Gather your supplier and stock details
Steps 1 to 5 work with a spreadsheet and Shopify’s own purchase orders. Step 6 covers what an app adds, and when it is worth it. Before building anything, collect:
- Suppliers: name, email, and how many days they usually take to deliver (lead time).
- Reorder levels: any minimum stock levels you already keep, in a POS system or a spreadsheet.
- Open orders: anything you have ordered but not yet received. This matters more than history, because stock that is already on its way changes what you should order next.
- Past purchase orders: useful for checking how long each supplier really takes. You do not need them to start.
Supplier details and settings such as reorder points are often spread across emails, invoices and spreadsheets, so collect them in one sheet first. A two-column sheet with SKU and Reorder point columns is enough to begin.
Work out daily sales for each product you buy in
For each product, divide the units it sold by the days it was available to sell:
Daily sales = units sold ÷ days the product was on sale and in stock
- Established products (on sale for the whole period, such as the last 30 days): divide by every day in the period, including days with no sales. For a product that sells now and then, the quiet days are part of its normal demand. Example: 90 units in 30 days = 3 a day.
- New products launched during the period: divide by the days since launch, not the whole period. Example: 30 units in the 10 days since launch = 3 a day.
- Stockouts you have recorded: leave out the days a product was out of stock, because it could not sell then. Example: 60 units in 30 days with 10 days out of stock = 60 ÷ 20 = 3 a day. Only do this for stockouts you can date.
- Not enough history: if a product has been on sale for fewer than 14 days, do not trust the number. Set a manual reorder point instead (see step 4).
This is a simple method for a spreadsheet. Reorder apps, Replenly included, calculate demand in their own ways, so their numbers can differ from yours.
Set a reorder point from lead time and safety stock
Reorder point = daily sales × (supplier lead time + safety days), rounded up
Worked example: you sell 3 units a day. The supplier takes 10 days. You keep 5 days of safety stock. Reorder point = 3 × (10 + 5) = 45 units.
Reorder when on hand plus already on order is at or below 45. If you count only what is on the shelf, you will order twice for the same gap. Count only orders you have actually sent to the supplier, not drafts.
Work out how much to order
Order quantity = daily sales × (lead time + safety days), minus on hand, minus on order, rounded up
Then raise it to the supplier’s minimum order quantity if they have one, and round up to a full case.
Continuing the example: on hand 20, on order 0, so 45 minus 20 = 25. With a case size of 12, round up to 36.
For products with no sales history yet, set a manual reorder point instead, for example “reorder when I am down to 10”, and order an amount you trust.
Order and receive in Shopify
Shopify admin has its own purchase order and inventory workflows, so you can record what you ordered from each supplier and update inventory when it arrives. Two habits keep the numbers honest:
- Receive only what physically arrived. Leave the rest of the order open. If you adjust stock by hand instead, “on order” stops being true, and step 3 breaks.
- Count against the packing slip before you change anything in Shopify.
Once a quarter, do a physical count by location and fix the differences. Reorder math is only as good as the on-hand number.
When a spreadsheet stops being enough
The spreadsheet works. It also takes time every week: export sales, update the formulas, copy numbers into purchase orders, and remember what is on order.
A reorder app does the arithmetic for you. Look for:
- suggestions that use on hand plus inbound, not on hand alone
- case sizes and minimums applied to suggested quantities
- partial receiving that writes back to Shopify inventory
- a way to bring in the data you already have
- estimates you can override, not a black box
Replenly is one option. Disclosure: I built it. It does steps 2 to 5 inside Shopify admin:
- reorder suggestions listed by supplier, from your recent sales, lead times, safety stock, and available and inbound stock, with the math behind each one, days of cover and a projected stockout date
- supplier minimum order quantities and case sizes applied to suggested quantities
- one draft purchase order per supplier from the rows you select (one per supplier and receiving location if you receive at more than one)
- full or partial receiving that writes received quantities back to Shopify inventory
- Auto or manual reorder points per product, including manual points for products without enough sales history, and a slow movers view
- CSV import for suppliers and product settings from templates, and for Stocky purchase order exports, with a row-level error report
- location stocktakes with a USB or Bluetooth barcode scanner
Suggestions are estimates, and you can change any quantity before you order.
The free plan covers reorder suggestions for up to 100 SKUs and 5 submitted purchase orders a month (drafts do not count). PDF and email purchase orders are on paid plans. If you get stuck importing, email me and I will help.
Replenly's suggestions start from the same reorder point and order quantity formulas, but work out daily sales their own way (see step 2), and the app shows the math behind each suggestion. Neither this guide nor the app accounts for seasonality or promotions, so adjust safety days for those.