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There is an old carboard box that used to contain fanfold paper right next to me.

Using the label on the box for the number of sheets and a ruler to measure the box, a stack of 2200 sheets was 7 1/2 inches thick, which is near enough 300 sheets per inch. or 0.085mm per sheet if you prefer those units.

This would have been plain white paper, the stuff we used by the ton for program listings, core dumps, etc - and probably the thinnest grade that would work reliably with high speed line printers.

In the US, paper is normally measured in pounds (lbs.) with the actual measurement defined a little differently depending on the type of paper - "cover", "text", etc. That is a very long discussion, and not that relevant here, here is one sample chart comparing types of paper/weights.

The end result is the usual way to refer to office paper, including, in my experience, fan-fold computer printer paper, is in lbs. for "bond" paper, with 20 lbs. being "typical ordinary copier paper", 15 lbs. being "lightweight" and 24 lbs. being "a little heavier, often for stationery". I remember typically 15 or 20 lbs. for fan-fold paper. You wouldn't normally go heavier than 20 lbs., but sometimes you would go lighter to save money and space.

Staples still sells green-bar paper and it is 15 lbs.

As far as overall thickness, a ream (500 pages) of paper is about 2" thick, or ~ 250 pages /inch. 15lb. paper should be ~ 3/4 as thick, or around 330 pages/inch.

The typical stuff we used to use for program listings, etc., came 3000 sheets, fanfolded, to a box that was 12 inches high. That would make one sheet 0.004 inches or about 0.1 mm thick.

This jibes well with Dave's answer, which refers to paper that's already been out of the box, through the printer, refolded, and squeezed. If it had not been squeezed it would have "measured" a bit thicker. So I think this is a pretty good answer.

n.b.: Personally I preferred blue-space-bar to green-space-bar. It disappeared when copied better than the green did, at least on the copiers we had at the time.

Approximately 0.1mm, based on the first fanfold printout I could lay my hands on in my office (vintage early 1970s). This has 10 sheets and squeezes down to 1mm, as near as I could eyeball it with a desk ruler. As far as I can recall, this was a fairly sturdy paper as such papers went. The printout was from an ICL 1903A computer, I never knew the printer's designation.

I have half of a box of Moore Paragon Paraflow 12-inch pin-feed fanfold for ASR 43 Teletypes. 400 sheets measures 37mm. It will compress only a little, to 36mm,but only when pressed hard at the point of measurement. So THIS paper is 0.0036 inches thick, but no thinner than 0.0035 inches. And it feels about the same as the wider line printer paper (enough of which I do not have, for accurate thickness measurement).
Kevin T, Sydney, Australia.

This question makes a lot of unnecessary and more harming non related assumptions. Most notably the exclusion of its grammage as base value, as that is the way paper handling is done.

For all practical purpose Tofro's answer already covers a good estimation as a bit less than 0.1 mm.

Paper thickness is related in a linear fashion to its weight. All it needs to go from the ubiquitous notation of grammage (weight per surface size) to thickness is to include its specific weight (weight per volume). Dividing these will easily deliver thickness. 4th grade math assumed.

With grammage in g/m^3 and specific weight in g/cm^3 this gets transformed into a simple

grammage * specific weight / 1000

The specific weight of a certain paper is defined by its style and how 'smooth' (*1) it is. For most practical purpose it ranges from 0.6 to 1.3 ) g/cm^3. As a result 80g/m^2 paper may range at 0.048 to 0.104 mm

Now, having done the basic math, there is also a less theoretical way. People like to have simple numbers, so paper maker also came up with a simple definition system for thickness of standard papers, defining a standard 'Volume' specifier which gets given out by paper makers to further define their paper.

It got set (*2) to 1 for a paper of a specific weight of 1 g/cm^3. Sounds superfluous, but using the term Volume as unit less classification instead of a specific weight with a complex unit seems to improve usage by non-nerds (*3).

For practical purposes, a volume 1 paper has a thickness in mm at exactly its grammage divided by 1000. So a volume 1 paper with 80 g/m^2 will have a thickness of 0.08 mm. Similarly, a 100 g/m^2 paper will have a thickness of 0.1 mm. Paper of different thickness is noted with a different 'Volume' like 1.5 for a thicker one, which with the same 80 g/m^2 will measure 0.12 mm. (80*1.5/1000)

This is especially great, as average office paper is of 'Volume' 1 type.

Bottom line: Just take the grammage, divide it by 1000 and apply the 'Volume' specifier. If none is given then 1 is a good value to start with for fine office paper, while cheaper and/or simple recycles one goes up to 1.2-1.4 (*4).

(Classic) Printer use, they are made to work well with paper of 0.2 mm or more (0.17 being a punch card type paper) and more importantly, several layers of such. Usually the transport mechanics (straight vs. bend over some rollers) and the stiffness was more important than thickness itself. Mainframe chain printers could work well with forms with fanfold 'paper' of 8 (!) layers paper and 7 intermediate carbon layers (*4) - although, one had to be rather exact when positioning the paper box and quite careful mounting the paper.

*1 - The German term paper makers use here is 'Gestrichen'. I have heard the term 'coated' in similar situations in English. Thicker papers are often called double or triple gestrichen/coated. Keep in mind, I'm definitely no expert about paper making, especially not in multiple languages.

*2 - Yes, as with everything there's a DIN fixing it, just don't ask me about the number.

*3 - It's also a real great example why the metric system is so easy. If everything is noted down using the same unit system, conversion usually breaks down to simple shifts - in this case shift right by 3.

*4 - Yes, that felt more like a diaper than printing paper:)

I measured the program listing from my master's degree project, printed on a line printer at London University in 1975. It is 44 sheets. The total thickness is 4.19 mm, giving 0.095 mm per sheet. I made no special requests, so the paper was typical line printer paper of its time.

It has been stored, usually flat, for over 40 years. I don't know whether that will have affected the thickness.

The fanfold paper I used varied from about 100 g/m², in stuff sold for home computer use, and thus for writing letters, to really thin, maybe 40 g/m², at penny-pinching employers.

The g/m² unit means "grams per square meter", which is the standard measure for paper weight in the UK and the EU. Ordinary photocopier paper is usually 80 g/m², nice letterhead paper is up to 200gsm.