Inventory Planning for Size Runs on Shopify

To plan inventory for size runs on Shopify, set a size ratio from your own sales, split every purchase order across sizes by that ratio, and track each size as its own variant. A size run is the set of sizes you stock for one style, such as XS to XL.

Good size advice at the point of sale helps the ratio hold, which is why many stores pair planning with virtual try-on on Shopify. This guide covers the planning itself.

What is a size run, and what is a size ratio?

A size run is the range of sizes for one style. A size ratio says how many units of each size go into a purchase order.

For example, a ratio of 1:3:3:2:1 across XS to XL means that for every nine units, you buy one XS, three S, three M, two L and one XL. The ratio changes with your customers and the garment.

How do you set a size ratio for a new style?

Use your own sales first. If you have no history, use the nearest similar style.

1. Export orders from your Shopify admin and group sold units by size.
2. Pick a similar product group, such as women's tops.
3. Convert units into shares of the total.
4. Adjust for sizes that were sold out, since their sales were cut short.
5. Write the result as a ratio you can reuse.

Our guide to forecasting demand by size explains the stockout adjustment in more detail.

How do you split a purchase order by size?

Multiply the total units by each size's share, then round to your supplier's minimums.

Here is a hypothetical example using round numbers. You plan to order 90 units of a top with a 1:3:3:2:1 ratio.

Size	Ratio part	Units
XS	1	9
S	3	27
M	3	27
L	2	18
XL	1	9
Total	10	90

Each ratio part equals nine units because 90 divided by ten is nine. Check that your rounded numbers still add up to the order total.

How should you set up variants in Shopify?

Make each size a variant of the same product, with its own SKU and stock count. This keeps counts accurate and lets shoppers see which sizes are available.

Use a clear, consistent SKU pattern, such as style code plus size. Consistent codes make counting, exports and reordering faster.

If a size sells out, decide how you will handle it. The guide to handling out-of-stock sizes on Shopify lists the options.

How do you keep a small budget under control?

Small brands often cannot buy a full run in every style. A few rules help:

• Buy the sizes that sell deeper. Look at your own data to see which sizes sell most.
• Buy the ends shallower. Extreme sizes sell less, but stockouts there lose loyal customers.
• Test before you commit. Order a smaller run first, then reorder the sizes that sell.
• Use pre-orders for the ends if your supplier allows small follow-up orders.

For more cost-saving ideas, see running a Shopify clothing store on a small budget.

How do returns and sizing affect your plan?

If shoppers pick the wrong size, your sales by size are distorted. A size that sells well but comes back often is not true demand.

Track returns by size and look at what shoppers exchanged into. AI size recommendation for fashion stores makes your sales data more reliable, because shoppers keep the sizes that fit.

ZeDaq.ai recommends a size from a 30-second body scan, matched to your own size chart. You enter your chart in the Shopify app, one per garment type. Install it from the ZeDaq.ai Shopify app listing.

What about plus sizes?

Plus ranges need their own ratio. The customer base and the fit are different, and the supplier may grade the range differently.

Read how to sell plus-size clothing on Shopify before you set the ratio.

How do you plan reorders by size?

Reorder each size on its own signal, not the style's total. A style with plenty of stock overall can still be sold out in the size most shoppers want.

Set a reorder point for each variant: the stock level at which you place a new order. Base it on how fast the size sells and how long your supplier takes to deliver. Check these two numbers against your own data, and revisit them whenever a lead time changes.

What should you review each month?

Keep a short review routine:

• Units sold by size for each product group.
• Sizes out of stock and for how many days.
• Returns and exchanges by size.
• Reorder points for the sizes that sell fastest.

Update the ratio at every reorder.

FAQs

What is a good size ratio?

There is no universal ratio. Build it from your own sales for similar products.

Should every size be a separate variant?

Yes. Separate variants give each size its own stock count and SKU.

How often should I update my size ratio?

Update it at each reorder, and after changes such as a new size chart or supplier.

What do I do with leftover sizes at the ends of a run?

Bundle them into a sale or move them to a clearance collection. Learn from them by shrinking those sizes in your next buy.