The formula, and where the 0.2 comes from
The single-infusion strike relation used here is
Tstrike = (0.2 / R) x (Tmash − Tgrain) + Tmash
where R is the water-to-grain ratio in quarts per pound. The 0.2 is the approximate ratio of the specific heat of crushed malt to that of water — grain absorbs roughly one fifth as much heat per unit weight as water does for the same temperature change. Work the standard case through it: 11 lb of grain at 70 degrees F, mashing at 1.25 qt per pound, targeting 152 degrees. That gives (0.2 / 1.25) x (152 − 70) + 152, which is 0.16 x 82 + 152, or 165.1 degrees F. That is the number the published tables give for the same inputs, which is a reasonable check that the form is right.
The relation is an energy balance with two convenient simplifications. It treats the specific heat ratio as a constant when it varies a little with moisture content and crush, and it ignores the mash tun entirely, which is the assumption that actually costs you degrees on brew day. That is what the tun allowance field is for.
The tun steals heat, and it steals a different amount every time
An empty cooler at room temperature absorbs heat from the strike water the moment it goes in, and it keeps absorbing until the plastic reaches mash temperature. A ten gallon insulated cooler typically costs one to three degrees. A stainless kettle used as a mash tun costs more, and it keeps costing because it also loses to the room. A vessel you preheated with a gallon of boiling water for ten minutes costs close to nothing.
The honest way to handle this is not to trust the default. Brew one batch with the allowance set to zero, record where the mash actually landed, and set the field to the shortfall for every batch afterwards. Once you have that number for your equipment it is stable, and it is the single change that moves strike temperature from a guess to a repeatable setting. Two degrees is a starting guess for an insulated cooler, not a value to keep using.
Ratio, and what it actually changes
| Ratio | Character | Consequence |
|---|---|---|
| 1.0 qt/lb and below | Stiff | Very stable temperature, hard to stir, dry pockets, leaves plenty of room for sparge |
| 1.25 to 1.5 qt/lb | Standard | Stirs cleanly, holds well enough, the range most published recipes assume |
| 2.0 qt/lb and above | Thin | Easy to stir and mix, swings with every addition, crowds the tun and cuts the sparge |
People argue about whether ratio affects fermentability. The effect exists and it is small compared with mash temperature, which is why this calculator treats ratio as a volume and equipment decision rather than a flavour one. What ratio definitely changes is how much strike heat you need — a thinner mash has more water buffering the grain, so the strike temperature sits closer to the mash temperature. At 1.0 qt/lb the example above needs 168.4 degrees; at 2.0 qt/lb it needs 160.2.
Absorption and deadspace are equipment numbers, not constants
Grain holds onto water and does not give it back. The figure most commonly used is 0.10 to 0.13 gallons per pound, and where you land inside that depends on your crush, how hard you squeeze the grain bed, and whether you sparge or just drain. A finer crush holds more. A batch sparge with a good drain gives it up more readily than a fly sparge that stops early.
Deadspace is whatever sits below the pickup and never leaves the tun. It is fixed for your equipment and you can measure it in ten minutes with a jug of water: fill, drain until it stops, and measure what is left. Neither of these is worth guessing at when both are measurable, and both of them land directly in your pre-boil volume and therefore in your gravity.
Where the mash temperature goes from here
Mash temperature sets fermentability, and fermentability shows up weeks later as final gravity. If you want to see what a given mash produced, feed the two gravity readings into the ABV and attenuation calculator — a beer that attenuates well past the yeast's published range usually mashed lower than you thought. The volume side of the same brew day is handled by the brewing water and boil-off calculator, which picks up where the sparge figure here leaves off.
Questions people ask
My mash landed below the target. What do I add?
Boiling water, stirred in immediately, and less than you think. Roughly speaking each quart of boiling water added to a five gallon mash raises it about two degrees F, but that depends on your ratio, so add half of what you estimate, stir thoroughly, and read again before adding more. Do it in the first ten minutes — after that the enzymes have already worked at the wrong temperature and correcting it changes less than you would like. If the mash landed high, cold water works the same way and is far less forgiving, because you cannot take water back out.
Should I preheat the mash tun?
It makes the strike temperature repeatable, which is the real benefit. A gallon of near-boiling water swirled in the cooler for ten minutes and dumped out brings the vessel close to mash temperature and removes most of the tun allowance from the calculation. The alternative is fine too: skip the preheat every time and set the allowance to whatever your tun actually costs you. What causes trouble is preheating sometimes, because then the offset changes between batches and your mash temperature wanders for reasons you cannot see.
Does grain temperature really matter, or can I assume room temperature?
It matters more than most people expect because it is multiplied by the ratio term. In the standard example, grain at 50 degrees F rather than 70 raises the required strike temperature by 3.2 degrees. Grain stored in an unheated garage in winter, or in a hot attic in summer, is not at the temperature of the room you are brewing in. Stick a thermometer in the sack an hour before you mash in rather than assuming, particularly if the grain was moved that morning.
Why does the calculator show grain displacement separately?
Because it decides whether the mash physically fits, and it is easy to forget. Crushed malt displaces about 0.08 gallons per pound once wet, so an 11 lb grain bill takes nearly a gallon of tun volume on its own before any water goes in. A 10 gallon cooler with 11 lb of grain at 1.25 qt/lb is holding about 4.3 gallons total, which is comfortable. The same cooler with 25 lb of grain at 1.5 qt/lb is at 11.4 gallons, which does not fit. Check it before brew day, not while holding a hot kettle.
Can I use this for a step mash or a decoction?
Only for the first infusion. This calculates a single strike that brings grain and water together at one target. A step mash needs the same energy balance run again for each infusion, with the current mash volume and temperature standing in for the grain and its temperature, and a decoction removes and boils part of the mash which the formula does not model at all. For those, work each step separately or use software built for it. The first infusion is the same arithmetic either way, so this gets you to the start of the schedule.