This calculator converts gravity reading between specific gravity (SG) and degrees Plato (oP).
In brewing Specific gravity (SG) is simply the density ratio between a sugar solution and pure water. For example, a sample with a specific gravity of 1.040 is 1.040 times more dense than pure water. As more sugars are dissolved the solution, the density, and thus the SG, increases.
In brewing, degrees Plato (oP) is the ratio of the weight of dissolved sugar in a solution to the total weight of the solution. In other words, oP represents the percentage of a solution’s weight that is made up by the sugar dissolved in the solution. A 10 oP solution would contain 10% sugars by weight and the rest water.
The formula used for this calculation was taken from Page 312 in “New Brewing Lager Beer” by Gregory J. Noonan. The most precise formula presented by Noonan is as follows:
oP = 135.997 * SG^3 – 630.272 * SG^2 + 1111.14 * SG – 616.868
To obtain the inverse relationship to convert from oP to SG, a series of points were plotted in Microsoft Excel using the formula presented by Noonan with oP as the independent variable and SG as the dependent variable. A 3rd order polynomial was fitted to the data, and the resulting equation was obtained:
SG = 6.253E-8 * (oP)^3 + 1.277E-5 * (oP)^2 + 3.868 * (oP) + 1.000