To calculate safety stock, work out your average daily use for a product and multiply it by its average lead time – how long it takes, in days, to arrive once you place an order. Then subtract this number from your maximum daily use times your maximum lead time. The result is the safety stock number for that product.
This guide dives into the safety stock formula: how to calculate it, the benefits of safety stock, and actionable tips for optimising your safety stock calculations.
In this safety stock formula guide
What is safety stock?
Safety stock is a buffer of inventory held to protect against stock-outs. Safety stock can be used if demand exceeds a sales forecast, production output is less than planned, or supply chain disruption results in long lead times.
Because safety stock ties up capital, supply chain managers will try to minimise the quantity they hold, balancing the risks of running out against the impacts of over stocking.
It’s important to note that the optimal level of safety stock varies product by product. That’s because each product will have a different rate of consumption and lead time – which directly affects how much safety stock is needed.
By apply the safety stock formula individually to each product, supply chain managers can optimise (i.e. minimise) the amount of stock buffer they hold across their portfolio.
What is the safety stock formula?
The safety stock formula is:
[maximum daily use x maximum lead time] – [average daily use x average lead time] = safety stock.
You can use the safety stock calculator below to calculate your safety stock automatically.
How to calculate Safety Stock in Excel
You can calculate safety stock in Excel by setting up your spreadsheet as per the image below. In the cell where you want your safety stock figure calculated for each product (SKU), type the formula:
- Column B is the maximum number of units you’ve used (sold) of a product in a day
- Column C is the average number of units you use (sell) of a product in a day
- Column D is the maximum number of days it takes to receive goods of that type once you place a new order
- Column E is the average number of days it takes to receive goods of that type once you place a new order
- 2 is the number of the row for that SKU
The safety stock formula can be calculated in Excel with a simple table.
Ideally, once a safety stock figure is set for each product, an inventory management system such as Unleashed is used to manage stock within ideal levels.
Understanding the safety stock formula
The safety stock formula is intended to work in conjunction with the reorder point formula. The reorder point is the level of stock at which you ought to reorder more stock (or components, in the case of manufacturers). By including a buffer based on the maximum number of sales made over the maximum number of days of lead time, the safety stock formula provides an important cushion. Essentially the safety stock formula answers the question:
“If – after I reorder my supplies – I somehow sell the most I’ve ever sold, every day, for the longest number of days my stock has taken to get to me, then how much stock will I need to avoid completely running out?”
Safety stock levels for manufacturers
If you’re in the business of manufacturing then you need to factor time taken for production into your maximum and minimum lead times. By looking at lead times in this way they will still be a measure of how long it takes you to get stock into your warehouse ready to be shipped out to a buyer.
Be sure to also include any time taken for setting up production, as well as transport times between your factory and your warehouse if these are in different locations.
- Learn more: The Complete Inventory Management Guide
The safety stock formula is used to set ideal stock levels for manufacturers, wholesalers, retailers and distributors.
Why is safety stock important?
The purpose of safety stock is to avoid stock-outs, whereby a product can’t be supplied for sale, its delivery after purchase is unreasonably held up, or a component isn’t available during manufacturing. Stock outs negatively affect a business in several ways.
1. Stock outs and customer satisfaction
Stock outs can be deadly to customer satisfaction, with some business types affected more than others. Running out of one shoe colour variation might not affect a clothing manufacturer unduly. Whereas a B2B business that relied on a small number of high value sales could be dramatically affected by not being able to fulfill a big potential sale.
2. Stock outs and revenue
Beyond customer satisfaction, stocks also directly affect a business’ bottom line – after all you can have the best marketing campaign in the world, but if your product is not available when the consumer wishes to buy, then their money will undoubtedly go to your competitors.
3. Stock outs and efficiency
Stock outs that occur during the manufacturing process are also very detrimental to a business. Running out of a single component typically halts production, with the run either held up – raising manufacturing costs relative to output – or else being split, so that something else can be made while you wait for the missing stock to arrive. Splitting a production run like this adds an inefficient extra breakdown and start-up stage.
Ordering excess safety stock can be costly.
The downsides of safety stock
While avoiding stock outs is important, it should be pointed out that there are serious downsides to holding safety stock as well.
Holding inventory costs money – both because the stock itself must be purchased, tying up capital – and because higher volumes of inventory require more warehouse space, as well as staff and other costs such as insurance.
Holding excess inventory can also lead to significant losses through wastage, as many types of inventory can spoil or devalue over time: foods, beverages and medicines all fall into this category. While others can break, go out of fashion, or become redundant. A company making toys or consumer electronics, for example, would need to carefully balance the risk of a potential stock out against the risk of holding excess inventory that never sells.
Who needs to calculate safety stock?
The actual job description of the person setting safety stock levels varies with the size and nature of a company. In small companies it may be the owner or general manager who decides on optimal safety stock levels. While in a larger business this might be set by production managers, warehouse managers, or a logistician.
Pain points that lead to setting safety stock levels
Often the person who decides on a safety stock figure does so because one of two things happens:
- The finance manager or CFO decides that too much capital is being tied up in inventory, and asks that stock levels be reduced as much as possible, or
- Regular stock outs are damaging the business.
Setting ideal safety stock levels is a balancing act.
Should you calculate safety stock for every SKU?
Not every product will require a buffer of safety stock – indeed it can be harmful to a business to hold so much inventory that they never run out. However this is a decision that must be made company by company, and the simplistic answer is that you should set a safety stock level for any product or component that you can’t afford to run out of.
Safety stock in inventory management software
Calculating correct safety stock levels for every product becomes very time-consuming as the number of products in a business grows. It also becomes very challenging in an environment where leads times are subject to a lot of change, as the data can become out of date very quickly. For these reasons safety stock levels are best maintained and updated with inventory management software.
In Unleashed’s inventory management system, safety stock comes into play when setting minimum stock levels for products. In Unleashed this can either be done manually or automated.
Manually setting safety stock
To manually set a minimum stock level for a product in Unleashed, you need to calculate the reorder point for each item. This is a level of stock that is calculated by factoring in both the safety stock level and the average quantity used between ordering stock and it arriving in the warehouse. Once this reorder point has been decided it can be set in Unleashed in the ‘min stock’ field for each product, with reordering then managed easily by running a key inventory report – the Reorder Report – as per this step-by-step video.
Automating safety stock
Manually setting optimised minimum stock levels can quickly become impractical, especially when lead times and demand vary, and the number of products being managed grows. This is when an automated min-max system comes in to its own.
Unleashed’s tool for calculating optimised safety stock levels is called Advanced Inventory Manager, or AIM. With AIM, supply chain managers are able to update and apply the correct levels of safety stock for any business, warehouse or supplier almost instantly, saving huge amounts of time as well as freeing up capital on overstocked lines – as the video below demonstrates.
Ready to optimise your inventory? Try Unleashed inventory management software free for 14 days.
Calculating safety stock in a growing business
A final point to consider when deciding how to calculate safety stock is whether your business is growing.
Because the safety stock figure is based on historic data it will only reflect what sales or production levels have been like in the past. However a company aggressively advertising or going through a period of expansion may do well to adjust its safety stock – and by extension its reorder point figures – accordingly.
In these situations an inventory forecasting tool comes in to its own.