Ask Mr. Wizard

More Mash Space


Marty Coatney • Lebanon, Oregon asks,

I’m looking at increasing the size/brewing capacity of my homebrewery. Right now I am using two 5-gallon (19-L) beverage coolers and they limit me to around 13 to 14 pounds (5.8-6.4 kg) of grain if I mash at a 1.25 qts/lb. ratio. When I upgrade, I want to be able to do bigger beers. How do I determine the grain capacity of any given mash tun of a certain volume?


Sizing brewing vessels is part of my job with the Paul Mueller Company. When I am sizing mash mixers for brewing applications where the mash is conducted in a stirred and heated vessel and wort separation is conducted in a separate lauter tun I focus on two primary parameters. The first is the range of grist weights used in the mash and the second is the typical mash thickness (liters of water/kg malt), which is usually between 3.0–3.5 in stirred mashes. The mash thickness that you are using is equal to 2.6 when you convert everything to weight. All of the routine brewing calculations I perform are done using metric units. If I want to switch to English units I do this after my primary calculations because metric calculations are much clearer to me.
Mash volume can be calculated by the following:

Mash Volume (liters) = (Mash thickness + 0.7) x kg malt

The 0.7 in the above equation accounts for the gain in volume that happens when 1 kg of malt is added to water and is empirically derived. Using an example based on your set up I will assume the mash thickness is 2.6 and the maximum grist weight is 14 pounds or 5.9 kg. The calculated mash volume is (2.6+0.7) x 5.9 or 19.5 liters (5.15 gallons). This agrees with your numbers.

Sizing a lauter tun is a different problem. The best way to think of a lauter tun or an infusion mash tun is like a filter. Filters are sized based on area and so are wort separation devices. The critical process parameter to consider is the grist load on the false bottom. This value varies from about 150 kg/m2 on the low end up to about 300 kg/m2 on the high end when brewing big beers with lauter tuns designed for normal strength (12 °Plato) lager beers. The grist load is used to determine the diameter of a mash tun or a lauter tun.

Let’s look at an infusion mash tun that will be built with a false bottom design (as opposed to a copper pipe manifold), and target a grist load of 250 kg/m2, a relatively normal load for this type of design (infusion mash tuns have higher grist loading than lauter tuns). I want to determine the diameter of this vessel and will begin by calculating the required area based on a maximum grist bill of 5.9 kg. To calculate area, simply divide 5.9 kg by 250 kg/m2 and the result is 0.024 m2 (0.25 ft2). This corresponds to a circle with a diameter of less than 7 inches (A=πr2). And this makes absolutely no sense in the eyes of the homebrewer. Why?

Commercial mash tuns and lauter tuns have grain beds that are considerably deeper than what is used at home. In the commercial world of really high-speed lauter tuns, grain beds are rarely any thinner than about 9 inches (23 cm), and most craft brewers using lauter tuns have grain bed depths ranging from 12–24 inches (30–60 cm). Infusion mash tuns have deeper grain beds ranging from 24–36 inches (61–91 cm). So when it comes to designing the homebrew mash tun looking at the commercial world is not as helpful because the design would lead one to build a very odd looking vessel that is tall and skinny. Although the vessel would function, it would be something that could not be purchased off the shelf and would be expensive.

The good news is that designing the mash tun with a thinner grain bed is not a problem, as long as the bed is no thinner than about 6 inches (15 cm) deep. This corresponds to a grist load of about 80 kg/m2 or a diameter of about 12 inches (30 cm). This is more what one would expect and is in-line with the typical 5-gallon (19 L), round water cooler like the one pictured on page 15.

This means that when you upgrade your system you have a few options. You can either scale up using a grist load of 80 kg/m2, or you can scale up using a higher grist load. The advantage of scaling up using a higher grist load is that you may be able to find something readily available that meets your needs rather than staying with the lower grist load value. Whatever you decide, good luck with the project!

Adding body, minimizing hop sludge

Response by Ashton Lewis.