Original Gravity is a key parameter for beer recipe design. The original gravity of a beer is a measure of the potential sugars, about 60-70% of which will be converted into alcohol during fermentation. So original gravity drives both the alcohol content and the residual body of the beer. This week we’ll cover how to estimate OG by hand, though in practice most people use brewing software to do the calculation more easily.
What is Original Gravity?
Original gravity is a measure of the sugars dissolved in the water in your unfermented wort. It is typically measured with a hydrometer or refractometer in the fermenter when brewing is complete but before fermentation has started. The gravity measurement is most often done on a unitless scale that measures the relative density of the wort compared to water. So water would have a specific gravity of 1.000, and beers often start in the 1.030-1.060 range.
OG can also be measured in degrees plato. You can do a rough conversion (its not exactly linear) by taking the “points” of a unitless measurement, and dividing by four. For example a reading of 1.048 is 48 “gravity points” which is roughly 48/4 = 12 degrees plato.
Understanding Grain Potential
First I would like to apologize in advance to those using metric units – this is one of the rare cases where working in English units (gallons and pounds) makes the math easy.
To calculate OG for a recipe, you need to know the “potential” contribution that each grain or extract in the recipe will make. This corresponds to the contribution that a pound of grain or extract will add if dissolved in a gallon of water. The maximum potential is approximately 1.046 which would be a pound of pure sugar in a gallon of water.
Liquid extracts typically have a potential of around 1.036, dry extracts run around 1.044, and pure sugar runs close to 1.046. Grains vary tremendously – from a low of 1.025 up to highs in the 1.040 range. Even Pale malt (which is often around 1.036-1.038) varies depending on the maltster.
You can find good values for the “potential” of various grains from the maltster’s web site or using software like BeerSmith which has hundreds of grains available (including dozens of add-ons).
Estimating Original Gravity Points
Once we have the grain bill and potentials for each extract or grain in the recipe, the next step is to calculate the “points” for each grain contribution and total them up. This is done simply by multiplying the potential points for each grain by the weight of the grain.
Recall that points are simply the fractional part of the potential – so an extract with a potential of 1.046 is simply 46 points. So for a simple stout with 8 lbs of pale malt (1.036 potential) and 1 lb of roast barley (1.025 potential) would give us:
36 points * 8 lbs = 288 points
25 points * 1 lb = 25 points
Total = 313 points.
The next step is to apply an “efficiency” factor to our process. The potentials given for the grain are the maximum possible amount you could draw from the grains if you crushed them under laboratory conditions with no losses. Real mashing processes and subsequent sparging, boiling and transferring are not ideal – so a typical brewhouse has an efficiency number far less than 100%. The brewhouse efficiency number includes all of the losses in the system into the fermenter including mashing, lautering, boiling, trub loss and transferring the finished wort to fermenter. A typical brewhouse efficiency number for a home system is 70-75%. In this case we’ll use 72%
313 points * 72% efficiency = 225.4 points
Now we just divide by the “into fermenter” volume which in this case is 5 gallons:
225 points / 5 gallons = 44.8 points/gal
And that is the original gravity estimate if we convert it back to specific gravity – 44.8 points gives us an OG of approximately 1.045
Using Original Gravity
Now that we have our OG estimate, we can measure our OG when we brew the beer and see how well we did. Assuming you hit your target volume (which also has a large effect on OG) and your OG comes in low, you can reduce your brewhouse efficiency number next time (which will make you use more grains) to drive it up again. Similarly if your OG comes in high you can raise your efficiency number (making you use less grains) to get back on target.
We can also use the OG to get a rough estimate of the final gravity (FG) for our recipe. The simplest way to do this is to apply the average yeast attenuation to the OG to get the FG. For example a yeast strain with an attenuation of 72% would leave about 28% of the remaining sugars unattenuated, giving us 44.8 * 0.28 = 12.5 pts which is a final gravity of 1.012.
What about Mash Efficiency?
You can also estimate the OG using mash efficiency (which accounts only for losses in the mash) instead of brewhouse efficiency. To do this we start with the estimated pre-boil gravity, which is done the same way as we did above, but using the pre-boil volume instead of the “into fermenter” batch volume. This calculation will give us a good estimate of pre-boil gravity.
Next you would need to account for all of the losses in the system from boiling forward. This would include boiling, which concentrates the wort (losing volume, but not gravity points), trub loss (which takes both gravity points and volume away) and any top up water added (which dilutes the wort). The calculation is a bit more complex, but can be done by tracking the changing volumes as well as gravity points remaining in the wort.
That is quick overview of calculating original gravity. As I mentioned earlier, most people use software like my BeerSmith application to make this a bit easier, but its good to know how to calculate by hand.
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