A step mash holds the grain at several temperatures in sequence, such as a protein rest, a saccharification rest, and a mash-out. Each rest favours different enzymes. Infusion stepping raises the temperature by adding hot water rather than applying direct heat.

How is the infusion volume calculated?

It comes from a heat balance. The water you add must supply enough energy to raise the whole mash from its current temperature to the target. The equation is volume added equals (target minus current) times (0.2 times grain plus current water) divided by (infusion temp minus target).

Where does the 0.2 come from?

0.2 is the specific heat of grain relative to water (water is 1.0). Grain absorbs heat at about a fifth the rate of the same mass of water, so the term 0.2 times grain weight converts the grain into its water-equivalent thermal mass.

What temperature should the infusion water be?

Boiling water at 100 degrees C is most common because it raises temperature with the least added volume, keeping the mash from getting too thin. Some brewers use cooler water for small steps. The tool works with any infusion temperature you enter.

What if the volume needed is too large?

If a single boiling-water infusion would make the mash too thin or overflow the tun, you have reached the limit of infusion stepping. Switch to direct heat or a decoction for that step. A negative or impossible result means the target is at or above your infusion water temperature.

Step Mash Infusion Calculator

This calculator tells you exactly how much boiling or hot water to infuse to step your mash up to each rest temperature, using the standard mash heat-balance equation. It is the tool that makes infusion step mashing repeatable instead of guesswork.

How it works

When you add hot water to a mash, the heat it carries must raise the entire mash — grain plus existing water — from its current temperature to the target. Balancing that energy gives the infusion equation:

Wa = (Tt - Tc) * (0.2 * G + Wm) / (Ti - Tt)

where:

Wa = water to add (litres)
Tt = target rest temperature
Tc = current mash temperature
G = grain weight (kg)
Wm = current mash water volume (litres)
Ti = temperature of the infusion water (e.g. 100 C boiling)
0.2 = specific heat of grain relative to water

The 0.2 * G term is the key idea: grain stores heat at about a fifth the rate of the same mass of water, so it behaves like 0.2 * G litres of extra water thermally. Once water is added, the new mash volume becomes Wm + Wa, which the tool carries forward to the next step automatically.

Worked example

You have 5 kg of grain in 13 L of water resting at 52 C (a protein rest) and want to step to 66 C for saccharification, infusing boiling water at 100 C:

Wa = (66 - 52) * (0.2*5 + 13) / (100 - 66)
   = 14 * (1 + 13) / 34
   = 14 * 14 / 34
   ≈ 5.76 L

So add about 5.8 litres of boiling water. The mash is now 13 + 5.76 ≈ 18.8 L, which becomes the starting volume for stepping up to a mash-out at 76 C.

Notes and limits

Boiling water (100 C) raises temperature with the least added volume, keeping the mash thicker; cooler infusions need much larger volumes.
If a step needs more water than your tun can hold, switch to direct heat or a decoction for that rest.
A target at or above the infusion water temperature is impossible by infusion — the tool flags it. All math runs locally in your browser.