Plenty of shop owners are great at selling but slip up quietly at one point: buying stock. You’re standing at the wholesaler, a by-the-case price looks cheap, so it goes in the cart — without checking how much profit you’ll actually make per piece. This guide shows you how to work out cost per unit from a case price so you can decide right there at the shelf.
Why a “cheap case price” isn’t always worth it
Shelf tags usually show the price per pack or per case, not per piece — so it’s hard to compare against your selling price in your head. A real example:
Canned soft drink, case of 24 cans, 315 THB
Sounds cheap. But per can that’s 315 ÷ 24 = 13.13 THB/can. If you sell at 15 THB, your real profit is only 1.87 THB/can (~12%) — not the big number it felt like at the shelf.
The key: always convert the case price into a cost per unit first, then compare it to your selling price.
How to work out cost per unit (one formula)
Cost per unit = price per pack ÷ pieces per pack
Then check profit per unit:
Profit per unit = selling price − cost per unit Profit % = profit per unit ÷ selling price × 100
Another example — canned coffee, pack of 30, 408 THB:
- Cost/can = 408 ÷ 30 = 13.60 THB
- Sell at 17 → profit 3.40/can (20%) ✅ worth it
- Sell at 15 → profit 1.40/can (~9%) ⚠️ thin, think twice
A simple rule for “should I buy it?”
There’s no one number for every shop, but as a guide:
- 20%+ margin — generally fine for groceries/drinks
- Below 10–15% — careful; after rent, power and spoilage there may be almost nothing left
- Fast movers can tolerate a thin margin (volume helps), but slow movers need a fatter margin, or your cash gets stuck in stock (see: what is inventory value)
3 traps that quietly lose you money
- Looking only at the case price, never per unit — cheap per case, razor-thin per piece
- Overbuying past your budget — you meant to spend 5,000 but drift to 8,000 and cash runs short. Set a budget before you go and stick to it
- Stocking up on slow movers because they’re cheap — great price, but if it doesn’t sell in two months it’s stuck cash plus expiry risk
Price from cost, not from a feeling
Once you know your cost per unit, pricing gets much easier — set it from cost + the margin you want, not by guessing off the shop next door (see: how to price products for profit). And remember real profit is after fixed costs too (see: gross vs net profit).
Coming soon: let Posless do the math at the shelf
Doing arithmetic in your head all day at the wholesaler is exhausting. Posless is building a stock-buying mode that helps you decide right at the shelf, inside LINE — rolling out to VIP first in Q4 2026:
- Snap the shelf price tag → AI reads the price per pack and pieces per pack (or type it yourself)
- See cost/unit + profit/unit + whether it’s worth it before it hits the cart
- Set a buying budget — it deducts automatically so you know what’s left
- Review the basket, edit any field, then confirm once to add to your inventory
So you know your profit before you buy — not after your shelves are full of stuck stock.
FAQ
Q: Cost per unit — before or after VAT? A: Use the price you actually pay (the final amount), because that’s the money leaving your pocket.
Q: How do I handle “buy 10 get 1 free”? A: Divide what you paid by the quantity you actually received — pay for 10, get 11 → cost per unit = price ÷ 11 (cheaper than it looks).
Q: What margin should I aim for? A: It depends on the product and how fast it sells; groceries/drinks are often around 15–30%. The rule of thumb: after all costs, there must still be something left.
In short: before it goes in the cart, convert the case price into a cost per unit and compare it to your selling price every time. Then you’ll know your real profit at the shelf — not after the stock is stuck. Make per-unit thinking a habit, and your shop will buy as well as it sells.