Under the commonly cited traditional rule, flour and water may sit for 18 minutes before the mixture is treated as chametz.
The rule concerns dough made from one of the five grains: wheat, barley, spelt, oats, or rye. Once flour is mixed with water, it must be kneaded, shaped, and baked promptly. If the raw dough remains unattended beyond the accepted limit, fermentation may begin and the dough can become chametz, which is forbidden to eat, own, or benefit from during Passover.
The 18-minute figure is a practical halakhic benchmark, not a universal stopwatch for every situation. Temperature, movement, the condition of the dough, and the opinions of different legal authorities can affect how the rule is applied. Matzah production therefore uses close supervision, rapid work, and frequent cleaning to prevent dough from remaining on equipment.
Another common misunderstanding is that all flour is automatically chametz. Dry flour is not chametz merely because it comes from wheat or another of the five grains; the concern begins when it is combined with water and allowed to stand. Kitniyot, such as rice or beans, are governed by a separate custom and are not themselves chametz.