Knowing a product is running low is the easy part. Knowing how many units to actually order is the work. Order too few and you stock out before the next delivery. Order too many and cash sits on a shelf. Here is the arithmetic that gets you to one defensible number.
The reorder quantity formula
There are two steps. First, work out how much stock you should be holding to cover the gap until your next order arrives:
Then subtract what you already have and what is already on the way:
Finally, round up to your supplier’s case pack or minimum order quantity, because you can only buy in the units they ship.
What each input means
- Average units sold per day: your real sell rate, measured over a recent window (the last 60 days is a sensible default). Long enough to smooth out noise, recent enough to reflect how the product sells now.
- Lead time: how many days the supplier takes to deliver after you place the order.
- Coverage: how many days of stock you want on hand beyond the lead time, so you are not reordering the moment the delivery lands.
- Safety stock: a buffer for demand spikes or a late delivery. Zero is a valid choice for steady products.
- On hand: units currently in stock.
- Incoming: units already on an open purchase order.
A worked example
Say House Blend coffee sells 6.1 units a day. Your supplier’s lead time is 14 days, and you want 14 days of coverage on top of that. You keep no safety stock on this one because it sells steadily.
You have 24 units on hand and nothing incoming:
So you order 147 units. Every number in that result has a label, which means if you disagree with the answer, you know exactly which input to change. Think the lead time is optimistic? Bump it to 21 days and the target rises. That is the difference between a suggestion you can trust and a black box.
Why “show the math” matters
A reorder number with no working behind it is just a guess with extra confidence. When the arithmetic is visible, you can sanity-check it against what you know: a seasonal spike coming, a supplier who is reliably two weeks late, a product you are quietly phasing out. The formula gives you a starting number; your judgement adjusts the inputs.
Doing this automatically
Restock Point runs this exact calculation on your live Shopify sales, for every product, and lays out the arithmetic behind each suggested quantity. You set lead time, coverage and safety stock as defaults, then override them per supplier or per product where reality differs. When you are happy, the selected products become one draft purchase order per supplier, with case packs and minimums already applied.
It is $29 per month with a 14-day free trial.