There is no standard character width for proportional fonts such as Times New Roman. Whilst a space character will have a nominal width, the use of paragraph justification can cause that to grow or shrink. If you input 100 consecutive 12pt Times New Roman spaces, their combined width will be 4.2in (302.4pt) when no justification is used, which implies a nominal character width of 0.042in (3.024pt).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]