Doing the Math

Don't panic, it's only multiplication and division!

First Number: How big do you want the picture to be, on the printed page, in inches, at the resolution you desire [using width in this example]:

S * dpi = pixels

• S = Size in inches
• dpi = Resolution, e.g., 300 dots per inch
• pixels = size in pixels, examples:
  • 4” wide @ 300 dpi = 1,200 pixels
  • 6.5” wide @ 300 dpi = 1950 pixels

Second Number: How big is your image in pixels: No math, just read the size in a graphics editor, or in the windows file manager. Remember size is both width and height and in this example we are working with Width as the critical dimension.

Compare Desired Width with Actual Width:

The actual size will either be bigger or smaller than the desired size [matching sizes only seem to happen about once a decade].

Actual <= Desired: Small graphic

Adjust the dpi to match the graphic: Divide the Actual pixel width by the desired width in inches. Example, with a graphic is 800 pixels wide for a 6.5” wide slot. 800 / 6.5 = 123.0769 which we will round down to 123. Set the graphics internal dpi to 123.

Desired < Actual: Big graphic

Reduce the size of the graphic and set dpi, which can be done as one step.

Setting the DPI

The following two examples show one graphics editors controls. Other editors are similar.