A cup of brewed coffee is mostly water. Even so, many home brewers spend years on beans and grinders before they look at the water. That is a missed step.
Water changes how a coffee tastes. It also changes how your machine ages. This guide explains what is in water, why reverse osmosis (RO) is a popular starting point, and how to try mineral kits without turning coffee into a chemistry class.
What is in your water
Water carries dissolved minerals. Two matter most for coffee.
General hardness (GH) mostly measures calcium and magnesium. These minerals help pull flavor compounds out of the coffee grounds. They also affect how the cup tastes.
Carbonate hardness (KH, or alkalinity) mostly measures bicarbonate. It acts as a buffer. It reduces acidity in the cup. Too much can make coffee taste flat. It can also leave scale in your machine.
Total dissolved solids (TDS) is a rough total of everything dissolved. It is easy to measure with a cheap meter. It does not tell you which minerals are present. Use it as a quick check, not as the whole story.
Too soft, too hard, or just right
Very soft water, such as distilled or pure RO, has almost no minerals. Coffee brewed with it often tastes thin, sharp or dull. The water lacks the minerals that help extraction.
Very hard water has the opposite problem. It can taste flat and chalky. It also builds scale inside kettles, boilers and heating elements. Scale can shorten the life of a machine.
Most good brewing water sits between the two. Coffee organizations publish target ranges. Use them as starting points, not strict rules. Your beans, roast level and brew method all shift what tastes best.
Why many people start with RO
RO water is stripped of nearly all minerals. That sounds bad, but it gives you a blank page. You can then add back the minerals you want, in amounts you choose. Starting from a known zero is easier than fighting your tap water.
Hoon looks at a commercial example in this video. It covers a high-efficiency RO system designed for the demanding needs of commercial foodservice.
Home options are simpler. Some people buy bottled RO or distilled water. Some use an under-sink or countertop RO filter. Pick what fits your budget and space.
Mineral kits and homemade recipes
There are two common ways to add minerals back.
Pre-made kits come as sachets or concentrates. You add them to RO or distilled water. They save time and give repeatable results.
Homemade recipes use food-grade salts, such as calcium chloride and potassium bicarbonate, mixed into a concentrate. You then dilute the concentrate into RO water. This gives you more control. It also needs a scale and care.
Hoon compares the two approaches in this video, where he sets Third Wave Water against his own homemade water recipe to see how big the taste difference is.
If you want a hands-on look at making your own, Hoon also has a video on a scale-free water recipe for an espresso machine and how to make a concentrate.
For a shortcut, see the Apex Water mineral kit in the gear library.
Test your own water
Try a simple, fair test. Keep everything the same except the water.
In his video on mineral kits, Hoon tests the Apex Water kit, with its Tonic, Lilac and Jam blends, against plain RO water. He brews a washed Ethiopian with notes of white peach, jasmine and lychee, then compares acidity, body and floral aromas.
You can copy that approach at home. Follow these steps.
- 1Pick one coffee with clear flavors. A washed, floral or fruity coffee shows differences best.
- 2Prepare two or three waters: plain RO, plus one or two mineral blends.
- 3Use the same dose, grind, brewer, water temperature and brew time for each cup.
- 4Brew each cup, label them, and let them cool a little.
- 5Taste in a blind order if a friend can pour for you.
- 6Write down acidity, sweetness, body and aroma for each cup.
- 7Keep the water you liked best. Repeat with a different coffee to confirm.
Water and the rest of your routine
Water works best when the rest of your brew is steady. Use fresh beans, a repeatable grind and a scale. The pour-over fundamentals guide covers that base.
Hoon also shares a brew routine in "Tricolate Routine with Perfect Water". If you are new to the idea of mineral kits, "Pro Tips - How to use Third Wave Water" is a short place to start.
A few practical habits help:
- Store mixed water in a clean, sealed container.
- Rinse your kettle and tank often.
- Descale on a schedule, even with good water.
- Taste your water by itself. If it tastes odd, so will your coffee.
Next steps
- Follow how to fix sour or bitter espresso if water is not your only suspect.
- Learn pour-over fundamentals and test your water there.
- Review the Apex Water kit and other tools on the compare page.
- Browse every water video in the library.
Written for this fan-built hub from Hoon's public videos; numbers are starting points, and your beans, grinder and taste have the final say.
