Why bytes, not characters
Amazon measures the search terms field in bytes. Plain English letters, numbers and spaces are 1 byte each, so for most listings bytes and characters match. Accented letters such as é, and symbols such as ° or ™, take 2 to 4 bytes. A field that looks like 245 characters can be over the limit once those are counted. The checker counts bytes the same way, using UTF-8.
What happens if you go over
Amazon's guidance says search terms should be less than 250 bytes and that if the field exceeds the limit, the attribute is ignored by Amazon Search. There is no warning on the listing. One extra word can switch off every backend keyword on that ASIN, so it pays to check after every edit.
The rules for the search terms field
- Stay at 249 bytes or less. Seller Central forum moderators confirm the generic keyword limit as 249 bytes.
- Do not repeat words from your title or bullets. They are already indexed. Repeating them wastes bytes.
- Do not repeat words inside the field. Amazon matches words in any order, so "steel bottle" and "bottle steel" are the same.
- No brand names. Neither yours nor a competitor's. Using other brands in hidden fields is treated as keyword abuse.
- No ASINs or product identifiers.
- No subjective or temporary claims such as best, cheapest, new or on sale.
- No offensive terms.
- Use spaces, not punctuation. Commas, semicolons and dashes are not needed. Amazon also suggests lowercase.
What to put in it
Use the bytes for words shoppers search that do not fit naturally in your visible copy: synonyms (flask, thermos), alternate names, abbreviations, uses (gym, hiking, office), audiences (kids, nurses) and related terms. Amazon's own seller blog recommends terms you could not include in the title, bullets or description.
Example
Before (with title "Acme Insulated Water Bottle 32 oz"): Water Bottle, insulated, best flask, Acme, thermos, thermos, B0ABCDEFGH
After: flask thermos canteen tumbler gym hiking travel school office cold drinks
Next, make sure your visible copy is clean: check your bullet points, your title length and your Item Highlights, or run the full listing analyzer.