A Weighty Subject: Beer Gravity

A good friend of mine, who drinks quality beer, but lacks the geek-love I have for it, asked me via text message:  “What do these crazy numbers mean on my beer?  Plato?  What is FG?”

That, my craft-drinking crew, is gravity.  Specifically degrees Plato (named after the German scientist, Dr. Plato) and Final Gravity.

As a general rule, the higher the °P (degrees Plato) or FG, the more alcoholic the beer.  Also, higher Plato or FG usually indicates a sweeter, fuller beer.  I say general rule, because yeast and temperature complicates this issue.  Both Wee Heavy Scotch Ales and Belgian Tripels are high gravity beers, but Tripels are quite crisp and dry, rather than full and sweet.

To provide an easy explanation as to the actual numbers, 10 °P means a wort (beer before it’s beer) is 10% solids.  To get a rough sense of what that means in terms of abv (alcohol by volume) 10 °P ~ 5% abv, like a typical German Pilsner.  12 °P is ~ 6% abv.  The ratio is not algebraic, but logarithmic, since 19 °P is ~ 10% abv.  Imperial Stout anyone?

The gravity measurement involves the density of the liquid in relation to water.  Water is 1.0 gravity, adding wort to the liquid raises the gravity.  There is a rough ratio, as with °P.  1.050 (or simply 50) beer will contain ~ 6% abv.  Again, it’s not a perfect relationship.  1.055 (or 55), and increase of just .005, results in an abv of ~7%.  That small increase moves a beer from say, a session-worthy amber lager to an American style IPA or stout.

To make things even more complicated, many European brewers, use the Balling scale, or like the Belgian brewers, use Belgian degrees.  When you see 8° on a cork and cage 750mL Belgian brew, it’s really 1.080, likely a Belgian Strong Dark, or perhaps a “Quad” coming in at ~10% abv (sip with care).

I hope this sheds a little bit of light on the topic.  For more info (and in order to cite my sources) visit:

Tasting Beer by Randy Mosher

Plato to SG Conversion Chart

Top Fermented (a blog)

Specific Gravity

Malting and Brewing Science