Unless some precision benefit accrues from multiplying the factors for each dimension individually, it would be better to compute the complete factor for the conversion once

[snip] Good point - I don't think there's a precision issue, but I should think about it some more. I'll take a look at this later this evening. I guess it can be rendered down even more: static quantity<unit<System2,Dim1>,Y> convert(const quantity<unit<System1,Dim1>,Y>& source) { static Y fac(compute_factor()); return quantity<unit<System2,Dim1>,Y>::from_value (source.value()*fac); }