What to know before your first fax

What a fax transmits

A fax turns a sheet of paper into tiny black and white dots and pushes them down a phone line. There is no colour and no grey. A dot is either black or white, nothing in between. That is why a photograph faxed as is comes out as a dark smudge.

Most services now carry the fax over the internet instead of a phone line, but if a fax machine receives it, it still ends up as the same black and white dots. Preparing the file for that shape makes the paper on the other end far easier to read.

That is all this tool does. It does not send the fax, because there is no phone line behind this page. It takes you up to the last step: a file in fax shape, with a cover sheet if you want one.

Resolution and page size

A fax scan line is 1728 dots wide. RFC 3949, which defines the internet fax file format, uses that single width for both ISO A4 and US Letter. The horizontal resolutions in use are 200 and 204 dpi, and the vertical ones are 98, 196 and 200.

Dpi means dots per inch. A vertical resolution of 98 is what people call standard mode and 196 is fine mode. Standard mode is coarse vertically, so the horizontal strokes of small type run together. That is why this tool does not offer it.

A4 is 210 by 297 mm. In fine mode a page becomes 2292 lines tall, and at square 200 dpi it becomes 2339 lines. One bit per dot works out to roughly half a megabyte per page, but long runs of white compress well, so the file you download is much smaller.

What this tool actually does

It expands every photo and every PDF page into an image. PDFs are rendered inside your browser. Nothing is uploaded.

Each image is placed whole onto a white A4 shaped page, centred, never cropped. The edges are kept clear by the margin you set. The default is 6 mm because fax machines tend to clip the outer edge.

The image is then reduced to grey and split into two values. Threshold mode paints every dot darker than the cutoff black, which keeps text crisp. Dithering passes the leftover error to neighbouring dots and scatters them, which suits a page with a photo on it.

Raising contrast rescues light pencil writing. Lowering it keeps paper stains from printing. A lower threshold gives you cleaner white, a higher one keeps faint text. Watch the first page preview while you decide.

You download the result as PDF or TIFF. The PDF holds one compressed black and white image per page. TIFF is the older fax format and holds every page in one file using PackBits compression. Which one you need depends on the service, so we build both.

The cover sheet is drawn separately, as a table of the fields you filled in, then converted to black and white exactly like the rest and placed first. It carries no name or address of ours, because we do not put our advertising on your paperwork.

There is no auto straightening. Rotating automatically cuts off text more often than it helps, so we adjust margins and contrast and leave the rest alone.

Photographing a document

Shoot from directly above with a window to one side so nothing casts a shadow. Shooting at an angle stretches the text, and converting to fax makes that worse.

Put something dark under the paper. White paper on a white desk has no visible edge, so you cannot tell where the margin should be.

Pencil writing and faded photocopies need more contrast. If the strokes still vanish, raise the threshold, but watch the preview: too high and paper stains print as well.

Files are converted in the order you select them. If pages come out reversed, choose the files again in the order you want.

What goes on a cover sheet

The cover tells the person at the machine who this is for and what it is. Large offices share one fax machine between departments, so naming the department and the person is what gets your paper to the right desk.

Fill in the recipient, the fax number, your name, your contact number, a subject, the date and the page count including the cover. The page count is filled in from your files, so you never have to update it by hand.

The count matters because a fax can cut out partway. Without it the person holding the paper has no way to know a page is missing.

Use the notes block for anything the recipient needs to act on. If you need a reply, say by when and to where. Leave it empty and the block disappears.

Where people get stuck

When someone says the text arrived smudged, it is usually contrast rather than resolution. If fine mode still smudges, the original photo was dark or shadowed. Reshoot it.

If the service asks for a file format, offer PDF first and TIFF if they refuse it. Internet fax services usually take both.

Free services cap the page count and some print their own branding on the cover. For paperwork going to a government office that branding can be a problem, so check first. The limits we verified are on the comparison page.

If the number is right and it keeps failing, the receiving machine may be off or out of paper. Waiting and retrying usually works, and a phone call settles it faster.

Mask ID and account numbers you do not need to disclose. A fax prints on paper in an office you do not control.