How Little Dark Malt It Takes to Change a Beer's Colour
Beer colour looks like the most subjective thing in a recipe and is in fact one of the most predictable. A small amount of dark malt does almost all of the work.
Malt colour units, then Morey
The SRM calculator works in two steps. Each grain contributes malt colour units in proportion to its weight and its colour rating in degrees Lovibond, divided by the batch volume. The total MCU then goes through the Morey equation:
SRM = 1.4922 × MCU0.6859
That exponent below one is the important part: colour saturates. Doubling the MCU does not double the SRM, because beyond a point the beer is already as dark as the eye can read.
A little roast goes a very long way
Take 5 kg of pale malt at 3 °L in a 20 L batch: MCU 6.26, SRM 5.25 — a pale gold. Now add roasted malt at 500 °L:
- +100 g → SRM 14.35, amber-brown
- +300 g → SRM 27.19, dark brown to black
Three hundred grams is 5.7% of the grist and it takes the beer from gold to nearly black. That is why dark malts are measured carefully and added last, and why a recipe's colour is far more sensitive to a small specialty addition than to the base malt.
Lovibond and SRM are not the same scale
A source of constant confusion. Degrees Lovibond describe the colour of a malt; SRM describes the colour of the finished beer. The calculator takes Lovibond for each grain and returns SRM for the batch.
EBC is the European equivalent of SRM and runs roughly twice the number — a 10 SRM beer is about 20 EBC. Mixing the two is an easy way to brew something twice as dark as intended.
What the number means to the eye
Roughly: 2–4 pale lager, 5–7 pale ale, 8–14 amber, 15–20 brown, 20–30 dark brown, above 30 effectively black. Above about 40 the eye cannot tell one from another in a glass — a stout and an imperial stout look identical until held to a light.
That saturation is the practical meaning of the Morey exponent. Adding more roast malt beyond that point changes flavour, not appearance.
The model has known limits
Morey is an approximation fitted to typical batches, and it is least reliable at the extremes. Very dark beers are compressed by the exponent, and the equation cannot know about kettle caramelisation, boil length, oxidation in the bottle, or how much colour the yeast drops out with the trub.
Treat the result as a planning figure accurate to a few SRM, not a specification.
Volume matters as much as weight
MCU is per unit volume, so the same grain bill in a smaller batch is darker. Missing your volume target changes the colour along with everything else — another reason to measure post-boil volume rather than assume it.
Design colour last
The practical order is to build the grist for the flavour and gravity you want, then check the colour and adjust with small specialty additions. Because dark malts are so concentrated, tens of grams are enough to move the number several SRM without meaningfully changing the fermentables.
Working the other way — choosing malts to hit a colour — tends to produce beers that look right and taste like a compromise.