Shoptalk: Paper

CALIPER: The measurement of a single ply of paper typically given in Mils or Points, meaning a sheet that is .0041 of an inch thick is stated as a 4.1 mils thickness or 4.1 points. The thickness is checked using online equipment while the paper is being produced and then rechecked on precision thickness testing equipment that is typically certified by outside vendors (not to be confused with hand held micrometers). It is also important to note that the industry standard for caliper variation is +/- 5%, so if you see a 65# cover sheet and the advertised caliper is 9.0, the reality is that the actual paper caliper could be in a range of 8.55 to 9.45 mils. This is important for postal regulations where a minimum caliper is required. So, when in doubt use 80# cover for 9 point post cards or ask your supplier if the sheet is USPS guaranteed.

PPI: This is typically the estimated amount of Pages Per Inch, or more importantly, the estimated thickness of a project with a given amount of pages. This becomes critical when ordering covers for a perfect bound book but is also very important when ordering three ring binders. Ever try to get 1.5” of paper in a 1” binder? One of the most important things to remember is that PPI references pages, meaning two pages per single sheet of paper, so if you have a book with 500 pages, a printer needs to find out how many of these pages are simplex, duplex or blank. As a printer, I was more concerned with the much less used term of “Sheets Per Inch” which is a even simpler calculation of: “1000 / C (caliper in mils) = S.P.I.”  so a 4.0 mil sheet = 250 sheets per inch. (1000 / 4.0 = 250).  I typically was more concerned about how many sheets in a book compared to pages. To convert PPI to SPI simply divide PPI in two.

I highlighted the word estimated because, as you may have found out, not everything is perfect in the printing world. Although the paper may be exactly 4.0 mils (see caliper variation above), there are a couple other factors: nesting of the sheets (two rough surfaces intertwine a little), reducing the overall caliper (-2 to-10%) or as many of you have seen fluffy paper as it has come out of the printer which then increases the overall thickness (+5 to+10%). The PPI or SPI numbers are intended to give a general rule to follow, but the only real way of knowing for sure is to make up a dummy book once the actual paper has been received.