Paper Weights - Equivalents - Is there a table?

[plevyadophy]

I have been looking at measurements for paper weights and thickness and getting quite confused.

My Canon BJC-85 printer's manual simply states paper weight in pounds.

But Canon papers have g/m2 and micron printed on the box.

HP papers talk of mils.

And HP and Canon papers can sometimes have similar micron measurements printed on the box but yet the g/m2 is different.

Is there some table that gives the different paper weight and thickness measurements used and their equivalents?

If there isn't doesn't anyone have an idea and would maybe like to produce a table for us?


Hope someone can help.

Thanks in advance.



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[Grandad35]

Plevyadophy,

Remember, you asked for this.

Before starting, it might be good to give a little description of why there are so many ways to give a paper's thickness and to explain some of the confusion. It would seem obvious that the best technique would be to just directly measure the paper's thickness. Unfortunately, this is not always as easy as it might seem. The background on this comes mostly from paper mills, where they make paper all the way from toilet tissue up to heavy board (the backs for tablets). You may or may not be aware of this, but the thickness of paper is not all that uniform on small samples, especially on thinner products. If you have a micrometer that is capable of a measurement accuracy down to 0.1 mils (2.5 microns) try to measure some toilet tissue to see this for yourself. The second problem is that there is a real skill involved in making such measurements accurately.

For these reasons, the paper mills needed a fast, simple, accurate and reliable way to measure the average paper thickness so that they could control the thickness of their products. What they came up with (and still use for the final Q/C check) is to cut a sample to a precise size and to weigh it. The size is selected so that the weight reading on the scale directly gives the "basis weight" of the paper. The metric units for basis weight are almost always grams per square meter (gsm) - sometimes shortened to just "grams". Note that basis weight measurements only tell you how much the paper weighs, not how thick it is. To convert to thickness, you need to know the density, which is typically in the 0.8-1.1 range for coated photo paper (http://www.paperonweb.com/density.htm).

Things aren't quite as simple in the US, where we use "pounds per ream", which many people shorten to just "pounds" ("lb"). A ream is 500 sheets, but different sheet sizes obviously give different basis weights for the same paper - see (http://home.inter.net/eds/paper/grammage.html) for more details. Fortunately, only a few papers are marked with "Lbs", since this requires you to guess at their "basic size" to get an idea what these numbers really mean. Personally, I usually ignore any value given in "Lbs", because it can mean anything. I did find a place in my Canon manual where they state that plain paper is "(64 gsm or 17 lb)", so this means that they are using a basic size of 17"x22". Just remember to multiply Canon's "lb" values by 3.76 to convert to gsm.

Thickness values are usually given in "mils" or "microns". A mil is a thousandth of an inch and a micron is a millionth of a meter. Both a human hair and regular "printer paper" are on the order of 3 mils (75 microns) thick. Most photo paper is in the range of 4 to 12 mils, but there are specialty products that are over 100 mils thick.

The following table assumes a density of 0.9 to convert basis weight values to thickness. I have included the Canon "lb" conversion for reference. Note that any conversion between thickness and basis weight will be affected by the paper's density, so these conversions are only approximate - they are marked with an "*".

To convert from: to: multiply by:
Microns mils 0.0394
Mils microns 25.4
*Gsm mils 0.0437
*Mils gsm 22.9
*Gsm microns 1.11 (1/density)
*Microns gsm 0.9 (the paper's density)
Lbs gsm 3.76
Gsm lbs 0.266

HTH - let me know if you need conversions that aren't listed.

 

[plevyadophy]

@ Grandad35

Hey, you old "smartarse" !!!!

Fantastic answer.

Funnily enough I used to work in the print trade (pre-press) and never knew all that.

The only thing I would quibble with you on is the definition of a ream, which I learnt to be 501 sheets (although, I have for many years been noticing how stationary suppliers are shipping 500 sheets and referring to it as ream).

Your answer is quality.

Thanks very much.

I will get back to you if I need anything else on the subject.

Thanks again.


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[Grandad35]

Plevyadophy,

I was involved in (among many other things) designing thickness control systems for continuous (roll to roll) paper coating, and those reams were either 1000 sq ft or 3000 sq ft, depending on the user. I really didn't get too involved in the 500 sheet count thing, but that's always the number that I was told - the second link in my original answer also specifies 500 sheets/ream. I suspect that your old paper supplier wanted to keep their customers happy, and included an extra sheet just in case the count was off by a sheet or the top sheet got damaged during shipment.

With today's "save every penny" business climate, someone obviously figured out that they could reduce their costs by 0.2% by eliminating the extra sheet. The CEO probably got a nice bonus for this "improvement".