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.