One of the things that annoys me about home brewing is the endless variation and confusion when it comes to units of measure.
To make things slightly clearer, I’ve decided to find out what connects degrees Plato to specific gravity.
Both specific gravity and °Plato are used to measure gravity, which is another way of saying the density, of a liquid, in our case beer.
Among the several units of measure are the main two used by home brewers: °Plato and specific gravity.
What Is Specific Gravity?
Specific gravity of a liquid is calculated by dividing its density by the density of water.
Water has a density of (approximately) 1000 kg/m3. Let’s say you have another liquid with a density of 1050kg. Using the above formula, you can calculate its specific gravity like this:
Specific Gravity and Brewing
All this is of interest to brewers because adding sugars, which yeast ferment to make beer, increases the density of water. By measuring this increase, we can estimate the beer’s strength.
Brewers measure beer and wort gravities at various points in the brew process. The two most commonly used terms are original gravity (density before fermentation) and final gravity (density after fermentation). For more about this have a look at this page.
Typical specific gravities for wort and beer are between 1.000 and 1.120.
To make matters more confusing, gravity is sometimes given in “brewer’s points”, which are the equivalent of 0.001 in a specific gravity figure.
For example, gravity of 1.001 is 1 brewer’s point, 1.010 is 10 and 1.100 is 100 brewer’s points.
In the Home Brew Manual I use the specific gravity system, i.e. 1.010 etc.
What Is °Plato?
°Plato were developed in the 18th century and are a more accurate measure for brewers.
Rather than general density they refer to the amount of sucrose in a liquid. Sucrose is used above other sugars because it increases the density the most.
°Plato refers to percent of sucrose in a liquid by weight. This is useful to brewers because it expresses the fermentability of the wort.
As with specific gravity, the scale is just another way of expressing percentages. 5 °Plato indicates that a given volume of water contains 5% sucrose, measured by weight.
What Connects Degrees Plato To Specific Gravity?
The relationship is non-linear and involves calculation.
However, as a rough rule of thumb, you can imagine that 1 °Plato corresponds to four brewer’s points. A gravity of 1.012 (twelve brewer’s points) for example is approximately the same as 3 °Plato.
This only holds true up to about 13 °Plato.
This article explains it more fully if you’re interested in calculating yourself.
Why Measure Gravity At All?
It’s useful to take gravity readings because the difference between original and final gravities can be used to estimate the alcohol content of the finished beer.
It’s also an indication of whether or not fermentation has taken place.
Efficiency of your setup can be monitored by comparing original gravity readings with predicted yields given by brew software or calculations.
There are many reasons to measure gravity, whether you use °Plato or not.