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A counter for each size, color, or time slot, not one for the whole product
You'll decide whether the stock for each thing you sell is tracked as one whole product or by each size, color, or time slot, setting each one up as its own listing with its own quantity, and you'll stop selling what you don't have left.
By Yodai · Published on September 6, 2026 · Updated on October 12, 2026
Before you worry about who grabs the last unit, make sure the counter you're checking is for that exact size or color, not the product lumped together. Say you've got a t-shirt with fifteen units in the stockroom: six in size S, none in M, and nine in L. Someone comes in, picks M, pays, and the site confirms it without blinking. The problem is there wasn't a single M left. The listing only looked at the total: fifteen units, go ahead with the sale.
Now you have to email the customer, issue a refund, or talk them into taking a different size. With luck, they won't be upset. Without it, they never buy from your site again, and they tell people about it too, and that comment gets read by the next person who was deciding between buying from you or from someone else.
One number for the product, not for what's actually running out
This isn't two people racing for the last unit, nobody's competing here, it's the counter that's lying. It says "fifteen" when the question that mattered was "how many M's are left?"
A product and a variant aren't the same thing when it comes to stock, even though the listing shows them together. The product is the t-shirt. The variant is the size, or the color, or for a service business, the shift or the time slot you work. When each variant has its own physical units, counting only the product gives you a number that adds up fine but doesn't answer what you're actually being asked. And if your store tracks stock by product, with no sizes or colors carrying their own quantity, you get a per-variant counter by setting up each size or color as a separate listing, with its own quantity.
This doesn't only happen with clothing. A hair salon that advertises "8 haircuts available today" without saying what time has the same problem: it can double-book the same hour and run out of openings by 4:00 p.m. even though the overall counter still shows availability.
When to count by variant and when the whole product is enough
The question that settles this is short: when someone buys one unit of a specific variant, does that leave the other variants out of stock, or not? If the answer is yes, you need a counter for each variant. If the answer is no, one counter for the whole product is enough, and building out the breakdown is extra work nobody's going to thank you for.
A t-shirt size only sells out that size, never the other two. An 11:00 a.m. salon appointment uses up that hour, not the other five slots left in the day. A phone case color only sells out that color, if they're packaged separately. In all three cases, the counter needs to be set per variant.
But say you sell candles in three scents, and the scent is an essential oil you add at the end of the process, from a bottle that lasts you two hundred units. There, the scent doesn't use up anything the other variants need: what runs out is the wax, which all three share. One counter for the whole product is enough, and setting up three separate scent listings would be inventing work where none is needed.
There's a middle case worth settling early: size and color at the same time, like a t-shirt in three sizes and two colors. There, you don't count by size or by color separately, you count by each combination: blue S is one listing, white S is another. Treat them as if they share stock, and you're back to the same problem as before.
- Set up in a minute: one number, done
- Works when the variant isn't tied to a physical unit that's different from the others
- Using it when there really are separate units per variant means you'll sell what you don't have without even realizing it
- Set up each size, color, or combination as a separate listing, with its own quantity
- Each sale subtracts only from that variant, never from the others
- When you restock, you update the variant that came in, not the product's total
- It takes more work to set up the first time, but it's the only one that doesn't lie to you
Which one to pick: If two units of the same product can run out at different times because they're physically separate pieces, count by variant. A single counter only works when no variant uses up something the others could also use.
Setting up a separate listing for each size or color takes more time the first time around. You make that half hour back in the first week you don't cancel an order or write an apology.
Exactly what to say to someone when the variant runs out while they're paying is the next lesson. Today what's settled is simpler: for everything you sell, whether the counter belongs to the size, the shift, the color, or the whole product.
- You know when to count stock by the whole product and when to count it by each variant
- You've applied this rule to what you sell: size, color, time slot, flavor, or a combination
- You know what to check before launching a campaign: the quantity of each size or color, not the total
Test yourself
Answer before opening each one. If one gives you trouble, go back to the section that explains it.
1. You sell handmade candles in three scents, all molded the same way and set apart only by an essential oil you add at the end from a bottle that lasts you two hundred units. Do you count stock by scent or by the whole product?
By the whole product. The scent doesn't use up a physical piece of its own; it's a final step that relies on a plentiful, shared ingredient. One single counter is enough; setting up three separate scent listings would be work with no payoff.
2. You rent out eight bikes by the hour. One person books the 10:00 a.m. slot and another books the 12:00 p.m. slot. Is a single 'bikes available today' counter enough, or do you need one per time slot?
One per time slot. Each hour is its own independent slot: if you only count the day's total, you can end up booking the same bike twice for the same hour even though the overall number seems to add up.
3. You sell a phone case in three colors, all on a single listing: twenty units total, twelve black, five white, and three red. Fifteen have sold and the listing shows 'five units' total. Can you keep selling any color based on that number?
No. The total of five doesn't tell you which color is left. If the three red ones sold first, right now there could be zero red and five split between black and white. You need to count what's left of each color before accepting an order for a specific color, and from there, one listing per color.
Even counting by variant won't save you every time: sooner or later you'll run out after someone's already paid, and what saves the sale then is what you write to them in that moment.
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