How strike water temperature works
Why cold grain lowers mash temperature, how the 0.2 grain heat constant works, and how to hit your rest temperature on the first try.
The heat exchange behind a mash
When you dough in, hot water and cool grain reach a shared temperature somewhere in between. How far the water falls depends on the mass of grain, the mass of water and how much heat each can hold. Water holds far more heat per unit weight than grain does, so a pound of grain only pulls the water down a little compared to a pound of cold water. The formula captures this balance with a single ratio, letting you predict the settled temperature before you ever pour.
Why the constant is 0.2
Grain has a specific heat of roughly 0.2 relative to water, meaning it absorbs about a fifth as much energy for the same temperature change. That is why the strike water can sit ten to fifteen degrees above your rest target and still land on the mark once the grain warms up. Dividing 0.2 by your water to grain ratio scales the effect to your particular mash thickness. A thinner mash has more water per pound of grain, so the grain moves the temperature less and the strike temp needed is lower.
Mash thickness and its trade offs
Mash thickness, expressed as quarts of water per pound of grain, changes both the strike temperature and the character of your mash. Thicker mashes near 1.25 quarts per pound resist temperature drops and give enzymes a concentrated environment, but they can be harder to stir. Thinner mashes near 1.75 quarts per pound are easier to mix and can favor a more fermentable wort, at the cost of holding heat slightly less well. Whatever you choose, enter the same figure here so the strike temperature matches your actual pour.
Dialing it in on brew day
Treat the calculated strike temperature as a strong first guess, not a guarantee. Preheat your mash tun with a splash of hot water and dump it before mashing in, since a cold tun can rob several degrees. Stir thoroughly and check the temperature after a minute once the grain has fully wetted. If you overshoot, stir in a little cool water; if you undershoot, add a measured amount of boiling water and stir again.