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Decoction Mash Calculator

Tell it where the mash is now, where you want the next rest, and how many decoctions you're running. It returns the share of the mash to pull and boil at each step — as a percentage and in whatever volume units you work in, from quarts and gallons up to hectoliters and barrels.

—Mash volume

—Temperature climb

—Decoction steps

—Boiling point

The mash

How much mash are you working with?

Units

Mash volume input

Volume unit — for the volume fields below

Total grain weight

Mash water added

gal

The schedule

Starting point, finish line, and steps.

Classic schedules — one tap loads the traditional rests; edit anything after

Current / starting mash temp

°F

Final target temp

°F

Decoction steps

Altitude — estimates boiling point

Boiling point — measured or estimated

°F

Set your altitude and the boiling point fills in automatically. If you've measured your kettle's actual boil temp, type it straight into the boiling-point field — that always wins.

Rest temp reached after each decoction — evenly spaced by default; edit any to match your recipe

Pull extra for transfer loss — optional safety margin, 0 to ignore

A returned decoction loses a little heat in transit and the tun soaks up some too. A few percent extra lands you on target without overshooting on the next infusion.

Pull schedule

Show volumes in — pick any combination

How the numbers are worked out

Each step is a straight energy balance. You pull a fraction of the mash, boil it, and stir it back in; the rest sits at the current temp. Setting the heat gained equal to the heat lost gives the share to pull: fraction = (target − current) ÷ (boil − current). Because it's a ratio of temperature differences, the answer is the same whether you work in °F or °C.

This treats the mash as one even thermal mass — i.e. the portion you pull is the same grain-to-water ratio as the whole. Real brewers pull a thick decoction (more grain, less liquid), which carries less heat per quart, so in practice you scoop a bit more than the bare formula says. That's what the transfer-loss margin is for, and why measuring the temp after returning each decoction beats trusting any calculator blind.

Boiling point is estimated from altitude as 100 °C − 0.0034 × meters (about 1 °F lost per 500 ft). Mash volume from grain assumes grain displaces roughly 0.08 gal/lb (≈ 0.67 L/kg) of liquid when submerged. Both are good working figures — override them whenever you have a real measurement.

Decoction Mash Calculator · A brewer's working tool

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Energy-balance method · verify against the mash thermometer

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