What alt text actually does for image SEO (and how to write it)

Alt text is the single image SEO factor Google has been the most consistent about: it's a direct, textual description of what's in an image, read by search engines that can't "see" a photo the way a person can, and used by screen readers for people who can't see it at all. Everything else in image SEO is a supporting signal. Alt text is closer to a primary one.

What alt text is actually for

Alt text – short for "alternative text" – lives in an image's HTML markup (<img alt="...">) and does two jobs that seem unrelated on the surface but end up reinforcing the same thing:

1. Accessibility. A screen reader encountering an image with no alt text announces "image" and moves on. One with good alt text reads the description aloud, giving a blind or low-vision visitor the same information a sighted visitor gets by looking at the photo. This is the original purpose of alt text, and it predates image SEO as a concept.
2. Search context. Google's crawler can't reliably interpret an image's content the way a person does. Alt text gives it a direct, human-written description to match against search queries – less ambiguous than inferring content from pixels alone.

Because the same text serves both jobs well, there's no real tension between "write it for accessibility" and "write it for SEO" – a specific, honest description does both at once.

What makes alt text good versus useless

• Bad: alt="image", alt="IMG_4821", or leaving it empty on a meaningful image – gives a screen reader and a search engine nothing to work with.
• Also bad: alt="mug coffee ceramic blue handmade buy cheap mugs" – reads as a keyword list, not a description, and both accessibility tools and Google recognize the pattern.
• Good: alt="A handmade blue ceramic coffee mug on a wooden table" – a plain, specific sentence describing what's actually in the frame, the way you'd describe it to someone on the phone who can't see it.

The test that actually works: read the alt text out loud, on its own, with no image in front of you. If it sounds like a natural description, it's doing its job. If it sounds like a list of words someone hoped would rank, it isn't.

Does alt text length matter

There's no hard character limit Google enforces, but screen readers and practical usability both push toward the same range: a sentence, not a paragraph – typically under 125 characters is the informal convention most accessibility guidelines converge on. Long enough to be specific, short enough that a screen reader doesn't read a full paragraph aloud for a single product thumbnail.

Decorative images are the one exception

Not every image needs alt text. A purely decorative background pattern or a spacer image adds nothing when described aloud – for those, an empty alt="" is the correct choice, telling screen readers to skip it entirely rather than announcing something meaningless. The mistake to avoid is applying that logic to images that actually matter: product photos, portfolio shots, anything a visitor would look at on purpose.

Where this gets tedious at scale

Writing one good alt text description is a two-minute job. Writing three hundred of them, one per product photo in a batch upload, is the part that turns into an afternoon nobody has – which is exactly the gap TagDuck is built to close: it generates a specific, accurate description for every photo in a batch, based on the image itself and a short note about what you do, and that same description doubles as the image's alt text.