The equation
Grain and water reach a common temperature somewhere between where they started, weighted by mass and by specific heat. Grain absorbs about a fifth as much heat per unit mass as water, which is where the 0.2 comes from.
Metric ratios convert at 1 qt/lb = 2.086 L/kg, which the calculator handles for you.
Ratio changes how much overshoot you need
A thin mash carries more heat relative to the grain, so it needs less overshoot. A thick mash needs more. At 3 L/kg with grain at room temperature you are looking at roughly 6 °C above target; at 2 L/kg it is closer to 9 °C.
This also means a thin mash is more forgiving. If you are new to infusion mashing or your temperature control is uncertain, mash thinner — the same error in strike temperature moves the mash less.
The tun will steal heat
The equation assumes only grain and water exchange heat. A cold mash tun is a third body and will take several degrees out of the mash, and a metal one takes more than a plastic cooler.
- Pre-heat the tun with hot water and dump it just before doughing in. This is the reliable fix.
- Failing that, add 1–2 °C to the calculated strike temperature for an insulated cooler, more for uninsulated metal.
- Whatever you settle on, it is repeatable for your equipment. Write it down and reuse it.
If you miss
Stir first and measure again — an unevenly mixed mash reads whatever the thermometer happens to be near. If it is genuinely low, add boiling water in measured amounts; if high, add cold water or stir with the lid off. Correct early. Ten minutes into a mash the starch conversion has already been shaped by the temperature it started at.