Where backend keywords live
Backend keywords (also called search terms or hidden keywords) are entered in Seller Central under the Keywords tab of your listing. Amazon gives you a single text field with a 250-byte limit — that's 250 characters for standard ASCII text. These terms are completely invisible to shoppers but fully indexed by Amazon's search algorithm, meaning they contribute directly to which search queries your listing appears for without cluttering your visible copy.
The byte limit is a hard constraint, not a suggestion. If you exceed 250 bytes, Amazon will not index any backend terms — the entire field gets ignored. This is one of the most costly and least-noticed listing mistakes: sellers add a long string of keywords thinking more is better, exceed the limit without realizing it, and lose all their backend indexing. Check your character count before saving, and stay under 249 bytes to be safe.
What should go in
Use backend keywords for terms that are relevant and accurate but didn't fit naturally into your visible listing copy. The best candidates are: synonyms and alternate product names ("tumbler" if your title uses "mug"), common misspellings ("stanless" for "stainless" — yes, buyers make typos and Amazon indexes them), Spanish translations for US listings that serve bilingual audiences, highly specific long-tail phrases that would read awkwardly in bullets ("keeps liquid hot in cold car"), and niche use cases that apply to your product but aren't your primary selling angle.
Format as a single space-separated string: no commas, no pipes, no quotation marks, no semicolons. Amazon treats each space-separated word as a separate indexable token, and it intelligently combines adjacent words to index multi-word phrases. You don't need to write "stainless steel" as a unit — writing "stainless steel vacuum insulated" will index "stainless," "steel," "vacuum," "insulated," and the phrases "stainless steel" and "vacuum insulated" automatically.
What should not go in
Never include competitor brand names or ASINs in backend keywords — Amazon's policies prohibit it and it can result in listing suppression or account action. Don't include words that appear in your title or bullets; they're already indexed and repeating them wastes precious byte space on terms that provide no additional indexing benefit. Avoid profanity, claims that violate Amazon's guidelines ("cures," "FDA approved" without actual approval), and subjective or unverifiable terms ("world's best," "most popular").
Also avoid stop words (a, an, the, by, for, of) — Amazon filters them out automatically, so including them just wastes bytes without adding indexing. Skip punctuation entirely. And don't include the words "Amazon," "Prime," or "free shipping" — Amazon's guidelines explicitly prohibit using platform-specific terms in keyword fields.
How to maximize the byte limit
Start by listing every keyword you want indexed that isn't already in your title or bullets. Then work backwards from 250 bytes, placing the most valuable unique terms first. A practical approach: take your full keyword list, remove anything already indexed in the visible listing, sort the remaining terms by relevance and search volume, then write them into the backend field as a space-separated string until you approach 240–248 bytes.
To fit more unique phrases, take advantage of the fact that Amazon combines adjacent words into phrases automatically. Rather than writing "insulated bottle" and "vacuum insulated" as separate entries, write "vacuum insulated bottle" — this indexes all three terms and the two-word phrase in fewer bytes than writing them separately with spaces. Think in terms of efficient token coverage rather than phrase-by-phrase listing.
Better division of labor
Think of your keyword placement as a three-tier system. The title handles your primary and most important secondary keywords — the terms worth fighting for in the algorithm because buyers also read them. The bullets handle the next tier of secondary keywords, woven into conversion-focused copy. The backend handles everything else: the niche terms, the synonyms, the long-tail phrases, and the alternate spellings that expand your indexing surface without requiring any visual real estate.
This division also makes ongoing optimization easier. When you identify new keywords from PPC search term reports or Brand Analytics, you have a clear home for each: if it's high enough priority to feature prominently, it goes in the title or bullets and you update the copy. If it's supplemental coverage, it goes in the backend without touching your conversion-tested visible copy. Maintaining that separation keeps your visible listing clean while continuously expanding your indexed keyword footprint.