Plotting OG with a Graph
Ever looked at the ingredients in a recipe and wondered what
the original gravity of the beer would be? Ever wanted to brew a beer of a
certain specific gravity and wondered how much malt you’d need?
There are formulas for calculating the expected original
gravity (OG) from a list of ingredients (and vice versa). But solving these equations can be tedious for some homebrewers. Fortunately, there’s another quick way to calculate specific
gravity from a list of ingredients. You can use this graphical method to help
you formulate recipes, modify recipes or figure out what OG to expect from a
list of ingredients. This method also works in reverse — you can calculate how
much malt you will need to hit a desired OG.
Single malt extract
To calculate the OG of a beer brewed with a single type of
malt extract, you need to know three things: the weight of your extract, its
extract potential and the volume of your wort. Extract potential is a measure
of how much extract or soluble solids can be extracted from a given weight of a
particular ingredient. The extract potentials of common ingredients are given
in Table 1 (at left). Using this information — and a graph like Figure 1 (see
below) — you can quickly estimate your beer’s expected original gravity.
In Figure 1, the amount of ingredients per unit volume (in
pounds of malt per gallon of wort) is plotted versus “gravity points.” Gravity
points are the decimal part of a specific gravity. For example, a barleywine
with an OG of 1.090 would have 90 gravity points per gallon. Lines are plotted
on the graph that reflect the extract potential for dried and liquid malt
extract.
To calculate the OG of a single-extract beer, locate the
point on the x-axis (the horizontal axis) that corresponds to the amount of
malt per gallon of wort you have. Trace a line straight up from that point
until it intersects the plotted line of the extract potential of your malt
extract. Then trace a line from this point horizontally until it crosses the
y-axis (the vertical axis). The value at that point on the y-axis is your OG in
gravity points.
Here’s an example. Suppose you had 6 lbs. of dried malt
extract to make a 5-gallon batch of beer. First you find the point on the
x-axis that corresponds to 6 lbs. of DME in 5 gallons of beer. That point is
(6/5) = 1.2 pounds malt/gallon of
wort and is shown with a circled one. Next, trace a line straight up until you
intersect the line that represents the extract potential of dried malt extract.
In Figure 1, the point where the two lines intersect is labeled with a circled
2. Finally, trace a line horizontally to the y-axis and read the value for OG.
This point is shown with a three. The value is 55 — so using 6 lbs. of DME to
brew 5 gallons of beer should yield a beer with an original gravity of 1.055.
Multiple malt extracts
Most brewers use more than one source of malt when they
brew. Extract brewers, for example, may mix dried and liquid extract to make a
wort. When brewing with multiple sources of fermentables, you simply repeat the
procedure for each ingredient, then add the results together.
Let’s say you want to brew 5 gallons of beer with 6 lbs. of
DME and 3.3 lbs. of liquid malt extract. In the previous example, we found that
6 lbs. of DME yielded an OG of 1.055 for a 5-gallon batch. Now, we need to find
out the gravity points expected from 3.3 lbs. of liquid malt extract. In Figure
1, point 4 corresponds to (3.3/5) = 0.66 lb./gallon of liquid malt extract. A
line drawn vertically intersects the liquid malt extract line at point 5. A
line drawn horizontally from that point intersects the y-axis at point 6. The
value at point 6 (24) corresponds to the gravity points from using 3.3 gallons
of liquid malt extract in 5 gallons of beer.
The final step is adding the two partial gravities together.
In this case, 55 plus 24 equals 79. So, the original gravity of the beer would
be 1.079.
Single, mashed malt
When you use only malt extracts, or adjuncts that don’t need
to be mashed, you need to know the weight and extract potential of your
ingredients. All-grain brewers also need to know their extract efficiency. If
you give several all-grain brewers 10 pounds of pale malt, each beer will
likely have a different OG because each system has different efficiency or
“brewhouse yield.” These differences could result from different mash tuns,
mashing procedures, water sources or many other factors. A brewer whose
efficiency is high might brew a 5-gallon batch with a gravity of 1.065. Another
brewer might end up with a 1.040 beer.
Different brewers express extraction efficiency in different
ways. There is “points per pound
per gallon” and the percent of the maximum amount of fermentables that could
have been extracted. You can use either way with the graphical method of
calculating OG.
If you know your efficiency in points per pound per gallon,
you would construct your extract potential line in the following way. Let’s say
your efficiency is 30 pt./lb./gallon; in other words, one pound of malt in one
gallon yields a wort with an original gravity of 1.030. Find the point on the
x-axis that represents 1 pound per 1 gallon. Trace a line up to 30 — the amount
of points you get for one pound of malt in one gallon of wort — and make a
point.
To make a line using this point, you need another point.
This point on the graph should represent the amount of gravity points you would
receive from another quantity of malt. Let’s pick an easy example; if you had
no malt, you would get no gravity points. In other words, using zero malt means
your beer is water. Water has a gravity of 1.000 (zero gravity points). So,
your line will be drawn from (0,0) to 1 lb./gallon and the SG equivalent of
your extract efficiency. In our example the second point would be (1,30).
If your extract efficiency is expressed as a percentage,
convert it to “points per lbs. per gallons” by multiplying your percentage by
the extract potential of the grain. For example, the extract potential for pale
malt is 38 pt./lb./gallon. If your extraction efficiency is 80%, your
efficiency in pt./lb./gallon is (0.80 * 38) = 30 pt./lb./gallon.
Once you have your own extract potential line plotted,
calculate original gravity as before. Pick a spot on the x-axis that
corresponds to the amount of malt you have. Move up until you intersect your
plotted line representing the extract potential of your malt in your brewery.
Then, move over until you cross the y-axis — and there’s your gravity.
Grains, extracts and adjuncts
To calculate the gravity of a beer made from a mix of
ingredients, find the amount of gravity points contributed by each and add them
together. You can lump together sources with the same or similar extract
potentials. For example, if you use 8 lbs. of pale malt (36 pts./lb./gallon) and 1 lb. of
Munich (35 pts./lb./gallon), count
it as 9 lbs. of pale malt.
Target gravity
You can reverse this method and calculate how much of a
certain type of ingredient you’d need to hit a target OG. Let’s say you want to
brew a barleywine with DME with a starting gravity of 1.100 — how much extract
would you need? To calculate this, start at 110 on the y-axis (labeled 1 on
Figure 2) and trace a line over to
the DME line. From this point, labeled 2, drop the line down to the x-axis. The
value where this line crosses the axis, labeled 3, is the amount of extract per
gallon you would need. Multiply this number (2.2) times the total gallons in
your batch. If you brewed a 5-gallon batch of barleywine, you’d need (2.2 * 5)
= 11 lbs. of DME to attain a gravity of 1.100.
You can still use the graph even if you want to mix
ingredients. For example, let’s say you want to brew the same barleywine with
an all-grain base and added extract. You could decide to make 70 points come
from the grains and 30 points come from malt extract. Determine how much grain
you need to make 70 points, and how much extract you need to make 30 points,
and there’s your recipe.
Calculating efficiency
All-grain brewers will have one final use for the graph —
calculating extraction efficiency. Pick a beer for which you know the amount of
grain used, the OG and wort volume. The best beers to pick are ones made almost
entirely out of pale malt, with no extracts or adjuncts and few specialty
grains. If less than 5 percent of your recipe is crystal malts or other lightly
kilned specialty malts, your estimate will be close.
Plot a point from the amount of grain used per gallon and
the gravity achieved. Draw a line from this point to (0,0). Where the graph
passes 1 on the x-axis is your efficiency in points per pound per gallon for
that beer.
Let’s say you brewed 5 gallons of pale ale with 7.5 lbs. of
pale malt (1.5 lbs. per gallon)
and your original gravity was 1.045. Plot a point at (1.5, 45) and draw a line
from the point to zero. As shown in Figure 3, the line crosses 1 — which
represents 1 pound per 1 gallon — at 30. Thus your efficiency is 30
pts./lb./gallon. When using the graph to calculate efficiency, be accurate when
calculating the pounds of ingredient per gallon of wort. Be sure to use the
volume of wort you actually ended up with in your calculation, not the amount
of wort you planned on.
If you are an extract brewer, you can also plot your
“efficiency” and see the actual number of points per pound per gallon you are
getting from your malt extract. If you actually get, say, 35 pt./lb/gallon when
using a specific brand of liquid malt extract, adjust your graph accordingly
the next time you use that malt extract.
Try it yourself
Figure 4 below is a blank graph that you can use in your brewing notebook. The curves for dried malt extract and liquid malt extract are supplied. Hopefully, it will save you some time calculating and let you spend more time brewing.
