Most platforms (and/or font handling libraries) provide a way to obtain the font-wide maximum advance width, which you could use to allocate column width if you absolutely need to fit. This should be a very inexpensive operation as you're simply looking up a value, then multiplying by the number of characters in the string(s), as opposed to actually rendering the strings.
Some platforms also provide a font-wide average advance width value; if you just need to get "pretty close" you could probably devise a formula that takes the average into account.
Under Win32, GetTextMetrics returns a TEXTMETRIC structure which contains both the max and average. As mentioned above, other platforms should have similar APIs but you'll have to figure that out yourself, or edit your question so we can help you.