Sorry I missed this thread earlier. The default rule thickness in is the eight font parameter of the fonts in math family 3. A bare context file with the standard 12pt fonts gives the value 0.47998pt. You can get the value via :
\edef\defautlrulethickness{\the\fontdimen8\textfont3}
Afaik, in all math setups three fonts in math family 3 are the same actual font so just the one value will do. But the actual value will definately vary a bit depending on the font setup. For example, palatino 12pt has 0.708pt.
OK. However, this doesn't seem to scale properly within the same font … try the following: --- \edef\defaultrulethickness{\the\fontdimen8\textfont3} \starttext Lorem ipsum dolor sit amet \defaultrulethickness \startformula \sum_{n=0}^\infty \frac{x^n}{n!} \stopformula \setupbodyfont[32pt] Lorem ipsum dolor sit amet \defaultrulethickness \startformula \sum_{n=0}^\infty \frac{x^n}{n!} \stopformula \stoptext --- So, your font parameter always returns the same value but clearly the fraction bar is scaled correctly … something else must be happening behind the curtain. If only I knew what it were ;-) Oliver