I am working in Indesign cc and i am using images with an actual ppi of 72 , and i need to know how much the effective ppi of the same images should come upto if i am to get the document printed in a good manner. I am working on a A4 sized document
Effective PPI needs to be 1.5 times the line screen (LPI) being used when printed.
In most cases a 150-175 line screen is used in offset printing. This means the effective PPI of an image should be roughly 225-265ppi.
- 175x1.5 = 262.5
- 150x1.5 = 225
It's not unheard of to use a more dense line screen though. If your print provider uses a 300LPI screen then you want the effective ppi to be 450. However 300lpi is rare.
In most cases, ensuring the effective ppi is 240 or higher will suffice. Commonly using the rounded 300ppi is easiest since that works for all the common line screens in offset printing and it's an easy round number to remember.
If you are printing on an end-user home inkjet printer and effective PPI of 150 is most often plenty.
The rule of thumb I've always been told to use by just about everyone is 300 pixels per inch. This ensures a sharp image in the average print job.
You have some leeway, though. For lower-quality print (like newspapers), 200 or even 150 dpi might be enough. Also, in regular print, if you use an image as a background, you don't need super-sharp quality and ~240dpi sounds acceptable to me.
Be warned that there's also printing techniques requiring even higher resolutions than 300 dpi. A coffee table art book, for example, might require dpi values of 400 or even 600.
Bottom line: always ask your printer if unsure.