How much boiling water do you add to raise the mash?
The same heat balance that sets your strike temperature runs the other way for an infusion: the water you add has to carry enough heat to lift the grain and all the water already in the mash. Palmer's form:
Wadd = (Ttarget − Tnow) × (0.2 × grain + water in mash) ÷ (Twater − Ttarget)
Grain in lb, water in qt, temperatures in °F — or grain in kg, water in liters and the 0.2 becomes 0.41. Boiling water is 212°F / 100°C at sea level; less up a mountain.
Palmer's own worked schedule: 8 lb of grain doughed in with 10 qt at 104°F. Raising it to 140°F takes 5.8 qt of boiling water; raising it again to 158°F takes another 5.8 qt — the second step needs as much water as the first for a smaller temperature jump, because there is now more mash to heat. The mash ends at 21.6 qt, 2.7 qt/lb: thin, and that is the whole story of infusion step-mashing in one number.
Mash-out infusion table
The step almost everyone actually does: boiling water to take the mash to 168°F / 76°C before running off. Per pound of grain, by where you're resting and how thick the mash is — with the thickness you'll end up at.
| Resting at | Mash thickness before, qt/lb → boiling water per lb (thickness after) | |||
|---|---|---|---|---|
| 1.25 | 1.50 | 1.75 | 2.00 | |
| 146°F · 63.3°C | 0.72 qt (2.0) | 0.85 qt (2.3) | 0.97 qt (2.7) | 1.10 qt (3.1) |
| 148°F · 64.4°C | 0.66 qt (1.9) | 0.77 qt (2.3) | 0.89 qt (2.6) | 1.00 qt (3.0) |
| 150°F · 65.6°C | 0.59 qt (1.8) | 0.70 qt (2.2) | 0.80 qt (2.5) | 0.90 qt (2.9) |
| 152°F · 66.7°C | 0.53 qt (1.8) | 0.62 qt (2.1) | 0.71 qt (2.5) | 0.80 qt (2.8) |
| 154°F · 67.8°C | 0.46 qt (1.7) | 0.54 qt (2.0) | 0.62 qt (2.4) | 0.70 qt (2.7) |
| 156°F · 68.9°C | 0.40 qt (1.6) | 0.46 qt (2.0) | 0.53 qt (2.3) | 0.60 qt (2.6) |
| 158°F · 70.0°C | 0.33 qt (1.6) | 0.39 qt (1.9) | 0.44 qt (2.2) | 0.50 qt (2.5) |
Boiling water at 212°F; multiply qt/lb by 2.09 for liters per kg (thickness columns 2.6 · 3.1 · 3.7 · 4.2 L/kg). Palmer's infusion equation from this page's engine. Cold or uninsulated tun: it will absorb a little, so expect to land a degree short.
Why infusion mash-outs run out of room
Look at the "after" numbers: a mash-out from 152°F at 1.5 qt/lb needs 0.62 qt of boiling water per pound and finishes at 2.1 qt/lb — you've added 40% more water to the tun. In metric: 5 kg in 15 L at 66°C needs 7.1 L to reach 76°C and ends at 4.4 L/kg. On a big grain bill in a cooler that was already comfortably full, that water isn't there to add — which is why the calculator tracks tun volume (grain displaces about 0.32 qt per pound, 0.67 L per kg, on top of the water) and warns before you find out with a mop.
It does two things: stops enzyme activity where you want it, and thins the wort so it runs off faster and rinses better. Batch spargers mostly don't need it — the first runnings are drained in minutes and the sparge water can be hot. Fly spargers benefit, because runoff takes an hour and viscosity matters. BIAB and all-in-one brewers with a heating element just raise the temperature directly. If your tun can't take the water, skip it or pull a small decoction: a third of the thick mash boiled and returned raises the rest without adding a drop.
Which rests do you actually need?
With modern, fully modified malt — which is nearly all of it — the honest answer is one saccharification rest, and maybe a mash-out. The classic ladder (acid rest ~95°F, protein rest ~122°F, beta rest ~145°F, alpha rest ~158°F, mash-out) was designed for undermodified malt that needed help breaking down proteins and cell walls; run a 20-minute protein rest on today's pale malt and you're mostly degrading the proteins that give the beer its head and body. The exceptions are real but narrow: a step schedule for a wheat beer heavy in raw or flaked adjuncts, a beta-then-alpha "Hochkurz" for a drier German lager, a ferulic-acid rest at 111°F for a hefeweizen's clove. Plan those here; skip the ritual.
Overshoot the strike and the same equation, run with cold water, brings you back: enter a target below the current temperature and a water temperature of 60°F / 15°C. Because cold tap water is so far from the target, the volume is small — a couple of quarts fixes most overshoots — and it barely changes the thickness. Do it in the first few minutes; a mash that spends ten minutes at 158°F when you wanted 150 has already set its fermentability, and cooling it afterwards won't give it back.