To plan purchasing and replenishment on Shopify, a fashion brand needs to look at each size separately: how much it sells, how many units are actually available to sell, what is on order with the supplier, how long it takes to arrive and how many days are left in the season.
With that, it decides whether to buy more stock or whether moving what it already has is enough, between the warehouse and the stores.
It sounds obvious, but it is an easy mistake to make, and an expensive one. A product can have plenty of units overall and be sold out in its best-selling size. If you decide based only on the product total, you buy the size you don't need and run out of the one you do.
In this guide you will see, with an example, how it happens, which data to check before each order, and what Shopify and Datcisions offer to make it less work.
Why does looking at total stock cost you sales?
Lucía runs a fashion brand with an online store on Shopify, a warehouse and one physical store. (This is an illustrative case, with made-up figures to explain the reasoning.)
Her best-selling trousers have 37 units left across sizes S and M. In the last 14 days she sold 42, about 3 a day, so the total lasts about 12 days. Her supplier takes 10 days to deliver. Lucía thinks she is fine.
| Size | Sales, 14 days | Daily rate | Stock | Coverage | Demand, 30 days |
|---|---|---|---|---|---|
| S | 14 | 1 | 25 | 25 days | 30 |
| M | 28 | 2 | 12 | 6 days | 60 |
| Total | 42 | 3 | 37 | ~12 days | 90 |
The problem is the M. It sells twice as fast as the S and has only 12 units left: it will sell out in about 6 days, and a new order would take 10 to arrive. During those days, anyone looking for an M won't find one.
Now imagine Lucía orders 60 pairs of trousers to last until the end of the season and splits them evenly: 30 S and 30 M. At the current rate, the S would need about 30 units and the M about 60.
She would end the season with about 25 S left over, probably headed for the sales, and would leave demand for about 18 M unmet, including the sales lost before the order arrived. She ordered 60 pairs, but split the sizes badly.
Looking by size, the decision changes: the S is covered almost to the end, and the order should be almost all M and go out as soon as possible. It is a first approximation that still needs a check of trend, pending deliveries and lead time.
Even so, splitting the order well doesn't prevent the stockout of the coming days: the M sells out around day 6 and the order would arrive on day 10. To avoid losing those sales, Lucía would have needed to see it sooner or get the supplier to bring the delivery forward. That is why you need to check the projection regularly.
There is a second, less visible effect: if a size barely has sales because it spent days sold out, its history doesn't show its full potential demand either. Whoever looks at the numbers weeks later sees a size that "doesn't sell much" and orders less again.
What data should you check before buying or replenishing?
Before each order, check six figures by size and by location: net sales, real sales rate, coverage, sell-through, days left in the season and pending orders. And before that, make sure your starting data is reliable.
Get the data ready
Start by checking that every combination of style, colour and size has its own SKU (the code that identifies each variant) and that it doesn't change over time. Review locations, returns and orders that are still pending.
If what the system says doesn't match what is on the shelves, any forecast will fail: it may recommend buying something you already have or allocating stock that isn't there.
The six indicators
| Indicator | What it is | Which mistake it prevents |
|---|---|---|
| Net sales | What was sold, minus returns. | Counting as sold items that come back to the warehouse. |
| Real sales rate | Net sales divided by the days the size was in stock. It gets closer to demand, although it doesn't recover all the sales lost during a stockout. | Thinking a size sells little when it was actually sold out. |
| Coverage | How many days your available units last at the current sales rate. | Ordering late: if coverage is shorter than the supplier's lead time, you are already late. |
| Sell-through | Units sold divided by the units available to sell in the period (those sold plus those left). | Comparing products or sizes without taking into account how much stock they had. |
| Days left in the season | Selling days left until the commercial cut-off date (sales, collection change). | Buying stock that arrives when it can no longer be sold at full price. |
| Pending orders | Units ordered from the supplier that haven't arrived yet, keeping those with a confirmed date separate. | Ordering the same thing twice, or counting on stock without a reliable date. |
Also note planned campaigns (Black Friday, sales, a launch): they change the sales rate, and the average of the previous weeks doesn't reflect them.
Watch out when comparing Shopify reports
Since 1 October 2026, Shopify has calculated its inventory reports using On hand quantity instead of Available quantity, as its inventory reports guide explains.
On hand includes units that are already committed or unavailable, so if the report goes up, it doesn't always mean you have more stock to sell.
Also check which sales and which days each report uses: the quantities sold don't subtract adjustments such as returns, and the days of inventory remaining are calculated from the sales of the last 28 days. They are not necessarily the net sales or the real sales rate defined above.
Buy from the supplier or move stock between stores?
First see whether you can cover today's need with the stock you already have; then work out how much to buy for the demand of the coming weeks. Buying adds stock to the business. Replenishing from the warehouse takes existing units to the store that needs them.
Back to Lucía. Of the 12 size M units, 10 are in the warehouse and 2 in the physical store, which sells one a day. Sending 6 to that store would raise its approximate coverage from 2 to 8 days.
It is a good move for the store, but it doesn't create a single unit: the brand still has 12 M and the order to the supplier is still needed. Before sending everything to one store, set aside what the other locations and the online channel need.
Set up a regular review, for example weekly, with one person in charge: first the sizes at risk of selling out, then each store's needs and finally the expected surplus. Write down why you adjust each quantity: the following week you can check whether the decision was right.
What does Shopify offer for inventory planning?
Shopify covers recording purchases and movements well, has reports by variant and suggests replenishment with its assistant. The rules of your business are for you to define.
From the Shopify admin you can record purchase orders with products, quantities and commercial terms, and track the movement and receipt of goods with inventory transfers.
Its inventory reports by variant include ABC analysis (which products contribute most to your sales; depending on your plan, it may require an app), sell-through rate and estimated days of inventory remaining.
In addition, in its guide to managing inventory after Stocky, Shopify documents replenishment and rebalancing recommendations with Sidekick, its AI assistant.
They are worth trying before adding anything else. What no tool decides for you are the rules: which period to compare, when it stops making sense to replenish an item and which store needs each size.
Those rules should be clear so that two people on the team read the data the same way. If you used Stocky, in After Stocky: how to organise purchasing and replenishment on Shopify we explain how to make the transition.
What does Datcisions add to what Shopify already does?
Datcisions is an AI merchandise planning tool for fashion brands. It connects to your online store or your ERP, such as Shopify or Holded, reads your products, sales and stock, and turns them into purchase and allocation proposals by size that your team reviews before confirming.
In Lucía's case, this is what changes:
- It calculates the purchase size by size. Open-to-Buy calculates each size's need until the end of the season and, when preparing the purchase, takes the expected delivery date into account. The sooner you look at it, the more room you have to order.
- Order what is missing, not what seems to be missing. The suggested quantity takes into account stock, orders already confirmed in Datcisions and the expected delivery date of the new purchase, so the suggested quantities are allocated to the sizes that need them, rather than split evenly.
- Know how far to trust the number. Each suggestion comes with a minimum and a maximum: the range that the size's sales data supports. If you choose a quantity outside that range, the tool flags it.
- Allocate without emptying the warehouse. Replenishment proposes how much to send to each store, size by size, based on what each one sells. You can reserve units in the warehouse, adjust the quantities and confirm the replenishment after reviewing it.
When the team confirms, Datcisions leaves the order ready to export and send to the supplier, and the replenishment shipment for each store ready to be prepared.
What in a spreadsheet would mean cross-referencing sales by size, stock, pending orders and delivery dates arrives already calculated in a proposal your team reviews.
Frequently asked questions about purchasing and replenishment on Shopify
Can you plan replenishment with Shopify alone?
Yes, but you will do the calculations yourself. Shopify records purchases and movements, has reports by variant and Sidekick suggests replenishment. Its days of inventory remaining are calculated from the sales rate of the last four weeks, not from the end of your season.
Knowing how much to order of each size to reach the end of the season, counting what is already on its way and when the new purchase will arrive, means cross-referencing all of that by hand, usually in a spreadsheet. Datcisions does that for you: Open-to-Buy proposes the purchase size by size until the end of the season, and Replenishment, how much to send to each store.
Do I always have to buy when a store is low on stock?
No. First check the warehouse and what the other locations need. A store may need an internal transfer even if the brand has enough units overall.
What should I do if a size seems to sell little because it was sold out?
Calculate its sales rate using only the days it had stock. If the M sold 10 units in 14 days but was sold out for 9 of them, it doesn't sell 0.7 a day: it sells about 2 (10 divided by 5 days).
If you look at the whole period, the M will look like a weak size and next time you will order less of exactly the one that sells most. And treat that rate as a minimum: the sales you lost while there was no stock don't show up in any report.
Does Datcisions place orders without review?
No. The team reviews, can edit and confirms each proposal. That is how campaigns, supplier terms and commercial decisions are taken into account. Afterwards, the order is exported so it can be sent to the supplier, and the replenishment is exported so the shipment can be prepared: Datcisions doesn't carry them out on its own.
Try it with your own collection: connect your Shopify store and review what you need to buy, size by size, until the end of the season. If you'd rather see it with someone from the team, book a demo.
Sources
Official Shopify documentation, consulted on 27 October 2026. It may change; check it before applying any process.