Understanding Mash Thickness

Brewing beer and baking bread share much in common. Both are believed to be tightly connected to the rise of human civilization and agricultural practices, both are made from cereal products, and yeast fermentation plays a vital role in both brewing and baking. In the world of home cooks and professional chefs, folks are reminded that it’s OK to go off script when cooking all sorts of foods: Casseroles, marinades, salads, pickled concoctions, soups, sauces, etc. However, that free-wheeling spirit doesn’t work so well with baking, where precision based on prior experience is required for great baked goods, especially when it comes to the ratio of liquid and solids used in bread dough.

We as brewers know that brewing also requires precision for certain key parts of the process. Unlike baking, however, most brewers don’t get too worked up about the ratio of brewing water-to-malt used in the mash. And that’s what this article is about. Whether you call it mash thickness, liquor-to-grist ratio, mash ratio, quarts per pound, or whatever, the ratio of brewing water to total grist weight is something to be mindful of in the brewhouse. Intrigued? Read on!

This article about mash thickness or liquor-to-grist (L:G) is divided into four sections: 

1. Influence of L:G on mash enzyme stability and wort fermentability.

2. Influence of L:G on first wort gravity, first wort volume, and sparge water requirement.

3. Effect of L:G on mash mixing and pumping, decoction mashing, and hot water requirements to increase mash temperature.

4. How L:G is used in strike water and decoction calculations.

One last introductory point is about the terminology of mash thickness. Mash thickness is a unitless number that relates the weight of mash water to the weight of malt and/or other grains in the mash. For example, a mash thickness of 2.5 means that 2.5 kg of water are used in the mash for every 1 kg of malt (or 2.5 lbs. of water per lb. of malt).

Because 1 kilogram of water occupies 1 liter of volume, using the metric system for mash calculations is so much easier than the Imperial system.  But for those that prefer to use the Imperial system only used in the U.S., Myanmar, and Liberia, 1 pound of water occupies 15.3 ounces (8.35 pounds per gallon). 

Enzyme Stability and Fermentability

Enzymatic reaction kinetics, or how quickly reactions occur, are important to the understanding of all enzymatic reactions. Factors influencing enzyme kinetics include enzyme concentration, substrate concentration, product concentration, pH, temperature, and co-factor concentration (for enzymes with co-factors). Mash thickness directly affects the concentration of enzymes and substrates at the onset of mashing and the concentration of products as mashing progresses. And because mashing reactions are hydrolytic, meaning that water is added across chemical bonds when polysaccharides and polypeptides/proteins are cleaved by enzymes, mash thickness affects how much water is available for these reactions.

Simply put, excessively thick mashes below about 2.5 slow mash reactions because water becomes limiting. Most infusion mashes are in the 2.5–3.0 range, mashes between about 3.0–4.0 are the norm for stirred mashes, and thinner mashes reduce enzyme stability at a given pH and temperature (as a frame of reference for American homebrewers, 2.5 = 1.2 qts./lb. while 3 = 1.4 qts./lb., and 4 = 1.9 qts./lb.). None of these generalizations are good or bad, yet understanding the basics allows brewers to gain control over the mashing process.

A general range for infusion mashing is holding the mash at 149–154 °F (65–68 °C), in the 5.4–5.6 pH range, at a thickness of 2.5–3.0, for 30–60 minutes. That’s a fair number of parameters for something that is generally simplified in homebrewing recipes as “mash for 60 minutes.” Temperature, pH, thickness, and time tweaks can all be used to control the mash. One of the challenges, however, is that modern malts tend to be well modified with a generous portion of enzymes. This fact has led many brewers, both home and commercial, to become blasé when it comes to mash control. In many cases, there is nothing wrong with not worrying, but in other cases a bit of worry can help solve some practical problems.

Assume that a brewer typically infusion mashes for 60 minutes at 154 °F (68 °C), at a mash thickness of 2.5, and a pH of 5.4. This works great for most brews, but this brewer wants to produce more fermentable wort for an upcoming Pilsner. My first move would be to simply use a thinner mash at about 3.2 parts water to malt. That’s it; a decision based on experience and backed by science. If this doesn’t get me to where I want to be, my next move would be to stick with the thinner mash and decrease the mash temperature to about 149 °F (65 °C) to lean a bit towards the optimal range for beta-amylase. Other tweaks include prolonging the mash rest and step mashing with an initial mash temperature at about 145 °F (63 °C). The last things I would consider include adding simple sugars to the wort, using exogenous enzymes like amyloglucosidase, and playing with mash pH.

Let’s look at a different problem and consider how a brewer using a stirred mash with a mash thickness of 3.2, rest at 158 °F for 60 minutes, and a pH of 5.4 can decrease their wort fermentability. The root problem is assumed to be “hot,” or highly enzymatic, pale malt. For several reasons, the brewer does not want to change their base malt. The easiest thing to do in this situation is to restrict enzyme activity or make the enzymes more vulnerable to heat denaturation.

The first thing I would do is shorten the mash rest to about 20–40 minutes before beginning wort collection. Even if the mash is not completely converted as measured by the iodine test, I am not going to have an issue with an abbreviated mash because enzymes remain active in the mash and in the kettle during wort collection when no mash-out step is used. This may sound a bit reckless, but it’s normal for many commercial brewers these days to mash-in, take a short coffee break, and then begin to vorlauf and collect wort. My day job is working with commercial brewers of all sizes and it’s amazing to me how common short, infusion mashing has become. And it’s one of the go-to methods used to brew beer with more body.

Another very easy way to pump the breaks on mash enzymes is using a thinner mash to make it easier to denature enzymes with higher mash temperatures. One of the reasons that thick mashes can be hard to control with hot malt is that the enzymes are well protected in a thick mash. Thinning the mash out gives brewers more mash control with smaller changes to temperature.

In the previous example, the brewery is using a stirred mash. This implies the ability to heat and also implies a separate lauter vessel (this is a commercial example). If the mash rest is simply shortened, enzymatic activity continues when the mash is transferred to the lauter tun and during vorlauf and wort collection. Because this total time is longer compared to infusion mashing, it’s important to halt enzyme activity by mashing-off at about 169 °F (76 °C) before transfer to the lauter. My strategy when using a stirred mash would be a thinner mash, shorter time, and mash-off after confirming a negative iodine test.

Yet a third strategy to reducing wort fermentability and boosting the final gravity of beer is to use adjuncts specifically to dilute malt enzymes. Unmalted wheat, oats, barley, corn, and rice can all be used for this purpose. Undermodified malts, namely chit malt, can also be used to dilute malt enzymes because these types of malts are more like barley than malt and lack normal levels of malt enzymes. Adjuncts can be used for lots of fun brewing tricks!

First Wort Gravity, First Wort Volume, and Fermentability

Mash thickness affects first wort gravity, where thinner mashes have lower first wort gravity than thicker mashes. And when you know the first wort gravity of a brew with a known mash thickness, you can estimate the first wort gravity of brews brewed using different mash thicknesses. In Wolfgang Kunze’s Technology Brewing and Malting, he provides 20 °Plato (1.083 SG) as a typical first wort gravity for mashes with 3 kg water-per-1 kg malt. These values can be used to estimate the mash thickness required to produce a target first wort gravity or they can be used to estimate first wort gravity given the mash thickness on a recipe.

Calculating and understanding the water-to-grist ratio helps to predict the outcome of the mash.

Why is estimating first wort gravity or being able to estimate mash thickness required to hit a given gravity useful? Say we are brewing a big beer with a target post-boil gravity of 28 °Plato (1.120 SG) and we want the pre-boil gravity to be at least 24 °Plato (1.101 SG). The easiest way to hit this high pre-boil gravity is to skip the sparge and only collect first wort. The calculation below shows how to estimate mash thickness using Kunze’s values:

Mash Thickness = 20 °Plato ÷ 24 °Plato x 3 = 2.5.

Mash thickness is the ratio of water weight-to-malt weight and can be read as 2.5 kg (same as liters) of water per kg malt or can be read as 2.5 lbs. of water per pound of malt. I like easy math and use metric because it is easier than the English system. The metric system is also the international language of science. To convert metric mash thickness to quarts per pound, the most common unit used for homebrewing and a unit used by zero brewing scientists in the world, simply divide by 2.09. In this example, 2.5 liters water per kg of malt is the same as 1.2 quarts of water per pound of malt (2.5 ÷ 2.09).

We can also easily estimate the volume of liquid left in spent grains by using a little hack based on grain weight. In normal brewing (as opposed to the no-sparge method being used for discussion) where the grain bed is sparged and the brewhouse yield (extract in wort compared to extract in malt) is between 85% and 90%, the weight of water contained in drained spent grains, the type of grain waste commercial brewers briefly store, is about 2–5% greater than the grain weight added to the mash. This is because malt contains about 80% extract by weight and contains about 4% moisture compared to spent grains that contain about 80% water and about 20% solids, mainly husk and protein.

For what it’s worth, I like checking my commercial references against homebrewing references and see there are all sorts of wort retention rates provided in the homebrewing literature, so I did a quick bench trial for reference. Turns out my bench trial is close to my rule of thumb above, but very different from some of the values found sprinkled around the web. Sources aside, my bench trial shows that 1 kg of malt contains 1.2 liters of water after wort is collected and the free liquid is drained from the mash bed (~0.6 quarts per pound). 

So what? Well, we can easily estimate first wort volume by assuming all the free-draining wort is collected in the kettle for boiling. In the previous example with a mash thickness of 2.5, we see that about 1.3 liters of free-draining first wort (2.5 – 1.2) should be able to be collected from each kg of malt used in our mash. Now let’s estimate how much malt we need if we want to collect 22 liters of wort (5.8 gallons) before the boil so we can net a full brew volume of about 19 liters (5 gallons). We know we will pull about 1.3 L of wort from 1 kg of malt, and we want 22 liters of wort before the boil. Therefore, kg malt = 22 liters of wort ÷ 1.3 liters wort. So 16.9 kg malt. This is for no-sparge brewing and that’s a lot of malt!

We can use this same information to determine how much sparge water is required. Let’s start with a new example based on a normal gravity brew with a typical brewhouse yield of 85% for a sparged mash. Our target gravity before boiling is 1.042 (10.5 °Plato), our pre-boil volume is 22 L (5.8 gallons), our recipe calls for 3.6 kg (7.9 lbs.) of malt, and our target mash thickness is 2.5.

The volume of water required by our mash is 2.5 x 3.6 kg or 9 L of water. To run this using English units, divide 2.5 by 2.09 to convert to 1.2 quarts per pound. And we can use this to determine we need 9.5 quarts of mash water to go along with our 7.9 lbs. of malt. The international language of brewing is metric and I unapologetically speak metric as my first language of brewing and science. I know I sound like a broken record but have decided to become more vociferous in my advocacy for metric brewing at
home. Go metric!

Because we plan on sparging this brew, let’s calculate our total water needs and subtract our mash water volume from the total to determine our estimated sparge volume. Our grain bed will retain about 1.2 L of water per kg of malt, so for this recipe that is equal to 1.2 L/kg x 3.6 kg malt or 4.3 L (1.1 gallons). Our total water is the pre-boil volume + volume retained in the mash bed or 26.3 L/6.9 gallons (4.3 liters + 22 liters or 1.1 gallons + 5.8 gallons). Subtract the mash water volume from the total (26.3 L – 9 L or 6.9 gallons – 2.4 gallons) and the sparge water volume is 17.3 L/4.5 gallons.

Mash Mixing, Pumping, and Mash Volume

Pragmatically, brewery mashes that are pumped and stirred work best if they are thin enough to easily move. For this reason, a minimum thickness of about 2.6–3.0 is used for stirred mashing. The best thickness for mixing depends on the mixer design and the vessel geometry; brewers with mash mixers quickly discover why thickness matters when playing around in the thicker end of the range. My go-to mash thickness for stirred mash brewing is 3.2.

Another very practical application of mash thickness is for calculating mash volume. Ever wonder if the brew you have planned will fit in your mash tun or lauter tun? Time to estimate mash volume using grist weight and mash thickness. In this example, we will use 3 for the mash thickness and 3.6 kg for the malt weight. This is a metric calculation that takes advantage of the fact that 1 kg of water occupies 1 L of volume, so if you want mash volume in quarts or gallons then liters must be converted after the volume determination.

Mash Volume (L) = (Mash Thickness + 0.7) x kg malt

Mash Volume (L) = (3 + 0.7) x 3.6 = 13.3 liters

In the previous example where we have 3.6 kg of malt and a mash thickness of 2.5, mash volume = (2.5 + 0.7) x 3.8 or 12.2 L (3.2 gallons). No problem for the typical homebrew system. However, in the first example where we estimated needing 16.9 kg of malt, the same mash thickness will occupy about 54 L (14 gallons). A definite deal breaker for even the most generously sized 19-L (5-gallon) systems!

Strike Water and Decoction Calculations

You need to know the mash thickness in order to calculate the volume to decoct to bring the rest mash up to the desired temperature when decoction mashing.

The last handy dandy things I will touch on about being deliberate with mash thickness is easily estimating strike water temperature (mash water temperature mixed with malt to provide the blended mash temperature) and decoction volumes. Because mash thickness is the ratio of water weight-to-malt weight, the strike water calculation is easy to show in °C and °F. The three variables required to calculate strike water temperature are: Mash thickness, malt temperature, and target mash temperature. For those engineering-minded readers, the specific heats of malt and water are in the condensed energy balances below and appear as 0.38 or the ratio of the specific heat of malt divided by the specific heat of water. In this example, we will use 3 for the mash thickness, 20 °C (68 °F) as our malt temperature, and 65 °C (149 °F) for our target mash temperature.

Strike Water Temp (°C) = 0.38 x [(Target Mash Temp – Malt Temp) ÷ Mash Thickness] + Target Mash Temp


Strike Water Temp (°C) = 0.38 x [(65 °C – 20 °C) ÷ 3] + 65 °C = 70.7 °C

Or in Imperial calculations:

Strike Water Temp (°F) = 0.38 x [(149 °F – 68 °F) ÷ 3] + 149 °F = 159.3 °F

When performing decoction mashes, we need to be able to calculate the volume of mash to boil to provide enough energy to heat the portion of the mash not boiled (known as the “rest mash”). Strictly speaking, we don’t need to know mash thickness for this calculation, but being able to estimate the mash volume sure makes things easy.

The five things we need for decoction calculations are: Mash thickness, total grist weight in mash, mash temperature before the decoction, desired mash temperature after the decoction is added back to the rest mash, and the temperature of the boiled mash when it is added to the rest mash. This math has been simplified by assuming that the portion removed for boiling and the rest mash have the same specific heat. Assumed in the example that follows are mash thickness of 3, total grist weight of 3.8 kg (7.6 lbs.), pre-decoction mash temperature of 65 °C (149 °F), post-decoction mash temperature of 70 °C (158 °C), and boiled mash temperature of 95 °C (203 °F) because things take time and the mash cools once the boil stops. Spoiler alert, this calculation is metric because the grist volume calculation is metric and converting this into an English version requires too many conversions. If you want to convert liters to gallons or quarts, this must be done after the decoction calculation.

Decoction Volume (L) = (Mash Thickness + 0.7) x Grist Weight x [(Post-Decoction Target – Pre-Decoction Temp.) ÷ (Post-Boil Mash Temp. – Pre-Decoction Temp.)]


Decoction Volume (liters) = [(3 + 0.7) x 3.8 kg] x [(70 °C – 65 °C) ÷ (98 °C – 65 °C)] = 2.13 L

Closing Thoughts

There is more than what initially meets the eye with mash thickness. It’s not as crucial as dough hydration, but not something to dismiss as trivial. Whether you are looking for a way to tweak mash enzyme activity, understand a bit more about brewing calculations surrounding mash and sparge water needs, determine mash volume, calculate strike water temperature, or figure out how much mash to boil for a decoction, mash thickness is a vital part of the conversation.

Issue: September 2023