate safety stock? This key performance indicator (KPI) 's primary goal is to ensure sufficient stock in the warehouse to avoid stockouts. Picture a stockout as a sudden dry spell when there's no water in sight; it's when you've accepted orders but can't fulfill them due to a lack of inventory. By maintaining a safety stock, you ensure a seamless flow of operations, even in the face of unpredictability.

4. Carrying Cost

Calculating the carrying cost is an essential aspect of inventory management. This formula helps determine the cost related to storing, handling, and financing your inventory. It specifically uses the average inventory level and the cost associated with maintaining a single unit of inventory per annum.

Businesses leverage the carrying cost formula to comprehend the expenses tied to their inventory storage to make informed decisions about inventory management.

The formula for Inventory Carrying Cost is:

Inventory Carrying Cost = Total cost of holding inventory / total value of inventory x 100

Let's try to understand this further with a practical example.

Consider a firm that aims to compute the carrying cost of its inventory. The total value of their inventory amounts to $500,000, while the total cost of holding the inventory, including storage, handling, and financing expenses, stands at $25,000.

Applying the above-mentioned inventory management formula, we can figure out the carrying cost as follows:

Inventory Carrying Cost = ($25,000 / $500,000) x 100 = 0.05 x 100 = 5%

In conclusion, the carrying cost of the firm's inventory is 5% of the total inventory value.

5. Stockout

A stockout event happens when a customer's order arrives at the warehouse, but there is insufficient inventory to satisfy the order.

The formula for calculating a stockout is as follows:

Stockout = number of unsupplied items × unit storage cost

For instance, if a business fails to meet 50 orders and each unsupplied item costs $5.00, the stockout cost would be computed as follows:

50 × $5.00 = $250 stockout expense

This metric is often expressed as a percentage, that is, the ratio of stockouts to the total orders received.

Stockout rate = (number of unsupplied items / total order quantity) × 100

Building on the previous example, if the business is unable to meet 50 out of 500 orders, we could compute the stockout rate percentage as follows:

(50 / 500) × 100 = 10% stockout rate

While this equation shows the percentage of orders that couldn't be fulfilled, keep in mind that failure to supply an order results in other issues, such as a lack of faith in the supply chain process.

6. Gross margin return on investment

The Gross Margin Return on Inventory Investment (GMROI) is a crucial metric that businesses use to assess their inventory's profitability. It helps businesses understand the returns they're getting on inventory investments by juxtaposing the gross margin against the average inventory investment.

GMROI = Gross Margin / Average Inventory Investment

Here, we need to understand two key components:

Gross Margin: It's the difference between sales revenue and the cost of goods sold.

Average Inventory Investment: It's the average of the beginning inventory and the ending inventory of a given period.

Let's illustrate this with an example. Suppose there's a bookstore that has a gross margin of $100,000 and an average inventory investment of $50,000 over a year.

Now, plugging these numbers into our GMROI formula, we get:

GMROI = $100,000 / $50,000 = 2

So, the GMROI ratio for this bookstore is 2. This implies that for each dollar invested in the inventory, the bookstore generates a gross profit of $2.

A high GMROI ratio reflects efficient inventory management and a solid return on investment. Conversely, a low GMROI ratio suggests that the business isn't generating substantial profit from its inventory. This could signal the need for changes in inventory management practices, such as inventory reduction or product mix enhancement.

By leveraging the GMROI ratio, businesses can evaluate their inventory's profitability, make informed decisions, and devise strategies to optimize inventory levels, pricing, and product mix, thereby maximizing profitability.

7. Inventory turnover rate

The Inventory Turnover Ratio (ITR) is a pivotal inventory management formula that quantifies how frequently inventory is sold and replenished within a specific period. It considers two major factors: the Cost of Goods Sold (COGS) and the average level of inventory held.

Bearing immense relevance in the business world, the Inventory Turnover Ratio formula is utilized by businesses to gauge their inventory management efficiency and to make informed decisions concerning their inventory levels.

The formula for the Inventory Turnover Ratio is given as follows:

Inventory Turnover Ratio = Cost of Goods Sold / Average Inventory Level

Where:

Cost of Goods Sold — Refers to the total cost of all goods that were sold during the specified period

Average Inventory — Represents the average of the inventory at the beginning and the end of the period. In mathematical terms, it is (Beginning inventory + ending inventory) / 2

Let's delve into a practical scenario to illustrate this better.

Consider a business entity boasting a COGS of $100,000 and an average monthly inventory of $50,000. Applying these figures to our formula:

Inventory Turnover Ratio = 100,000 / 50,000 = 2.0

Interpreting this, the Inventory Turnover Ratio of 2.0 implies that the company sells its entire inventory twice within the month. To ascertain the number of days it takes to achieve this, you can divide the number of days in the month by the turnover ratio. For instance, if the month in question is July with 31 days, the calculation would look something like:

31 / 2.0 = 15.5 days

In essence, it takes approximately 16 days for the company to sell off its inventory.

A high Inventory Turnover Ratio is indicative of efficient inventory management. This efficiency is often achieved through the optimization of inventory levels, minimization of obsolete inventory, enhancement of forecasting accuracy, and the implementation of effective sales and marketing strategies. Such efficiency can reduce carrying costs, improve cash flow, and ultimately, boost profitability.

It's important to understand that Inventory Turnover Ratios vary across different industries. For instance, industries dealing with perishable goods must maintain a high turnover to guarantee the freshness of their products.

8. Maximum Stock Level

It's important to understand the Maximum Stock Level when managing inventory. This number represents the maximum amount of items your warehouse can hold without incurring storage costs. Accurately calculating this can optimize inventory management, reduce costs, and prevent overstocking or understocking.

The formula for calculating your maximum stock level is as follows:

Maximum Stock Level = (Reorder Point + Replenishment Quantity) - (Minimum Demand × Lead Time)

Let's make this more concrete with an example. Let's say you're managing inventory for a business that has a reorder point of 5,000 units, anticipates a replenishment quantity of 8,000 units per order, and expects a minimum daily demand of 1,000 units. The lead time - the time it takes for an order to arrive after it's placed - is 4 days.

Maximum Stock Level = 13,000 - 4,000 = 9,000 units

So, in this scenario, your warehouse can hold a maximum of 9,000 units without incurring unnecessary storage costs.

This figure represents the maximum level of inventory that your facility can hold while still providing customers with a cost-effective and efficient logistics service.

9. Mastering the Reorder Point

Ever wondered how businesses seem to magically restock just when they're running out of an item? The magic wand behind this is the reorder point formula. This essential inventory management formula helps a company pinpoint the perfect moment to place an order with their suppliers. It ensures a seamless production flow and optimal usage of storage space.

So, how do you calculate the reorder point? Here's the formula:

Reorderpoint = safety stock + (average consumption x lead time)

Imagine this: Your business is a bakery, and you bake the best cupcakes in town. You've calculated that your average daily flour consumption is 1,000 units and your flour supplier takes 4 days to deliver. You also have a safety stock of 1,000 units, just in case there's a sudden cupcake rush in town. To ensure you never run out of flour, you would calculate your reorder point as follows:

So, when your stock of flour drops to 5,000 units, it's time to place a new order with your supplier. This ensures you always have enough to keep your ovens hot and your customers happy.

Do remember to revisit this calculation periodically. Why? Because it helps the logistics manager determine the ideal stock volume for goods procurement. It's a delicate balance between investing in merchandise and avoiding the risk of stockouts. And who wants to tell a horde of cupcake-craving customers that you've run out of flour?

10. ABC analysis

ABC analysis is a fundamental technique frequently employed in inventory management. This strategy helps businesses to systematically categorize items based on their overall value, thus allowing them to direct their inventory management efforts to areas where they matter most. It's an instrumental approach that assists firms in resource allocation, ensuring that operations are both effective and efficient.

This inventory management model is used to calculate the individual value of each item in the inventory, subsequently assigning them into one of three categories: A, B, and C.

The ABC analysis formula is as follows:

ABC = (Annual usage value of an item / Total annual usage value of all items) x 100

Here's what each term means:

Annual usage value of an item - This represents the product of the number of units sold or consumed over the year and the cost of each unit.

Total annual usage value of all items - This is simply the aggregate sum of the annual usage value of every item in the inventory.

Let's take a look at a real-life example:

Consider a retail company that sells only three types of inventory items: laptops, smartphones, and headphones. Each item has a unique price and annual usage value:

Laptop - Price $1000, annual usage value $500,000

Smartphone - Price $500, annual usage value $250,000

Headphones - Price $100, annual usage value $100,000

The total annual usage value for all the items will be:

500,000 + 250,000 + 100,000 = 850,000

Applying the ABC analysis formula, we can compute the relative percentage of the total annual usage value for each item:

Laptop - (500,000 / 850,000) x 100 = 58.82%

Smartphone - (250,000 / 850,000) x 100 = 29.41%

Headphones - (100,000 / 850,000) x 100 = 11.76%

Based on the calculations above, the company can categorize laptops as A items since they contribute to 59% of the total annual usage value. Smartphones, contributing 29%, can be classified as B items, and headphones, at 12%, can be listed as C items.

The ABC analysis formula is instrumental in helping companies pinpoint which inventory items are of utmost importance to their operations. This insight guides them in prioritizing their inventory management efforts. Category A items necessitate rigorous monitoring and frequent restocking, while items in categories B and C may need less stringent oversight.

The ABC analysis formula is a game-changer, helping businesses optimize inventory levels, minimize storage costs, improve cash flow, and ultimately enhance their overall profitability.

Let Brahmin Solutions automate all these formulas

If you find the idea of inventory computations overwhelming, you're not alone. Fortunately, there are tools available that can take the stress out of this task. While Excel formulas can handle basic inventory control, things can get complicated quickly.

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