Stockouts are not bad luck. They are a systems failure — specifically, a failure to calculate reorder points, set safety stock, and automate alerts before stock runs out. This guide gives you a step-by-step system to prevent them, starting with the formula every Shopify seller should know.
Most sellers think of stockouts as "lost sales." The real cost is higher and longer-lasting than a single transaction.
An inventory app at $21.99/month pays for itself by preventing a fraction of one stockout per year. The maths is not complicated.
Stockouts are almost never random. They follow a pattern — and fixing the pattern prevents them from recurring.
This is not a checklist you run once. It is a system you set up and maintain. Once it is running, most stockouts become impossible because you always have enough lead time to reorder before hitting zero.
The reorder point formula is:
Example: You sell 6 units/day of a product. Your supplier takes 14 days. You want 15 units of safety stock.
When stock drops to 99, place your reorder immediately. You will receive stock before hitting zero, with 15 units to spare if demand spikes or shipping is delayed.
Safety stock is not a fixed number — it depends on how variable your demand is and how reliable your supplier is.
Rule of thumb:
For the example above (6 units/day, 14-day lead time):
Under-setting safety stock is the most common cause of avoidable stockouts.
Running out of an A-item — your top 20% of SKUs driving 70% of revenue — is a major event. Running out of a C-item barely moves the needle. ABC analysis tells you which products need the tightest buffers and most vigilant monitoring.
Without ABC categorisation, you are applying equal attention to unequal products.
Most Shopify stores set alerts at a fixed number like "alert me when below 10 units." That is better than nothing but misses the point.
The alert should fire at your reorder point — the number that gives you enough time to reorder and receive before hitting zero. For a product with a 14-day lead time, an alert at 10 units is too late. An alert at 99 units (your reorder point) gives you the full lead time to act.
Low-stock alerts are a safety net. Reorder points are the system. Both working together means you almost never stockout.
Reorder points work well for stable demand. They are slower to respond to demand spikes — a product going viral, a seasonal surge, or a trend shift that doubles sales velocity over two weeks.
AI demand forecasting catches these patterns earlier than a static reorder point system. It analyses your sales trend, seasonal patterns from previous years, and current velocity to flag products that are likely to need earlier-than-usual reorders.
This does not replace reorder points. It works on top of them — as an early warning layer for the situations where your usual system would catch it too late.
Sales velocity changes. Seasons shift. A product that was a C-item becomes an A-item after a TikTok mention. Reorder points set 6 months ago may be completely wrong today.
Build a quarterly habit: review your top 20 SKUs, check whether their sales velocity has changed significantly, and update reorder points accordingly. Any good inventory app does this automatically — manual review is only necessary if you are managing everything in Shopify Admin or spreadsheets.
If you are reading this because you just had a stockout, here is what to do immediately — then come back and implement the prevention system above.
Turn on pre-orders in Shopify for the out-of-stock product. Clearly state the expected restock date. Some customers will wait — and you keep the sale rather than losing it to a competitor.
Stop paying for traffic to a sold-out product page. Pause any campaigns targeting that specific product until it is restocked. Redirect that budget to in-stock products.
If you have back-in-stock notifications set up on Shopify, use them. Email your list when the product is restocked — customers who wanted it and could not get it are your warmest leads.
For A-items, always have a backup supplier identified in advance. A higher cost rush order from a secondary supplier is almost always cheaper than 2 weeks of lost sales.
After restocking, check: what was the reorder point set at? When did stock actually hit zero? Calculate what it should have been and update it. Every stockout is data.
Every step in this guide can be done manually — and you should understand how each piece works. But doing it manually across 50-500 SKUs is unsustainable. Reorder points become stale, alerts get missed, and the system only works if you keep maintaining it.
Debnix automates reorder point calculation from live Shopify sales data, runs ABC analysis automatically, flags products approaching their reorder threshold, and generates draft supplier reorder emails so the action step is one click. All at flat $21.99/month — no per-SKU fees.
The most common cause is reordering by gut feeling rather than by a calculated reorder point. Without a formula-based trigger, stores either reorder too late (stockout) or too early (overstock). The fix is calculating a reorder point from actual daily sales velocity multiplied by supplier lead time, plus a safety stock buffer.
In Shopify Admin, go to Products, select a product, and in the Inventory section you can enable low stock notifications. However, Shopify low stock alerts fire at a fixed quantity you set manually — they do not automatically calculate your reorder point. For alerts that trigger at the right time based on your actual lead time and sales velocity, you need a third-party inventory app.
Safety stock is extra inventory you keep above your reorder point to absorb demand spikes and supplier delays. A simple starting rule: set safety stock at 25-30% of your lead time demand for stable products. For seasonal or trending products, go 40-50%. For your A-items (bestsellers), always err on the higher side — the cost of carrying extra stock is almost always less than the cost of a stockout.
Every 60-90 days, or any time you notice a product's sales velocity has changed significantly. Seasonal products need a review before each season starts. If you are using an inventory app that calculates reorder points from live sales data, this happens automatically. If you are doing it manually, build a quarterly calendar reminder.
Shopify does not automatically place reorders with suppliers. It can send you low-stock alerts, but the actual reorder decision and supplier communication is manual. Inventory apps like Debnix automate the alert and can generate draft supplier emails when a product hits its reorder point — but the final approval and send is always yours.
Debnix calculates reorder points from your live Shopify data, runs ABC analysis automatically, and alerts you before stock runs out — not after. 30-day free trial.
$21.99/mo. Built solo from Nepal.