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Reference

Decaf, and why it behaves differently.

Decaf is not a fourth roast — it is a separate axis. Decaf beans are roasted light, medium or dark like any others, so both settings apply at once. What decaffeination changes is how the bean behaves, not how it tastes.

The hardest combination in the app: light-roast decaf. It exists, but it is uncommon outside specialty roasters, and decaffeination strips some of exactly the delicate aromatics a light roast is bought for. If you have both, the fault is the combination, not your technique.

What decaf actually changes

It is not caffeine-free. Around 97% is removed, and measured decaf espresso shots have come in anywhere from about 3 to 16 mg each. That is irrelevant as a preference and can matter if you are avoiding caffeine on medical advice.
You cannot judge a decaf's roast level by its colour. Decaffeination darkens the bean before roasting even begins, so a decaf that looks dark may well be a medium. Go by what the bag says, not by what you see.
Decaf beans are more brittle and more porous. They shatter finer at the same grinder setting and they give up their solubles faster. Almost every decaf problem traces back to one of those two facts.
Decaf stales faster. Buy smaller bags more often. A decaf three weeks past roast is further gone than a regular coffee at the same age.
On processes: Swiss Water uses water and activated carbon, CO2 uses pressurised carbon dioxide, and sugarcane or ethyl acetate processing is common in Colombian decaf. All three are standard in specialty coffee, and the difference they make in the cup is smaller than the marketing around them suggests.

The adjustment, by method

Espresso

Expect shots to run fast. Grind two or three steps finer than your usual and accept a little less crema — decaf produces less of it.

Through milk

Grind finer and consider a slightly longer ratio. Decaf's lighter body is far easier to hide under milk than in a straight shot, which is why a decaf cappuccino is the easiest decaf drink to get right.

Filter

Two effects pull against each other here, so taste before you trust either. Decaf is more soluble, which argues for grinding coarser to avoid over-extraction; it is also more brittle and throws more fines, which speeds the brew and argues for grinding finer. In practice the difference on filter is small — start at your usual setting, drop the water to about 91 °C, and adjust one step in whichever direction the cup asks for.

Cold brew

Shorten the steep by two or three hours. Decaf's porosity means a full 16 hours can push it into woody, papery territory.

Flash brew and espresso tonic

Decaf loses most of the high florals that flash brew exists to capture. It works, but it is the least rewarding decaf application in the app.

Turkish and Vietnamese

No adjustment needed. These methods are so extraction-heavy already that decaf's faster extraction barely registers.

See it applied per recipe

The full roast-and-grind reference lets you set light, medium or dark and toggle decaf on top of it.

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