Checking stock by eye works when you have twenty SKUs. Once you're past fifty variants across sizes and colors, something sells out before you notice, and the reorder arrives two weeks after the stockout. Automated alerts fix that. You don't need a paid app to start: Shopify Flow is free and covers the basics well.
One prerequisite: alerts only work on variants where Shopify is tracking quantity. If the "Track quantity" box is off, Flow never sees a number to compare. Our guide to tracking inventory per variant covers that setting.
Shopify Flow (free)
Flow is Shopify's own automation app, free on every current plan. A low-stock workflow has three parts:
- Trigger: Inventory quantity changed. It fires every time a variant's stock moves, from a sale, a refund or a manual adjustment.
- Condition: the new quantity is at or below your threshold, and the old quantity was above it. That second check stops the same alert firing on every sale after you cross the line.
- Action: send an internal email, post to Slack, or add a tag such as
low-stockto the product.
Setup
- Install Shopify Flow from the Shopify App Store if it isn't already in your admin.
- Click Create workflow and choose the trigger Inventory quantity changed.
- Add a condition: product variant inventory quantity is less than or equal to 5.
- Add a second condition: the previous inventory quantity was greater than 5.
- Add the action Send internal email to whoever places orders with suppliers. Include the product title, variant title and SKU in the message so they can act without logging in.
- Turn the workflow on and test it by adjusting one variant's stock down by hand.
If your SKUs follow a clear pattern, the email is far easier to act on. Our guide to variant SKU naming helps there.
Per-variant threshold
One number for every variant rarely fits. Your black medium tee might sell ten a day while the lime XXL sells one a month. Give each variant its own reorder point.
Create a variant metafield definition called Reorder threshold (integer) under Settings → Custom data → Variants. Fill it per variant, then change the Flow condition to compare inventory quantity against that metafield instead of a fixed 5. Variants with no value can fall back to a default.
How to pick a threshold
A simple reorder point is daily sales × supplier lead time in days, plus a few days of safety stock. Some examples:
| Variant | Sales per day | Lead time | Reorder at |
|---|---|---|---|
| Black tee, M | 4 | 7 days | 28 + safety stock |
| Navy hoodie, L | 1 | 14 days | 14 + safety stock |
| Lime tee, XXL | 0.1 | 7 days | 1 or 2 |
Use the last 30 to 90 days of sales from your variant reports, and adjust before peak seasons. Our variant inventory forecasting guide goes deeper on demand planning.
Multiple locations
If you stock in more than one place, decide whether the alert should watch each location or the total. Flow's inventory trigger reports per location, so a workflow can warn you that the warehouse is low even while the retail shop still has units. The setup for location-level stock is covered in our multi-location inventory guide.
Instant alerts or a daily digest
Instant alerts suit small catalogs and fast movers. In a store with hundreds of variants they turn into noise, and people start ignoring them.
For larger catalogs, build a second workflow with a scheduled trigger (every morning at 8:00), a step that fetches all variants with inventory below the threshold, and one email that lists them. One message a day gets read. Our Shopify Flow with variants guide shows more workflows built on variant data.
Apps
Flow alerts you. It doesn't plan purchases. When you want sales forecasting, suggested order quantities and supplier purchase orders, a dedicated inventory app such as Inventory Planner is the next step. Compare current features and pricing on the Shopify App Store, because this category changes often.
Personalized and print-on-demand products
If your print partner holds the blanks, you don't track stock in Shopify for those products, so there's nothing to alert on. Watch your partner's stock notices instead.
If you print in-house, the alert should watch the blank, not the finished item. A personalized mug with 35 font choices is still one blank mug per order. With Print It My Way, the customer's text and font are saved as line-item properties rather than separate variants, so your inventory stays at the blank level and one threshold per blank is enough.
Frequently asked questions
How to set low-stock alerts for Shopify variants?
Use Shopify Flow with the Inventory quantity changed trigger, a condition on your threshold, and a Send internal email or Slack action. It takes about ten minutes to set up and costs nothing.
Best threshold for low-stock alert?
Multiply daily sales by your supplier's lead time in days, then add a few days of safety stock. Set it per variant, because fast and slow sellers need very different numbers.
Free vs paid alert setup?
Shopify Flow is free and handles alerts, tags and digests well. Paid inventory apps add forecasting, suggested order quantities and purchase orders.
Slack alert for low stock?
Yes. Flow has a Slack action that posts to a channel you choose. Connect your Slack workspace once and pick it in the workflow.
Per-variant threshold?
Yes. Store a Reorder threshold value in a variant metafield and have the Flow condition compare stock against it instead of a fixed number.
Alert frequency: every change or daily digest?
Instant alerts work for small catalogs. Larger stores do better with one scheduled morning email that lists every variant under its threshold.
Alerts for POD (unlimited inventory)?
Not needed for products your print partner stocks, since you don't track inventory for them in Shopify. If you print in-house, alert on the blank items instead.
Related reading
Sources & further reading
Primary sources referenced in this article. We link to official platform documentation and standards bodies rather than secondary summaries so you can verify claims directly.
- Shopify Help Center: Product variants
- Shopify.dev: Product variant limits
- Shopify Help Center: Line item properties (customization)
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