Safety Stock for Shopify: How Much Buffer to Keep, With 3 Formulas and One Worked Example

By Vladislav Gerasimchuk, founder of Extensions Market, which builds EZstock

Safety stock is the guess inside a reorder point. Lead time demand is plain arithmetic: what you sell per day times how long the supplier takes. Get the buffer on top wrong and a busy week empties the shelf, or cash sits in stock for months. Shopify shows your counts and how many days they'll last, but the buffer is a number you choose. This guide compares three ways to size it on one product with made-up numbers, then covers which to use and what throws each one off.

What is safety stock?

Safety stock is the stock you keep on top of what you expect to sell while an order is on its way. Wikipedia's article describes it as "a buffer to protect organizations from stockouts caused by inaccurate planning or poor schedule adherence by suppliers." For a Shopify store, that means two things: a week when sales run faster than usual, and a delivery that turns up later than promised.

Too little safety stock and a good week empties the shelf before the delivery lands. Too much and cash sits in boxes. Every unit of buffer is a unit you paid for and haven't sold, so the right amount depends on how painful running out is for that product and how much holding it costs.

Where does safety stock fit in the reorder point?

Average daily sales times lead time is called lead time demand: the units you expect to sell while you wait. Wikipedia's article on the reorder point sums up the formula as "Normal consumption during lead-time + Safety Stock." When available stock plus anything already on order falls to the reorder point, you place the next order.

Lead time here means the days from placing the order to having the units ready to sell: the supplier's processing time, the shipping, and the day or two you need to receive and shelve the delivery. Our guide to low stock alerts and reorder points walks through setting the reorder point itself. This guide is about the buffer inside it.

Three ways to size safety stock

A few days of sales

Pick a number of days you're comfortable with, often 3 to 7, and multiply by average daily sales. It's easy to explain to staff and easy to change. It treats a steady seller and a jumpy one the same way, which is its weak spot.

Worst case minus normal case

This sizes the buffer for the busiest day and the slowest delivery happening on the same order. It's simple and safe. It usually holds more stock than you need, because both rarely happen together.

A service-level formula

Z sets how often you accept running out before a delivery arrives. Wikipedia's safety stock article describes this service-level approach and notes that "a 95% service level could result in stockouts, but is at a level that is acceptable to the company." It needs a spreadsheet and some sales history, and it gives each product a buffer that fits how much its sales swing. If your lead times also vary a lot, the full formula adds a term for that.

One product, three answers

One candle variant, with made-up numbers. It sold 112 units in the last 28 days, so it sells 4 a day on average. The supplier usually takes 14 days from order to shelf, and 18 at worst. The busiest day in the window sold 7, and daily sales swing by about 2 units either way (a standard deviation of 2).

Lead time demand is 4 × 14 = 56 units. The table shows the safety stock and the reorder point with each method:

The four-day buffer and the 95% service level land close together. The worst-case method nearly doubles the reorder point, because it plans for the busiest day and the slowest delivery on the same order. At a $9 unit cost, the gap between 13 and 70 units of buffer is about $510 of stock sitting on the shelf for this one variant.

Which method should you use?

• Slow movers and cheap items: a few days of sales. The buffer is small either way, so the simple rule costs little.
• Steady sellers with some history: the service-level formula at 90 to 95%. It keeps the buffer lean where sales are predictable.
• Hero products, long lead times, or anything a customer would leave over: the service-level formula at 98 to 99%, or worst case minus normal if you'd rather overstock than run out.

You don't need one method for the whole catalog. Our ABC analysis guide splits products by revenue, so the top sellers get the careful buffer and the long tail gets the simple one.

Where do the numbers come from in Shopify?

• Sales per day. Shopify's help page says the Days of inventory remaining report calculates "the average quantity of items sold per day" for each variant from "its sales over the last 28 days." For a longer window, export sales by variant from Shopify Analytics and divide by the number of days.
• How much daily sales swing. Export daily units sold for the variant and use STDEV in a spreadsheet. That's the standard deviation in the service-level formula.
• Lead time. Look at your last few purchase orders with the supplier. Note the day you sent each order and the day its stock was ready to sell. The average gap is your lead time, and the longest is your worst case.
• Stock already on order. Shopify's purchase orders can be saved as drafts or marked Ordered once the supplier confirms, so the Ordered ones are your stock on the way. Count them before you reorder.

Since June 2026 you can also ask Sidekick, the assistant in the Shopify admin, what to reorder. Shopify's announcement says it "looks at what's been selling, what you have left, and forecasts demand to recommend what to buy and how much." A safety stock you've chosen on purpose is a good check on any suggestion, from Sidekick or an app.

What throws safety stock off?

• Days when the product was sold out. A variant that was out of stock for a week sold nothing that week. That lowers both the average and the swing, and the buffer comes out too small. Leave those days out.
• Promotions and seasons. A sale or a holiday rush can double daily sales for weeks. Size the buffer from last year's busy period, because a quiet month's numbers leave it too small. Our holiday inventory planning guide covers the timing.
• New products. No history means no average and no swing. Borrow a similar product's numbers and replace them after a few weeks of sales.
• A supplier's lead time changing. The square root of lead time sits in the formula, so a switch from 14 to 35 days raises the service-level buffer by more than half. Update it the day it changes.
• Several locations. If each location orders its own stock, each needs its own buffer from its own sales.
• Bundles. If a bundle uses the variant, count the units sold inside bundles too.

How does EZstock handle it?

EZstock is our Shopify app for reorder planning. You set a reorder point for each variant, so your safety stock goes into that number, and every plan, including Free, sorts everything at or under its reorder point by urgency on the low-stock dashboard. A daily low-stock email sends the list as a CSV. Each supplier has its own lead time, and the On Order column counts stock you've already ordered, so the same units aren't ordered twice.

On Growth, EZstock works out how many units of each variant sell per day from up to 365 days of your Shopify orders and shows the days of stock left. Anything that will run out before its supplier's lead time is flagged first, and it suggests an order for each supplier with the math shown. Every install gets 14 days of every feature with no credit card. The plans are Free, Starter at $19 a month and Growth at $49 a month.