Blog / Margins and pricing

How to avoid selling at a loss on Amazon: the margin calculation you're missing

By the SellerKing team · Updated July 2026

Many Amazon sellers find out they're losing money on a product months after they started selling it. Not because the business is doing badly, but because the initial margin calculation fell short: it was done with a calculator that didn't include everything that needed to be deducted.

The most common mistake: calculating margin using only cost price

The math most sellers do when starting out is simple: sale price minus supplier cost price. The problem is that between those two numbers there are at least three more costs Amazon automatically deducts before you see a single euro:

• Amazon's commission, which varies by product category (typically between 8% and 15% of the sale price).
• Weight-based shipping cost, especially relevant if you use FBA (Fulfillment by Amazon), where cost depends on package size and weight, not just destination.
• VAT, which must be deducted from the sale price before calculating real margin, not added separately.

A product that "seems" to leave a 20% margin over supplier cost can end up at a real 3-4%, or outright negative, once these three items are deducted.

Why this goes unnoticed for months

The reason this mistake takes time to detect is that sales keep coming in normally — the problem isn't demand, it's invisible profitability. Without a clear breakdown per sale, it's easy to confuse "I'm selling a lot" with "I'm making money", when in reality it can be exactly the opposite for some products in the catalog.

This gets worse as the catalog grows: calculating real margin by hand for 50 products is tedious but doable; for 2,000 listings it's practically impossible without automating it.

How to calculate the real net margin of each product

The correct formula always starts from the final sale price and deducts, in this order: Amazon's commission based on category, shipping cost based on weight and dimensions, applicable VAT, and the real supplier cost (including possible volume discounts). What's left after those four deductions is the real net margin — the only number that should matter when deciding whether a product is worth it.

At SellerKing, this margin calculator runs automatically before publishing each product: if the net margin doesn't reach the minimum you've defined, the product simply isn't published. No need to check it by hand or discover it three months later.

What to do if you already have products with negative margin

If you suspect part of your current catalog is selling at a loss, the first step is to audit the real net margin of every active listing, not just new ones. From there, you have three options: renegotiate cost with the supplier, raise the sale price if the market allows it, or simply discontinue the product if neither of those two is viable.