How does the dialog manager calculate the average width of a character?

Raymond Chen

Some time ago, I explained that the Map­Dialog­Rect function requires the handle to a dialog box because the mapping from dialog units to pixels is dependent upon the default font of the dialog box, so you need to know which dialog box you are converting.

I noted that if you don’t have a dialog box, and you don’t want to find or make one, then you can simulate the calculations yourself using the standard formulas:

8 vertical dlu =  1 character tall
4 horizontal dlu =  1 average character wide

According to Knowledge Base article Q145994, the calculation of the character height and width are performed as follows:

For height, call GetTextMetrics and use the tmHeight.

For average width, get the text extent of the string


and divide it by 52, rounding to the nearest integer. Do not use the tmAveCharWidth from the text metrics. Despite its name, it is not the average of anything. It’s just the width of the character x.

Bonus chatter: Maybe the font people interpreted it to mean “the width of an average character”, rather than “the average width of a character.”