What is a decoction mash?

A decoction mash raises the mash temperature by pulling out a portion of the mash, boiling it, and stirring it back in. The boiling portion adds heat to lift the whole mash to the next rest. It is the traditional way to step-mash without direct heat and adds malty depth and colour.

How is the decoction volume calculated?

The volume to pull equals the total mash multiplied by the temperature rise divided by the gap from the current rest up to boiling. In symbols, decoction equals total mash times (target minus start) over (boil minus start). The boiling portion carries the extra heat needed to reach the target rest.

Should I pull a thick or thin decoction?

Pull a thick decoction, meaning more grain than liquid. Keeping most of the liquid and its enzymes in the main mash protects conversion, while the grain-heavy pull provides the malty flavour and colour from boiling. Thick decoctions are standard for step-up rests in traditional schedules.

What temperatures do the classic rests use?

Common rests include a protein or ferulic acid rest near 45 to 52 degrees Celsius, a beta-amylase rest around 62 to 65 degrees, an alpha-amylase rest near 70 to 72 degrees, and a mash-out around 76 to 78 degrees. A triple-decoction schedule steps through several of these by boiling successive pulls.

Why does the returned decoction need to be boiling?

The formula assumes the pulled portion is at boiling, since that is the maximum heat it can carry back. If you return it cooler, it adds less heat and the mash falls short of the target rest. Boil the decoction fully, then stir it back in gradually while watching the main mash temperature climb.

Decoction Volume Calculator

The Decoction Volume Calculator tells you how much of your mash to pull, boil, and return in order to step up to the next rest temperature. Decoction is the traditional, heat-source-free way to run a step mash, and it is essential for authentic German lagers, Pilsners, and Hefeweizens.

How it works

In a decoction mash, you remove a portion of the mash, bring it to a boil, and stir it back into the main mash. The boiling portion carries enough extra heat to raise the whole mash to a higher rest. How much you pull depends on how far you want to climb and how far the boiling pull is above the current rest:

Decoction volume = total mash * (target - start) / (boil - start)

The numerator is the temperature rise you need, and the denominator is the full gap from the current rest up to boiling. The larger the step, the bigger the fraction of mash you pull. The calculator shows the volume, the fraction, and how much mash stays in the tun.

Worked example

A 20 L mash resting at 52 °C, stepping up to 64 °C, with a 100 °C decoction boil:

Decoction = 20 * (64 - 52) / (100 - 52)
= 20 * (12 / 48)
= 20 * 0.25
= 5 L

So you pull 5 litres of thick mash, boil it, and stir it back in to lift the whole mash from 52 to 64 degrees Celsius.

Tips and notes

Always pull a thick decoction with more grain than liquid. This keeps most of the enzyme-rich liquid in the main mash, protecting conversion, while the grain-heavy pull delivers the malty flavour and colour that boiling produces. Stir the returned decoction back in slowly while watching the mash temperature, and stop once you reach the target rest, since a vigorous return can overshoot. The formula assumes the decoction is returned at a full boil, so boil it completely. For a multi-step or triple-decoction schedule, run the calculator afresh at each rest, using the new current temperature as the start each time.