The GetCharABCWidths API function for TrueType fonts, and the GetCharWidth for other fonts, allow to calculate the width of characters in logical units.

The following is a sample code featuring the GetCharABCWidths function:

FUNCTION GetTextWidth(hDC, cText)
* hDC - device context handle, cText - a string
	DECLARE INTEGER GetCharABCWidths IN gdi32;
		INTEGER hdc, INTEGER uFirstChar,;
		INTEGER uLastChar, STRING @lpabc
	LOCAL ii, ch, nResult, cBuffer, nLen
	nResult = 0
	cBuffer = Repli(Chr(0), 12)
	FOR ii=1 To Len(cText)
		ch = Asc(SUBSTR(cText, ii,1))
		= GetCharABCWidths(hDC, m.ch, m.ch, @cBuffer)
		nLen = buf2dword(SUBSTR(cBuffer, 1,4)) +;
			buf2dword(SUBSTR(cBuffer, 5,4)) +;
			buf2dword(SUBSTR(cBuffer, 9,4))
		nResult = nResult + nLen
	ENDFOR
RETURN nResult

FUNCTION buf2dword(lcBuffer)
RETURN Asc(SUBSTR(lcBuffer, 1,1)) + ;
	BitLShift(Asc(SUBSTR(lcBuffer, 2,1)),  8) +;
	BitLShift(Asc(SUBSTR(lcBuffer, 3,1)), 16) +;
	BitLShift(Asc(SUBSTR(lcBuffer, 4,1)), 24)

The problem with this code is that a TextBox control in FoxPro is not linked to a device context, at least you can not reach that device context if it ever exists.

There is a solution, rather bulky, -- creating a virtual device context, selecting into it a font that is used in the TextBox, and then calling the GetTextWidth function. All in all it's going to be 50..100 lines of code. Myself, I found that GetCharABCWidths gets quite accurate results for TrueType fonts.

* * *
The GetTextMetrics function returns average dimensions for characters in TEXTMETRIC structure. Certainly you can not predict a content of a TextBox. So may be this function is more suitable for you.

* * *
FoxPro code samples:

Reading metrics for the currently selected font
http://www.news2news.com/vfp/?example=339&function=522

Vertical Label control (members area)
http://www.news2news.com/vfp/?example=398&function=522