Compress image to 150KB

One hundred and fifty kilobytes is an awkward number to hit by hand and a comfortable one for an encoder. It is roughly the point where a 1600-pixel-wide photograph settles at a quality in the high seventies — artefacts are present in the file, but you have to zoom past the displayed size to find them.

The practical consequence is that you rarely have to reduce the dimensions. At 100 KB a large photograph starts to need scaling down; at 150 KB the same image usually fits at full resolution. If a form has handed you this limit, you can almost certainly satisfy it without touching the pixel size.

It turns up most often in two places. Application and registration forms use it as a middle ground — generous enough for a readable scanned document, tight enough to keep a database small. Email templates use it for header and banner images, because a heavy banner is the single most common reason a marketing email is slow to render.

It is also a sensible self-imposed budget for inline images in an article. A page carrying six photographs at 150 KB each is under a megabyte of imagery, which is the range where Largest Contentful Paint stays healthy on a mobile connection.

Encoders do not take a file size as input — they take a quality setting, and the size falls out of it. Getting to a specific number therefore means searching.

The tool encodes your image at a mid-range quality, measures the result, and then moves the quality up or down depending on whether it came in under or over your limit. It repeats that, narrowing the range each time, until it finds the highest quality that still fits. That is a binary search, and it converges in about eight passes.

If nothing fits even at minimum quality, and you have allowed the dimensions to change, it scales the image down by a fifth and searches again. If you have locked the dimensions, it stops and tells you the target could not be met at that pixel size — rather than quietly handing you a file that will be rejected.

The quality number in a JPEG or WebP encoder is not a percentage of anything. It is an index into a set of quantisation tables that control how aggressively detail is discarded. The relationship to file size is steep and non-linear.

For a typical 12-megapixel phone photograph the curve looks roughly like this:

• Quality 95–100 — near-original appearance, but the file is often only 20–35% smaller. Rarely worth it for the web.
• Quality 80–90 — the sweet spot for photography you care about. Typically 50–70% smaller with no artefact you would notice at normal viewing distance.
• Quality 70–80 — the sweet spot for ordinary web images. Usually 70–85% smaller. Artefacts appear only if you zoom in on flat sky or skin.
• Quality 50–70 — visible softening and blocking in smooth areas, though thumbnails and background images survive it well.
• Below 50 — obvious blocking, ringing and colour banding. Useful only for placeholders.

Diminishing returns set in fast at the top of the range. Going from quality 100 to quality 90 usually halves the file for no perceptible change; going from 90 to 80 saves far less and costs slightly more. If you only remember one number, use 80.

Every time a JPEG is decoded, edited and saved again, the encoder re-runs its quantisation step on data that has already been quantised once. Errors from the previous pass are treated as real detail and get their own new errors layered on top. This is called generation loss, and it accumulates in one direction only.

Saving an already-compressed JPEG at quality 90 does not restore anything — it just spends more bytes preserving the artefacts of the earlier pass. Saving repeatedly at a low quality is what produces the smeared, blotchy look of an image that has been through a dozen chat apps.

Two habits avoid it. First, always compress from the highest-quality original you have, not from a copy that has already been through a compressor. Second, decide on your output size once and do a single pass, rather than nudging the quality down repeatedly and re-saving each time. If you need to try several settings, run each of them against the original file.