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BSR stands for Best Sellers Rank and is Amazon’s way of showing how a product sells versus other products in the same category. A product with a BSR of 1,000 is the 1,000th best selling product in that category at that moment. BSR’s continually fluctuate with better selling products being re-ranked more frequently.
Since categories are so large, an easier way to reference BSR is the percentile. The lower the percentile, the better the BSR. So a BSR percentile of 2% means that product is in the top 2% of sellers in its category.
To give you a good perspective on BSR and the number of products in different categories, SellerAmp have a Sales Rank table available for use. It is free and updated daily!