Converting a colour photo to black and white is usually thought of as a creative choice, but it has a practical side too: removing colour information can genuinely reduce a file's size, sometimes significantly. Whether it is worth doing depends heavily on what the image actually is and what it will be used for, since the size benefit varies a great deal by content type.
The Image Compressor can compress a greyscale image the same way it handles a colour one; converting to greyscale first is a separate step worth understanding on its own.
When greyscale genuinely helps with size
- Scanned text documents see the biggest benefit — a page that is genuinely black text on white paper carries no meaningful colour information to begin with, so converting to greyscale (or true black-and-white) removes data the compressor would otherwise have to encode for nothing.
- Simple line drawings and diagrams with no meaningful colour content behave similarly.
- Faxed or photocopied documents, which are often effectively greyscale already even if saved in a colour format.
When it makes little practical difference
For a genuine colour photograph — a portrait, a landscape, a product shot — converting to black and white removes the colour channels but the underlying tonal complexity (the range of light and dark across the image) remains largely the same, so the size saving is often much smaller than people expect, sometimes barely noticeable. The bulk of a photo's file size in this case comes from the detail and gradation within each tone, not specifically from colour itself, so removing colour alone does not solve a size problem the way it does for a scanned document.
Steps
- Convert the image to greyscale or black-and-white using a basic photo editor (most include this as a simple filter or adjustment).
- Upload the converted image to the Image Compressor.
- Compress and check the resulting size against your target.
True black-and-white versus greyscale, and why the distinction matters for size
Greyscale still allows for a full range of grey tones between black and white, which is appropriate for most photographs and detailed scans. True black-and-white (sometimes called “bitonal” or 1-bit) allows only pure black or pure white for each pixel, with no grey in between at all, and is dramatically smaller as a result — but it is only suitable for content that is genuinely just black marks on a white background, like a typed document or a simple line drawing. Applying true black-and-white to a scanned photo or anything with real tonal gradation produces a harsh, poor-quality result, so match the choice to the content rather than always reaching for the smallest option.
A worked comparison
A typical scanned text page in colour, saved as a photo, might be 2–3 MB. Converting the same scan to greyscale before compressing can bring that down significantly further than compressing the colour version alone would, often to a fraction of the greyscale-converted size once compressed, because there is so little genuine colour information being thrown away that was actually needed. A colour photograph of similar starting size, converted the same way, typically shows a much more modest reduction, because most of its size comes from tonal and textural detail that greyscale conversion does not remove.
Deciding whether the aesthetic trade-off is worth it
For a document scan, converting to greyscale or black-and-white is rarely a meaningful aesthetic trade-off, since the document likely had little colour information worth preserving in the first place. For a genuine photograph, converting to black and white is a real visual change that affects how the image looks and feels, not just its file size — worth doing deliberately as a creative choice if that suits the purpose, but not something to do purely chasing a size reduction that, for a real photo, is often smaller than expected anyway.
Applying this to a batch of mixed scanned documents
If you are processing a batch of scanned pages that are genuinely plain text with no meaningful colour content — letters, forms, printed pages — converting the whole batch to greyscale before compressing is one of the more effective, low-effort size reductions available, and is worth doing as standard practice for this kind of content rather than only when a specific size problem comes up.
Checking the result before relying on it
Once converted, open the result at the size it will actually be viewed or printed at, rather than only checking it zoomed in during editing. A greyscale conversion that looks fine at a large preview size can occasionally reveal a slightly muddy tone once printed or viewed at a smaller, real-world size, so a quick check at actual scale is worth doing before finalising a whole batch of documents this way.
Deciding this at the point of scanning versus after the fact
It is more efficient to decide on greyscale at the point of scanning, using your scanner or scanning app's own greyscale mode, than to scan in colour and convert afterwards, since the scan itself is smaller and faster to produce in the first place when the scanner never has to capture and store the colour data at all. Converting after the fact is a reasonable fallback for documents you have already scanned in colour, but building the choice into your scanning habit going forward saves the extra conversion step for every future document of this kind.
A final sanity check before archiving in bulk
Before applying a greyscale conversion across a genuinely large batch you plan to archive long-term, convert a handful first and file them exactly as you would the full batch, then come back and check them again after a day or two with fresh eyes. This small delay catches problems a same-session review sometimes misses, before the setting is applied irreversibly across an entire archive.
Frequently asked questions
Will converting to greyscale make a document harder to read?
No, for a document that was mostly black text on white paper to begin with, converting to greyscale changes essentially nothing about legibility.
Does this work the same way for a scanned photo within a document?
A scanned photograph behaves like any other photograph — converting it to greyscale will change it visually and may not meaningfully reduce its size, so this technique is best reserved for genuinely text-based content.
Can I convert back to colour later if I change my mind?
No, once colour information is removed it cannot be restored; keep the original colour file if there is any chance you will want it later.
Is greyscale always smaller than true black-and-white for the same content?
No, true black-and-white is typically smaller for content that suits it, since it stores only two possible values per pixel rather than a full range of grey tones.
Does my scanner have a greyscale setting so I do not need this as an extra step?
Most scanner apps do offer a greyscale or black-and-white scan mode, which is worth using directly at scan time if you know the content does not need colour, saving the conversion step afterwards entirely.
Will this work well on a document with a coloured stamp or logo?
Converting to greyscale removes that colour along with everything else; if the colour of a specific element matters, keep the document in colour despite the size cost.
Is the tool free and private?
Yes. No sign-up, no watermark, and files are removed from the server automatically about an hour after processing.