How Much Should I Reorder? A Simple Formula Using Sales Speed, Lead Time and Cover Days
Three numbers decide a reorder: how fast it sells, how long the supplier takes, and how long the order should last. Here’s the formula, with a worked example.
Most reorder mistakes come from guessing: ordering “about the same as last time”, or waiting until a product is nearly gone and then ordering a big round number. A reorder amount only needs three numbers, and you already have two of them in Shopify.
The formula
Reorder = sales per day × (lead time + cover days) − in stock − on the way
- Sales per day is how fast the product sells right now.
- Lead time is how many days your supplier takes from order to delivery.
- Cover days is how long you want the new stock to last once it arrives.
- In stock is what you have now. On the way is anything already shipped to you.
The idea is simple: from today until the new stock arrives (the lead time), and then for as long as you want it to last (the cover days), you’ll keep selling at about the same speed. Order enough for that whole stretch, minus what you already have.
A worked example
A speckled ceramic pour-over sold 28 units in the last 28 days and 11 in the last 7. There are 12 in stock and nothing on the way. The supplier takes 14 days, and you’d like each order to last about a month (30 days).
Sales per day. The 28-day average is 1.00 a day. The 7-day average is 1.57 a day. It’s selling faster lately, so use a blend: half of each gives 1.29 a day.
Days left. 12 ÷ 1.29 ≈ 9 days. The supplier takes 14, so this product will sell out before a new order could arrive. Order today.
Reorder. 1.29 × (14 + 30) = 56.8. Take away the 12 in stock and nothing on the way: 44.8, so order 45.
If 20 were already on the way, the order would drop to 25, and you would still want to watch it: until those 20 arrive, the shelf still has only 12.
Getting each number right
Sales per day: use recent sales, but not only recent sales
A 28-day average is steady but slow to notice change. A 7-day average notices change fast but swings on one big order or a quiet week. Blending the two (half and half) catches a product that’s picking up speed without overreacting to a single odd day.
Two things will throw it off:
- Days it was out of stock. If a product sat at zero for a week, it sold nothing because it couldn’t, and its average looks lower than real demand. Be generous with products that recently ran out.
- Seasonal swings. If you’re heading into your busy season, last month’s speed understates next month’s. Raise the cover days for those products, or order by hand.
Lead time: count the whole trip
Lead time isn’t just the supplier’s production time. Count from the day you place the order to the day the stock is on your shelf and sellable: production, shipping, customs if it’s imported, and the day or two it takes you to receive it. If a supplier says “10 to 14 days”, use 14.
Shopify has no built-in supplier lead-time field. Its guide for stores moving off Shopify’s Stocky app suggests storing lead times in custom metafields. A note in a spreadsheet works too.
Cover days: a cash decision
Cover days are how long each order should last. Longer cover means fewer orders and fewer stockouts, but more cash sitting on shelves. Shorter cover frees cash but means ordering more often.
Some ways to choose:
- 30 days is a reasonable starting point for most products.
- Longer for suppliers with high minimums or shipping costs that only make sense in bulk.
- Shorter for anything perishable, trend-driven, or expensive to hold.
When the formula needs your judgment
- Minimum order quantities. If the formula says 45 and the supplier’s minimum is 60, the formula can’t decide whether the extra 15 are worth it. You can.
- New products. With only a week or two of sales, the speed is a guess. Order small and look again soon.
- Products that stopped selling. If sales per day is zero, the formula says reorder nothing, which is right. The better question is why it stopped and whether to discount what’s left.
- Safety stock. Some stores add a fixed buffer on top for suppliers that are often late. Raising the lead time a few days does the same job with one less number to track.
When to reorder, not just how much
The same numbers tell you when to act. If days left is less than the lead time, the product will sell out before a new order arrives, so order now. If days left is within a week or so of the lead time, put it on this week’s list.
Doing this for every product, every day
The math takes a minute for one product. It doesn’t scale to a few hundred variants, and the numbers change every day as you sell. You can work it out in a spreadsheet from Shopify’s sales reports, or let a tool run it for you.
StockDigest does exactly this calculation every morning and emails you the products that need ordering, with the amounts. It starts with a 14-day lead time and 30 days of cover, which you can change, and it subtracts stock that’s already on the way. To try the formula on one product first, use the reorder calculator.