It completely depends on what exactly you are measuring. Different chemicals have different densities. For this reason I use small scale that measures to 0.1g. Mass is more accurate then volume.
I agree with Chris: get a simple digital scale. I purchased a small one that measures up to 150g in 0.1 increments.
First thing I found when I got it was that I was underdosing a lot of my ferts by using the tsp to gram conversions found on the web for different chemicals. It varies quite a bit depending on how packed it is, etc, etc...
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