No. Weights and volumes are sometimes but not always convertible. It's a question of density. Water is the density standard. 250g of water = 250ml. English/American measurements can be ambiguous, because a unit ounce can refer to either weight or volume. With water, "a pint's a pound, the world around." However, a pint (16oz volume) of white wheat flour weighs about 9oz, because, you see, uncompacted flour is less dense than than water.
Consequently, when you convert a unit of mass to one of volume, as you do when converting 1.4g to 1/2tsp, a density similar to water is an underlying assumption.
I assumed that it was well understood that weight/volume conversions -- no matter whether the units are metric, "American," or mixed -- would not always be 1 to 1. However, it appears my assumption was not well founded and not everyone understood. My apologies.
Back to coffee: If you do not understand how moisture loss in the roasting process alters mass more than volume, I'll be happy to explain that too. Just ask. Further, volme/weight conversions for coffee beans are complicated by a great deal more than mere roasting. Perhaps that's a topic for discussion in some other thread. Again, you have but to ask.
The subject here is conversion from metric units to "American" and vice versa. It is not coffee roasting. But: If 250g is the equivalent of 1/2 a pound -- which is part of what I said -- then 250g of roasted coffee and 250g of green beans are both about half a pound, to the same degree of accuracy.
Edited by boar_d_laze - 7/28/12 at 8:18am