How to Prevent Stockouts on Your Best-Selling Shopify Products

Preventing stockouts on your best-selling Shopify products comes down to three things: knowing your daily sales velocity per SKU, building a reorder point that accounts for supplier lead time, and getting a signal early enough to act. Most merchants skip the math and reorder by gut feel, which is why stockouts on top-sellers are so common even in otherwise well-run stores.

Key takeaways

• Stockouts on Shopify affect 53% of products in a given year, with the average out-of-stock period lasting 35 days.
• The true cost of a stockout is typically 2 to 5 times the value of the direct lost sale, once you factor in wasted ad spend, expedited freight, and customer churn.
• A reorder point formula (daily velocity x lead time + safety stock) is the single most reliable way to stop reactive reordering.
• ABC analysis lets you focus stockout-prevention effort on the SKUs that actually move the needle.
• A daily digest of SKUs ranked by days-of-stock-remaining replaces hours of manual spreadsheet work.

Why best-sellers go out of stock first

Counter-intuitively, your fastest-moving products carry the highest stockout risk. High velocity means stock depletes quickly. A SKU selling 20 units per day will hit zero in 10 days on 200 units of remaining inventory. The same math on a slow-mover could take months. Meanwhile, reorder cycles built around slow-movers are simply too sluggish for your A-items.

Products that experience stockouts remain unavailable for an average of 35 days, giving competitors ample time to capture displaced customers permanently. For your best-sellers, 35 days of zero revenue is catastrophic. According to retail industry research, 43% of consumers will go to a competitor when faced with an out-of-stock product. Many of those customers do not come back.

The true cost of a stockout is typically 2 to 5 times the value of the lost sale itself, because stockouts trigger a cascade of consequences that compound over time. A product with a $50 AOV selling 15 units per day, out of stock for 7 days, loses $5,250 in revenue at the surface. Add wasted ad spend, emergency air freight (which costs 4 to 10 times more per unit than ocean shipping), and the churn of customers who never return, and the real number is far higher.

The reorder point formula (do the math once, save it forever)

A reorder point (ROP) is the stock level at which you must place a new order to avoid hitting zero before the replenishment arrives. The formula is:

ROP = (Average Daily Sales x Supplier Lead Time in Days) + Safety Stock

Here is how each variable works in practice:

• Average Daily Sales: Pull a rolling 30-day or 90-day average from Shopify's sales reports. Use 90 days if you have seasonal patterns; use 30 days for trend-driven or promotional SKUs.
• Supplier Lead Time: The actual calendar days from PO send to stock received. Include transit time, not just production time.
• Safety Stock: A buffer for demand spikes or supplier delays. A common starting formula is (Max Daily Sales - Avg Daily Sales) x Max Lead Time, but even a flat 1 to 2 weeks of average sales works as a starting buffer for most merchants.

Example: A skincare serum sells 12 units per day on average, with a 14-day lead time from a supplier in South Korea. Safety stock is 48 units (4 extra days buffer).

ROP = (12 x 14) + 48 = 216 units

When inventory hits 216, a purchase order goes out. If you wait until you notice it is low (say, 50 units), you are already 12 days into a stockout by the time stock arrives.

ABC analysis: focus where stockouts hurt most

Not every SKU deserves the same vigilance. Bestseller status alone is not enough to prioritize demand; velocity, sell-through rate, and margin all matter. A slow-moving, high-margin product may warrant more stock than a fast-moving, low-margin one that generates frequent returns.

ABC analysis tiers your catalog by revenue contribution:

Set reorder points 50 to 100% higher than normal levels for your A-items to prevent stockouts during the highest-traffic periods of the year. This single adjustment, applied only to your top tier, can eliminate the majority of revenue-damaging stockouts without inflating overall inventory carrying costs.

The five actions that prevent stockouts before they happen

These are operational habits, not one-time fixes:

1. Calculate and save an ROP for every A and B SKU. Review it quarterly, or after any significant sales trend change.
2. Factor your lead time variability, not just the average. If your supplier sometimes takes 10 days and sometimes 21, size safety stock around the worst-case scenario, not the average.
3. Sync your reorder calendar to your marketing calendar. A promotion or influencer drop that doubles daily velocity will blow through safety stock sized for organic demand. Look at sales trends, seasonality, and your marketing calendar to anticipate demand. If you are running a promotion that drives a traffic spike, make sure your top products have enough inventory to handle it.
4. Check days-of-stock-remaining daily, not weekly. A weekly check on a 5-day lead time item can already be too late.
5. Centralize supplier lead times. If lead time data lives in an email thread, it will not get used at reorder time.

By the time a staff member manually creates a purchase order by emailing a supplier and updating a tracker by hand, it may already be a stockout. Manual processes are the single biggest reason merchants with good instincts still get burned.

How to get a daily signal on what to reorder first

The operational problem most merchants face is not that they lack the math. It is that pulling daily sales velocity, checking current stock levels, and cross-referencing supplier lead times across dozens or hundreds of SKUs takes more time than anyone has. Once your Shopify store exceeds 100 SKUs, you need automated systems to prevent stockouts, trigger reorders, and maintain accuracy.

This is exactly the gap that Stockcast: Inventory Forecast is built to close. It monitors your Shopify stock levels daily, applies the reorder point math with transparent, shown calculations (not a black box score), ranks every SKU by urgency, and delivers a daily email digest so you wake up knowing exactly which products to reorder first. There is no guessing, and no spreadsheet to maintain. The free plan covers up to 25 SKUs, and paid tiers include a 14-day trial with no lock-in.

Reorder method comparison: gut feel vs. formula vs. automated forecasting

The spreadsheet approach works until it does not. One missed update, one promotional spike, one supplier delay, and the whole model is stale. A McKinsey report found that companies using accurate forecasting methods reduced inventory costs by 20 to 50%. The gains are large enough to justify getting the infrastructure right, even for mid-sized merchants.

FAQ

What is a reorder point and how do I calculate it for Shopify?

A reorder point is the inventory level at which you place a new purchase order so stock arrives before you hit zero. The formula is: (Average Daily Sales x Supplier Lead Time in Days) + Safety Stock. Pull your average daily sales from Shopify's sales reports over a 30 to 90-day window, confirm your actual lead time including transit, and add a safety stock buffer equal to roughly one to two weeks of average demand.

How much safety stock should I keep for my best-selling products?

For A-tier products (top 10 to 20% of SKUs by revenue), a common rule of thumb is to carry enough safety stock to cover your longest recent lead time at average daily sales velocity. During peak seasons like Q4, many merchants increase that buffer by 50 to 100% above normal levels. The goal is to absorb both a demand spike and a supplier delay happening simultaneously without hitting zero.

What is the real cost of a stockout on a best-seller?

The direct cost is your average daily sales velocity multiplied by the days out of stock multiplied by your gross margin. But the true cost is higher: add wasted ad spend on campaigns pointing to an unavailable product, expedited restock shipping (which can cost 4 to 10 times standard freight), and the customer lifetime value of shoppers who switch to a competitor and do not return. Research suggests the total is typically 2 to 5 times the surface-level lost margin figure.