How do I calculate original and final gravities?

 

Back to Ask the Brewmaster.

 

I’ll cover the original gravity first, and then go on to the final gravity.  If you’re an extract brewer, determining the original gravity is a pretty straightforward process.  Just multiply the pounds of extract by 38 and then divide by the batch size. As an example, if you’re using 6 pound of extract syrup in a 5-gallon batch, your gravity will be 6 x 38 / 5 = 46, or a specific gravity of 1.046. Note that this assumes extract syrup. Dry malt extract will give you 42 points per pound, for a total of 50.  So how about that crystal malt? Add in another 15 points per pound per gallon. Say you’re putting in 1 pound. That will add 1 x 15 / 5 = 3 points. Add that to the 46 from the extract, and you get a gravity of 49, or a specific gravity of 1.049.  If you’re a partial mash or an all-grain brewer, you need to know your extraction efficiency.  This will depend on your technique. See, for example, my article in the April Gazette. Most brewers will get between 25 and 30 points per pound of pale malt. If this is your first attempt, use 25. So for that 5-gallon batch with 9 pounds of pale malt grain, you’ll get 9 x 25 / 5 = 45. If you add crystal to the mash, some of the enzymes from the pale malt will help extract some of the sugars a bit more than a simple steeping, so you’ll get about 20 points. Hence, one pound will contribute 4 points. Other types of grain will give a slightly different value. See the table for details.

 

Dry Malt Extract

42

Malt extract Syrup

38

Pale Malt

25 - 30

Crystal Malt (Steeped)

15

Crystal Malt (Mashed)

20

Munich or Vienna Malt

22

Dextrin malt

15

 

Final gravity depends on how completely the yeast ferments the available sugar. Most yeasts will ferment glucose, sucrose and maltose, sugar types which have one or two sugar units. A three-unit sugar called maltotriose is fermentable, but another called raffinose is only fermentable by lager yeast.  Sugar compounds with 4 or more units (called dextrins) won’t ferment, thus leaving residual gravity, and some residual sweetness. Different yeasts have slightly different abilities to ferment. This character is called attenuation. For the Wyeast labs liquid yeast products, the attenuation can be found on a big chart at the brewshop, or at the Wyeast website at http://www.wyeastlab.com/beprlist.htm.  Attenuation is the amount of gravity that disappears.  Hence, if the attenuation is 75%, and your starting gravity is 49, the final gravity will be 49 – 0.75 x 49 = 12, or a specific gravity of 1.012.  Note that the Wyeast chart shows a range of attenuation.  For American Ale 1056 for example it ranges from 73% to 77%. This is because the mashing process and grain types affect the fermentability. Extract brews will hit the higher end of this range. All-grain mashes will be somewhat lower. High mash temperatures add to final gravity. Subtract a percent from the attenuation for each two degrees above 150oF. Adding dextrin malts, and to a lesser extent, crystal or Munich malt raises the final gravity. Subtract a per-cent for each pound of crystal or Munich malt and subtract two percent for each pound of dextrin malt.

A couple of final notes: First of all, the gravity readings are temperature sensitive. Your hygrometer is calibrated to 60oF. Warm or hot wort will read low.  Secondly, make sure your final wort volume is correct.  If you miss your volume by half a gallon in a 5-gallon batch, your gravity will be affected. Your anticipated 1.049 starting wort will come 10% low, or only 1.045. 

Malt Type Gravity pts per lb. in 1 g