ABV vs ABW not sure the difference.

So i use an online beer math calculator (link won't cut and paste right) to calculate my alcohol percentage in my beer. It seems to work well but i had a question about the alcohol by weight vs alcohol by volume.

What is the difference? Why do we not just pick one and use that one? And we are not measuring the alcohol by weight or by volume. We measure by density when we us a hydrometer (which I guess is kind of like measuring by weight but if you ask a chemist it is not the same.)

I know 1 is used mostly for beer and the other for spirits, but i can not remember which is which. Can someone clear that up?

Alcohol by volume - Wikipedia, the free encyclopedia

Homebrewing For Dummies - Google Book Search

No pun intended. The "dummies" link is what google came up with 2nd.

abv is used mostly in the US abw is a european thing. and yes you are measuring by volume with the hydro.

That's like asking why can't we agree on how many ounces to a pint (or even on whether to use ounces or milliliters to begin with). These things develop on a country by country basis, mostly.

After I spent half an hour trying to explain my confusion I think i get it now. It seems so obvious now I feel stupid that I did not get it.

If you and 3 grams of salt to 97 grams of H2O the weight will increase by 3 grams. Thus the solution created would be 3% salt by weight.

But the salt will not increase it by the same percentage by volume.

This is the part I struggled with the most. I feel dumb now.

Actually, ABW is a USA thing. Most everywhere else uses ABV. Back before the craftbrew era, many people thought that many foreign beers - particularly Canadian if you grew up near the border - had more alcohol in them. This was in fact not true, because the US beers were reported in ABW so if you just went by the value of the number and weren't aware of the ABW vs ABV difference, you thought they had more alcohol, when in fact they didn't.

In fact, it wasn't until recently that brewers in many states were even allowed to use ABV. New legislation had to be passed to allow it's legal use.

They used ABW in bassackwards South Carolina until recently. They also repealed the law that made brews with equivalent the ABW equivalent of higher than 5.5%ABV legal too.