Fax preview: see what the recipient's fax machine will print
A fax is printed in black and white at low resolution, so faint text, colour and dark photos can look very different on the other end. Add a PDF or a photo of a document and see each page at real fax resolution next to your original, with plain-language checks and how to fix what they find. Free, no sign-up, and your file never leaves your device.
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Fax resolution
Your file stays on this device. It is opened and converted in your browser and is never uploaded.
Drop a PDF or a photo here
PDF (up to 20 pages checked), or a JPG, PNG or HEIC photo
How this works
Everything runs in your browser. These are the methods behind each result; all of them are estimates from the pixels of your page.
- Fax resolution
- Each page is drawn 1,728 dots wide, the width of a fax line, and resampled to 196 lines per inch (Fine) or 98 lines per inch (Standard). Pages of any size are scaled to that width, as fax machines do. Photos are placed on a Letter page the same way FaxUp's send page places them.
- Black and white
- Every dot becomes pure black or pure white. The cut-off is chosen per page with Otsu's method (it separates ink from paper using the page's own brightness histogram) and kept between 43% and 63% brightness, close to the fixed 50% cut many fax converters use. Some fax services turn grey into dot patterns instead; where they do, light areas look speckled rather than vanish.
- Faint marks and colour
- A mark counts as lost if it is clearly darker than the paper on your page but prints white, is darker than the area around it (so a soft shadow that drops out doesn't count), and is not just the soft edge of a letter that does print. Marks with strong colour (a big difference between their red, green and blue values) are counted as colour; the rest as faint. A page is flagged when lost marks cover more than 0.03% of it.
- Dark areas
- The share of black dots on the page, and the share of the page covered by solid areas: 0.12-inch squares that are more than 80% black. A page is flagged above 30% black overall or 4% solid.
- Edges
- Fax machines commonly lose up to about a quarter inch at the edges, and the sending service prints a header line across the top. Content inside that 0.25-inch band is flagged.
- Text size
- Connected black shapes the size of letters are grouped into lines; a line's height, from the top of its tallest letter to the bottom of its lowest, is about 0.9 of the type size. Text is flagged when its lowercase letters would be under 6 rows of fax dots tall: below about 8.5 pt in Standard and about 4 pt in Fine. Handwriting is not measured.
- Transmission time
- The page is encoded line by line with the Modified Huffman code from the fax standard (ITU-T T.4) and the size divided by 14,400 bit/s, the usual top speed of a fax call, and by 9,600 bit/s, a common fallback. Many services use the more compact 2-D codes, so real pages are often quicker. Dialing, ringing and the handshake come on top and don't depend on the document.
Questions
Is my document uploaded anywhere?
No. The file is opened, drawn and converted by your browser on your device. It is not sent to FaxUp or anyone else, and it is gone when you close the tab. Our analytics record that the preview was used, with the number of pages and warnings, never the file or anything in it.
Why does a fax look different from my PDF?
A fax has no grey and no colour: every dot is black or white, at about 204 dots per inch across and 98 or 196 lines per inch down. Light grey text, pencil, highlighter and pastel colours can turn white, dark photos turn into black blocks, and very small text loses detail.
What is the difference between Standard and Fine?
Both are 204 dots per inch across. Standard sends 98 lines per inch down the page and Fine sends 196, so Fine keeps small text and signatures sharper but has twice as many lines to send. FaxUp sends in Fine; if a fax keeps failing, its later automatic retries switch to Standard.
How do I make faint text or a light signature show up?
Go over it in black or dark blue ink, or rescan the paper with a darker setting or in black-and-white mode. For a PDF you created, change light grey text to black before exporting. Then check it again here.
Can I fax a photo of a document?
Yes. Put the page on a light surface in bright, even light, hold the phone straight above it and fill the frame with the page. A dark table around the page prints as a black border, and shadows can hide text, so crop tightly. The FaxUp app's scanner crops and cleans the page for you.
Is the preview exactly what will print?
It is a close estimate, not a guarantee. Fax services convert documents in slightly different ways, and the receiving machine's printer, paper and toner add their own changes. The checks flag likely problems so you can fix them before sending.
What is the fax-ready PDF?
A copy of your document as the black-and-white fax image, one page per page, at the resolution you picked. You can keep it as a record of what you sent, or send it with any fax service.
Ready to send it?
Upload your document on the send page, enter the fax number and follow the status until it is delivered. You pay per fax, any number of pages.
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