Efficient way of calculating design dimensions based on print DPI

Here is the thread:

https://community.adobe.com/t5/indesign-discussions/total-ink-coverage-plug-in-for-indesign/td-p/13685179 

I guess I'll pile on here as well. I don't think anyone has pointed out that reducing the DPI just makes the dots larger, so they are not using any less ink.

I suspesct @Robert at ID-Tasker is probably right that this has to to with page area coverage, but changing print resolution will have no effect on that at all.

Thanks everyone for your replies. I will research more about my problem and if I still don't find the solution to my problem, I will talk to my senior.

I think it has something to do with the EU directive - can't give any more details, just that someone some time ago asked about such calculator - that "taxes" printed materials based on how much ink was put on those materials... 

Usually the pages that have smaller images and more text get printed at lower DPI.

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So I have to calculate the dimensions for the each brochure pages in pixels based on the PPI...

By @Booze385499119sr8

I can only agree with the already expressed opinion that it all sounds bizarre.

For example, as others also noted, why would your bosses ask you to calculate pixel page dimensions for pages that contain both images and text? This could, potentially, make sense if they wanted to convert entire pages into images. Which isn't the goal here, obviously.

As noted in prior comments, this is a very confusing set of claims. At a technical level, DPI has nothing much to do with print cost; it matters only when a more costly process must be used to achieve a higher DPI result. Which doesn't quite align with what you say the issues and goals are.

For one thing, printing part of a brochure on one line and other parts on other lines is almost certainly going to be more expensive in marrying, binding and trimming the result than just "hitting print" on a single print line. I've done a lot of work in this general realm, and unless you're leaving out some critical aspects, it sounds as if your CEO has some faulty ideas about how printing works, and you're not likely to come up with a solution that reduces costs (much, if at all) by trying to use such a complex print workflow.

I'm sure the expertise here can help with the technical issues of achieving the (technical) goal, but someone in your food chain should really work through the actual print costs for the various options to see if all the effort (now, in working out the methods) and down the road (in splitting up and managing each project with this complex print process) actually saves any money, or at least enough to pay for all the man-hours it will take to execute them.

TL;DR version, which a new employee on his first job really can't use, boils down to "Boss? Ya gotta be kidding, right?" 🙂 But that's far too common in firms where the brass has erroneous ideas about how a production process actually works. So gentle, humble, fact-based reporting back up the chain might be the right approach.

Having worked in the design and print industry for over 20 years, the past 10 of which have been spent in a printing company I genuninly don't understand the the logic behind what you've been asked to do nor how it could result in a significant discount on the print costs.
During pre-press process your files will be RIP'd at the optimum settings for the press regardless of what resolution you've created them at and only bitmapped images have an effective resolutions. Text, gradients and graphics created in Illustrator are all vector elements and ignore the output resolution of your pdf settings. 
If you've been supplying files for print with hundreds of very large image files which are scaled down to a thumbnail size in your Indesign documents it's possible that when the printers are running these files through their pre-press software its taking a very long time to process and therefore they may have offered a discount as an incentive for your company supply lower resolution files but they won't be running different pages in a brochure at different settings unless perhaps they are printing the covers on a different paper stock.
It sounds to me like your CEO and Senior have either misunderstod what the printers have asked or it's the equivelent of sending you to the hardware store for stripped paint or a long wait.

Eugene is right. I'll resist the temptation to expand on it.

Well - bluntly - this sounds ridiculous. 

This approach sounds quite misguided.

DPI (dots per inch) is determined by the output device and the printing method used.

For instance, in lithographic printing, different LPI (lines per inch) settings lead to varying results. Newspapers typically use coarse paper, which absorbs more ink.

To calculate DPI for lithographic printing, many people mistakenly use double the LPI. However, this isn't entirely accurate because halftone dots are rotated 45 degrees, impacting the effective measurement.

Using the Pythagorean theorem, where A^2+B^2=C^2 , the diagonal measurement yields a factor of about 1.41.

The actual number to multiply by is 1.41 - which is harder than just x2 which is why people use x2

For Newspaper printing - the LPI is set quite low - which is around 80-120 

For newspapers then it's 80x1.41 = 112.8 dpi up to 120x1.41= 169.2

And you'd need to ask your newspaper what their LPI is 

Magazines could use 120-150 = again multiply the LPI by 1.41 to get the optimal DPI

This is where the magic 300PPI or DPI comes in for print
It's a simplification of the 150 LPI output at x2 = 300 

And that's why people say it must be 300!!!! 

High end coffee magazines etc could use 175-200 LPI 

So you're limited by the output device LPI in Lithographic Printing

For digital printing, it's a bit more forgiving in a sense
You could go below 225 as low as 200 or even 175

When dealing with various types of images such as grayscale images, line art, and bitmaps—different DPI considerations apply. Line art, for example, typically requires higher settings in around the 1200DPI to maintain sharpness, while grayscale images might vary based on the desired print quality.

You could have a foggy scene or photo and where your LPI might be 150 on the output device - and you set your image to 211.5 DPI - that might not be necessary - you could go lower for foggy images, or images which are not very sharp - even to 150 or 125 DPI

Then you have large format printing - you typically don't need 300 DPI for billboards - usually 30DPI is fine for billboards.

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All that being said -  you won't know what DPI to make anything until you ask the printers what LPI their devices use (if they even know) - given for Litho it's usually 150 - then you are safe around the 225 mark or 250 or 300 - 

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Finally - INDesign you can select your DPI of images when exporting  
In the Compression Settings for the PDF

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In short
Ask your designated printer what DPI is optimal - and set your compression settings to suit the PDF export.