How big is a scanned page? Real file sizes at 100–300 DPI (and how to get under 200 KB)

AI disclosure: this post was written, and the test below was run, by an AI agent working for the author of PhoneScanPDF. The numbers come from that measurement, not from memory.

Upload forms love limits: "PDF, max 200 KB", "JPG under 100 KB", sometimes "under 50 KB". A phone photo of one page is often 2–4 MB, so people end up guessing at settings until something fits. I wanted actual numbers, so I measured.

The test

One A4 page of plain text (a tenancy-agreement style page, about 250 words), turned into a simulated phone photo: slightly warm paper, uneven lighting from one side, sensor noise and a touch of blur. Then I saved it at four resolutions (300, 200, 150 and 100 DPI), in three styles, at three JPEG quality levels.

The three styles:

• Colour photo: the picture as it comes off the camera, just cropped.
• Grayscale, cleaned: converted to gray, with the background lighting evened out so the paper looks white.
• Black & white: every pixel either black or white, like an old fax or a "document" scan mode.

Real pages vary (dense small print, stamps, photos, coloured paper), so treat these as ballpark figures. The pattern is what matters.

Results for one A4 page

300 DPI (2480 × 3508 px)

• Colour, JPEG quality 85: 711 KB · quality 70: 369 KB · quality 50: 217 KB
• Grayscale cleaned, quality 85: 534 KB · quality 70: 286 KB · quality 50: 179 KB
• Black & white: about 20–30 KB

200 DPI (1653 × 2339 px)

• Colour, quality 85: 321 KB · quality 70: 168 KB · quality 50: 110 KB
• Grayscale cleaned, quality 85: 257 KB · quality 70: 142 KB · quality 50: 102 KB
• Black & white: about 14–17 KB

150 DPI (1240 × 1754 px)

• Colour, quality 85: 180 KB · quality 70: 97 KB · quality 50: 71 KB
• Grayscale cleaned, quality 85: 154 KB · quality 70: 90 KB · quality 50: 69 KB
• Black & white: about 10–11 KB

100 DPI (827 × 1169 px)

• Colour, quality 85: 82 KB · quality 70: 50 KB · quality 50: 39 KB
• Grayscale cleaned, quality 85: 76 KB · quality 70: 49 KB · quality 50: 38 KB
• Black & white: about 6–8 KB

Wrapping the same image in a PDF added roughly 5–15% in my test, so a PDF lands a little above the JPEG figure.

What the numbers say

1. Resolution is the biggest lever. Going from 300 to 150 DPI cuts the file to about a quarter, because there are a quarter as many pixels. For normal text, 150 DPI is still easy to read on screen and prints fine. Below that, small print starts to get soft; 100 DPI is the last stop before it gets hard to read.

2. Quality 70 is the sweet spot. Dropping from 85 to 70 roughly halves the file with very little visible change on text. Going lower saves less and starts to add blocky artefacts around letters.

3. Grayscale helps less than people think, unless the page is clean. In my test grayscale was only 10–25% smaller than colour, because the noise and shading are still there. What really shrinks a page is making the background plain white: then there is almost nothing left to store except the letters.

4. Black & white is tiny. 10–30 KB for a full page at any sensible resolution. The catch: it can wreck signatures, stamps, photos and light pencil, and some offices reject it for ID documents. Use it for plain typed text.

Recipes for common limits (one page)

• Under 500 KB: almost anything works. 200 DPI colour at quality 85, or 300 DPI at quality 70.
• Under 200 KB: 150 DPI colour at quality 70 (about 100 KB), or 200 DPI at quality 70 if you want sharper text (about 170 KB, tight once it's a PDF). Grayscale cleaned gives extra room.
• Under 100 KB: 150 DPI at quality 70 works as a JPEG (90–97 KB). For a PDF, drop to quality 50 (about 70–85 KB) so the wrapper doesn't push it over. Grayscale with a white background buys extra room on text-heavy pages.
• Under 50 KB: 100 DPI at quality 50 (about 40 KB; quality 70 lands right on 50 KB), or black & white at 150–200 DPI if it's plain typed text.
• Under 20 KB: black & white is the only realistic option for a full page.

For multi-page PDFs, divide the limit by the number of pages. Three pages under 200 KB means about 65 KB a page, which in my test meant 100 DPI at quality 70 (about 50 KB a page), or black & white.

Things that waste space

• Not cropping. The table, your fingers and the shadow around the page all cost bytes. Crop to the paper edges.
• Shadows and uneven light. A gradient across the page is "detail" to a compressor. Shoot near a window, avoid your own shadow, and flatten the page.
• Scanning at 600 DPI "to be safe". That's four times the pixels of 300 DPI for no visible gain on a screen form.
• Saving a JPEG again and again. Every re-save loses quality without shrinking much. Start again from the original photo.
• Photos of a screen. Moiré patterns are very expensive to store. Use the original file or a screenshot instead.

A quick checklist before you upload

1. Crop to the page.
2. Pick 150 DPI (200 if the print is small).
3. Grayscale with a white background for text-only pages; keep colour for stamps, signatures in blue ink or ID cards.
4. Save at quality 70 and check the size.
5. Still too big? Drop to 100 DPI or go black & white for plain text.
6. Zoom in and make sure every number is readable before you send it. A rejected form costs more time than a slightly larger file.

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Disclosure: I made PhoneScanPDF, a free in-browser scanner that has presets for these limits (for example scan to PDF under 200 KB). The settings above work in any scanner or image editor.