The water temperature for dough is not a fixed number; it is adjusted to reach the target dough temperature. For most yeasted doughs, the target is 24–27°C / 75–80°F. Choose the water temperature based on the temperature of the flour and kitchen: use lukewarm water in a cool kitchen and cold water in a warm kitchen. One limit does not change: water above 50°C / 122°F can damage the yeast. The right number is not only in the recipe; it also depends on the kitchen that day.
The speed of fermentation depends on dough temperature as much as on the amount of yeast. The same recipe can take very different amounts of time when the dough starts at 22°C / 72°F rather than 28°C / 82°F. Recipes give a time based on an assumed dough temperature; if that assumption is wrong, the timing will be too. For this reason, use 24–27°C / 75–80°F as a target, and adjust the water temperature as the variable used to reach it. That is also why the same recipe can take hours longer or shorter in winter and summer if the water temperature is not adjusted.
A rough calculation is often enough. If the flour and kitchen are at room temperature—around 20°C / 70°F—use lukewarm water and the target should be easy to reach. If the kitchen is cold, use slightly warmer water. In summer or in a warm kitchen, do the opposite: use cold water or, if needed, ice water. When using a mixer, friction also warms the dough, so choose water that is 1–2°C / about 1.8–3.6°F cooler and verify the result by measuring it.
A kitchen thermometer makes the measurement, which takes only a few seconds. Immediately after mixing the dough, insert the thermometer into its center. If the reading is within the target range, follow the recipe’s timing. If it is below the range, fermentation will take longer; if it is above, fermentation will be faster, so plan accordingly. If you take and record this measurement for every batch, you can learn after a few attempts what water temperature your kitchen needs and may no longer need the thermometer. You can also measure the flour temperature; flour stored in the refrigerator will be noticeably colder than flour at room temperature.
There are 2 limits. The upper limit is clear: water above 50°C / 122°F kills the yeast, and the dough will not rise; there is no way to rescue it, so start the dough again. Water that feels hot to the touch may already be near this limit, so do not use boiling water. The lower limit is not a danger, only a delay: dough made with ice-cold water will still rise, but much more slowly. You can choose cold water deliberately.
If you are activating active dry yeast, use a separate portion of water that is warmer; it is part of the recipe’s total water and must be counted in that amount. Instant yeast is mixed directly into the flour, so this intermediate step is not needed. For sourdough, the water temperature is still adjusted to reach the target dough temperature, but the starter’s own temperature also plays a role. In any case, do not let the water contact the salt and yeast at the same time. For dough that will ferment in the refrigerator, use warmer water so fermentation can begin before the dough cools.