In a world that includes black IPAs, why get hung up on the color of your beer? Sure, in a homebrew competition, where beer is evaluated against style guidelines, the difference between 5 and 10 SRM can determine whether your Munich Helles (expecting 3–5 SRM) is in style. It turns out, though, that even for the casual drinker, color can impact how a beer is perceived. A deeper hue can set the expectation of a richer malt profile. At the other end, a lighter tinted stout may seem thin-bodied, regardless of its gravity or roast profile.
That’s why I still pay attention to the color and try to control it, even when I brew beers that don’t fall into a classic style. If you’re already using brewing software such as BeerSmith, it might automatically present a color swatch based on the recipe, but you can work this out with nothing more than a calculator. It’s just math, not magic.
Running the Numbers
Beer color is generally assessed as Standard Reference Method (SRM) values, which are determined by measuring the attenuation of light passing through the beer. Without special equipment, we can match a beer against a color guide to estimate the SRM value. That’s fine when you already have the beer, but we want to figure out the color in advance.
The simplest rule of thumb is to take the Lovibond ratings for each grain addition, relative to the amount of grain and volume of wort. This leads to a very simple calculation for Malt Color Units (MCUs):
MCU = (number of pounds of grain) x (grain color in degrees Lovibond) / (volume in gallons)
So, if you used 10 pounds of 1.8 L 2-row barley in a 5-gallon batch, this would work out to:
MCU = 10 x 1.8 / 5 = 18 / 5 = 3.6
For a more complex recipe, the MCUs can be individually calculated and added together. Let’s add a pound of 60 L crystal malt to the mix:
MCU = (10 x 1.8 / 5) + (1 x 60 / 5) = 18 / 5 + 60 / 5 = 3.6 + 12 = 15.6
Unfortunately, MCUs aren’t quite the same as SRM, especially once you get higher than 6 or 8 SRM. That doesn’t mean that MCU calculations are useless, though. We just need some refinement.
Honing In
The MCU calculation takes the right basic factors into account and the relationship between them seems sensible; the problem is that the color doesn’t follow a linear scale. Randy Mosher and Ray Daniels each took a shot at fine-tuning the formula. They both maintained the mathematical simplicity of a linear approach by multiplying the MCU value and adding a constant, and, in turn, each calculation yielded closer approximations.
Dan Morey also started with the MCU value, but he decided to fit it into a power equation to better match the non-linear aspect of the SRM. His formula looks a little more complex but is easy if you have a calculator:
SRM = 1.4922 x [ MCU ^ 0.6859 ]
You take the MCU value and raise it to the 0.6859 power, then multiply the result by 1.4922. This is less intuitive than the MCU formula, but Morey derived this by analyzing beer recipes and their measured SRM values. His equation holds well for beers that are lighter than 50 SRM, which covers every BJCP style (50 SRM is effectively black). Your eye won’t detect a difference for higher values, especially in a beer glass.
We can apply Morey’s formula to our 2-row/crystal malt example above to get a better approximation of the SRM:
SRM = 1.4922 x [ 15.6 ^ 0.6859 ] = 1.4922 * 6.5821 = 9.8
Practical Application
Morey’s equation can give you a good estimate of how dark your beer will be, but keep in mind that some malts offer their own nuanced shades, such as the orange hue of rauchmalt or roast malt’s reddish tint. Also remember that a longer boil will darken your beer further.
Next time you develop a new recipe, try running the numbers to see what the formula predicts and tune it to the shade you want. Once the beer is brewed, pour a sample and see how well the math worked for you.