Beer Colour (Morey's Equation)

Colour does not add up. Two pounds of roasted barley do not make a beer twice as dark as one pound does, because light passing through wort is absorbed exponentially — the Beer-Lambert law, named for an entirely different Beer — and the darker the liquid already is, the less difference each additional absorber makes. Every beer-colour correlation exists to bend the linear MCU figure back onto that curve, and Morey's is the one most recipe software uses.

Its form, $SRM = 1.4922 \times MCU^{0.6859}$, is a power law with an exponent below one, which is what produces the flattening. At 5 MCU it returns about 4.5 SRM, a modest correction. At 50 MCU it returns about 21, not 50 — the linear estimate would have been more than twice too dark. Randy Mosher and Ray Daniels published earlier fits with the same shape and different coefficients, and Dan Morey's is a least-squares fit that reconciles them across a wider range; all three agree closely in the middle and diverge at the extremes, which is the honest signature of extrapolation.

SRM and EBC both measure the absorbance of a wort sample at 430 nm and differ only in the multiplier applied, so $EBC = 1.97 \times SRM$ exactly, by definition. That makes them true units, convertible without an equation, and it is why this page's answer can be read on either scale. Degrees Lovibond cannot join them. It is a nineteenth-century visual comparison against tinted glass, and its relation to SRM is itself an approximate fit — usually quoted as $SRM = 1.3546 \times L - 0.76$. Grain is still sold in Lovibond, beer is measured in SRM or EBC, and the two scales quietly coexist on the same spreadsheet.

Past about 40 SRM every correlation is guessing, and the reason is physical rather than mathematical: the beer is opaque. A stout and a considerably blacker stout differ by a great deal of roasted malt and by almost no measurable absorbance, because the sample has already stopped transmitting light. The classic mistake is to trust the second decimal place of a predicted colour and adjust a recipe by a few grams of chocolate malt. Boil length, kettle geometry, wort pH and lot-to-lot malt variation each move the outcome by more than that. Use the number to choose between amber and brown, not to argue about 17.4 against 18.1.