Inventory how-to

When should you reorder a product

Reorder when your stock on hand will run out before new stock can arrive and be ready to sell.

Short answer: Reorder when your stock on hand will run out before new stock can arrive and be ready to sell. In practice that means ordering by a specific date, called the order-by date, which you calculate from your daily sales rate, your supplier's lead time, and a safety buffer.

The core idea: reorder point

Your reorder point is the stock level at which you need to place an order right now so you do not run out. It is not a date, it is a quantity: reorder point = daily sales x (lead time + safety days). Lead time is how long the supplier takes to deliver. Safety days cover delays and demand spikes.

Turning that into a date

A quantity is useful, but a date is easier to act on. Take your current stock on hand, divide by daily sales to get days of stock remaining, then compare that to lead time plus safety days. If days of stock remaining equals lead time plus safety days, today is your order-by date. If days of stock remaining is higher, you have time. If it is lower, you are already late.

Why daily sales rate matters more than gut feel

The daily sales rate should come from recent sales, not a guess or last year's number, unless the product is strongly seasonal. A product that sold steadily for months can suddenly speed up or slow down. Using a short recent window, like the last 28 days, keeps the reorder point close to what is actually happening now.

Per-SKU, not per-product

Reorder timing has to be calculated per variant, not per product. A shirt in size M and size XL can have very different sales speeds and different days of stock left, even though they are "the same product" in the catalog.

Step by step

  1. List daily sales for each SKU using recent sales history, not a hunch.
  2. Get the lead time and safety days for each supplier.
  3. Calculate days of stock remaining: on hand divided by daily sales.
  4. Calculate the trigger point: lead time plus safety days, in days.
  5. Compare the two numbers: if days of stock remaining is less than or equal to lead time plus safety days, order now.
  6. Factor in any stock already on order so you do not double up.
  7. Repeat this per variant, not just per product, since sales speed varies by size and color.

Worked example

Say a mug sells 4 units a day on average, the supplier's lead time is 10 days, and you keep 4 safety days. The trigger point is 4 x (10 + 4) = 56 units. If you currently have 70 units on hand, that is 70 / 4 = 17.5 days of stock, more than the 14-day trigger, so you can wait. A week later you have 40 units on hand, which is 10 days of stock, below the 14-day trigger, so you should have already ordered.

How SteadyShelf helps

SteadyShelf calculates this per SKU automatically, using your recent sales window and each supplier's lead time and safety days, and tells you the order-by date directly so you do not have to run the math by hand. It does this per SKU store-wide, not per location, so if you stock multiple warehouses or locations you still need to track that split yourself.

$29/month flat, unlimited products and suppliers, 30-day free trial. Run the numbers now with the free reorder point calculator.

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FAQ

What if a product has no sales history yet?

With little or no sales history, you cannot calculate a reliable daily sales rate, so use a cautious manual estimate based on similar products or planned marketing, and recalculate as soon as real sales data comes in, ideally after the first few weeks.

Should safety days be the same for every supplier?

No. Safety days should reflect how reliable and how far away each supplier is. A supplier with inconsistent delivery times or a long shipping distance needs more safety days than a local supplier who ships next day.

Let SteadyShelf do this math for every SKU

Order-by dates per supplier, seasonal forecasts and draft POs for your Shopify store. $29/month after a 30-day free trial.

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