Why a coffee puzzle, why today
29 September is National Coffee Day in the US, and coffee chains are handing out free cups all over the country. Two days later comes International Coffee Day on 1 October. It's the first one since the UN General Assembly formally designated the date, in Resolution A/RES/80/248 on 10 March 2026. That makes three days when a huge number of people are thinking about the drink they have every morning.
Most of us never learn why one cup is sour and the next one is bitter. Baristas fix it with a loop called dialling in: brew, taste, adjust one thing, brew again. Dial It In turns that loop into a puzzle you can play in a few minutes, and by the end you'll know how to fix your own coffee at home.
The brewing control chart
Coffee people describe a cup with two numbers:
- Extraction yield (EY) is the share of the dry coffee that ended up dissolved in your cup. The Specialty Coffee Association's target is about 18–22%. Under-extracted coffee tastes sour, salty and thin, because acids dissolve first. Over-extracted coffee tastes bitter and drying, as harsher compounds keep coming out.
- Strength (TDS), or total dissolved solids, is how concentrated the drink is. Filter coffee usually sits around 1.15–1.45%. Espresso is far stronger, roughly 8–12%.
Put EY on the x-axis and TDS on the y-axis and you get the brewing control chart. Every brew in the game lands on it as a numbered dot, so you can see exactly where you missed.
How the model works
Behind the scenes, js/sim.js uses a deliberately simple model tuned to match real-world reference points:
- Each bean has a maximum extractable amount (about 25.5–29.5%) and a rate. Darker roasts are more soluble, and dense light roasts are less so.
- Extraction climbs towards that maximum as
EY = max × (1 − e−X). X grows with a finer grind, hotter water, longer contact time and more water per gram of coffee. - For espresso and pour-over, brew time comes from the grind: finer grounds slow the flow. For immersion brewers you set the steep time yourself.
- Strength is simply
dose × EY ÷ drink weight. Paper and grounds hold back about twice the coffee's weight in water (less for an AeroPress, none for espresso, where you weigh the liquid).
The reference recipes land where the textbooks say they should. A shot of 18 g in and 36 g out in about 28 s at 93°C gives roughly 20% EY and 10% TDS. Fifteen grams of filter coffee to 250 g of water gives about 1.35% TDS. Real coffee also depends on water chemistry, grinder quality, channelling, freshness and much more, so treat the game as a way to build intuition, not a lab instrument. All 25 campaign orders were checked against the model to make sure each one is solvable and none is solved by the starting recipe. Rush Hour builds every order from a working recipe, so it can always be solved too.
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Sources
Every fact on the site comes from these sources, checked on 29 September 2026:
- Coffee extraction (EY ranges, TDS strength norms, brew ratios): Wikipedia
- Espresso (1884 Moriondo, 1901 Bezzera, 1948 Gaggia, 9 bar, caffeine per shot, drink definitions): Wikipedia
- Coffee (arabica/robusta, caffeine, peaberries, 1582 etymology, Kaldi legend, kopi luwak): Wikipedia
- History of coffee (Yemen, Mecca 1511, Istanbul 1554, Venice 1645, Oxford 1650/1654, de Clieu, Palheta, Saint-Domingue, 1852, 1999): Wikipedia
- Production figures, 2023: List of countries by coffee production
- Coffee culture (fika, Italian bar customs, Vienna, Japan 1888, Finland): Wikipedia
- International Coffee Day and the 2026 UN resolution: International Coffee Organization
- SCA extraction guidance, and sour vs bitter: Clive Coffee, Espresso Outlet
- Turkish coffee on the UNESCO list (2013): UNESCO
- Viennese coffee house culture (2011): Austrian Commission for UNESCO
- Egg coffee, 1946: Café Giảng
- Irish coffee, 1943: Foynes Flying Boat & Maritime Museum
- Greek frappé, 1957: Wikipedia
- Melitta Bentz, 1908: German Patent and Trade Mark Office
- Moka Express, 1933: MoMA. Chemex, 1941: MoMA
- AeroPress, 2005: AeroPress. Hario V60: European Coffee Trip
- French press, 1929 patent: Wikipedia. Vacuum brewer: Wikipedia
- Finnish consumption: thisisFINLAND
- National Coffee Day 2026: TODAY
Credits
Concept, code, illustrations and writing are original to this site. The fonts are Fraunces, DM Sans and DM Mono, from Google Fonts. The customers and the bean lots in the game are fictional, but the growing regions they're named after are real. Nothing here is health or nutrition advice.