The limit is in bytes
For Amazon US, the search terms field must stay under 250 bytes. Plain letters and numbers use 1 byte each, but accented letters use 2 — “café crème” is 10 characters but 12 bytes.
Going over the limit can stop the field being indexed, so stay safely under it.
What to include
- Synonyms and alternative names (flask, canteen, tumbler).
- Common spelling variants and abbreviations.
- Terms in other languages your customers use.
- Related uses that don’t fit naturally in the title or bullets.
What to leave out
- Words already in your title — they are indexed from the title anyway.
- Commas and other punctuation — separate words with spaces.
- Repeated words, brand names you don’t own, ASINs and claims like “new” or “on sale”.
A cleanup example
For the title “Steel Water Bottle”, the terms “water bottle, insulated; Insulated flask gym” (44 bytes) clean up to “insulated flask gym” (19 bytes) — punctuation, repeats and title words removed.
The Backend Keyword Byte Counter does this cleanup automatically, and the Listing Checker checks your full listing.