How do you compute the right and left margins in typewriter?

First you need to know the print size. Typewriters came in pica and elite. Pica is 10 characters to the inch, elite is 12 characters per inch. Then you need to know the width of your paper. Letter and legal size paper are 8.5" across. So to get the total number of characters, you would multiply by the characters per inch. So pica typewriters have a maximum of 85 characters per line on letter-sized paper, while elite has a maximum of 102.

If you want to know how many characters is the margin, then decide on the margin size and multiply by the characters per inch. So a 1" pica margin would be 10 characters, while a 1" elite margin would be 12. Likewise, at 1.5" the margin would 15 (pica) or 18 (elite) characters.

To get the number of usable characters, you would add the characters of the two margins and subtract from the maximum characters. For instance, for pica, lets assume a left margin of 1.5" (15 characters) and a right margin of 1" (10 characters). That would be 25 characters. Subtracting from a total of 85 leaves 60 characters (or 6 inches) for typing. Or for elite, the 1.5" is 18 and 1" is 12, for a total of 30 characters, leaving 72 characters (still 6") for typing from 102 in all. Or to make it easier, just subtract the margin sizes in inches from the paper width to get the number of usable inches. Then multiply by the characters per inch.

Now if you are doing center-justified, then you should manually compute the margins per line. You would subtract the number of characters you plan on using from the total possible characters (85 for pica, 102 for elite, assuming 8.5" paper). Then you would divide by two. Lets say you want to center a line with 25 characters on a pica typewriter, that leaves 60 characters of white space, and if you split that to center the text, then you would start typing at 30 characters (or 3" in).