The kind of water to use for baby formula should be safe, clean, and free of harmful contaminants. The best water options for preparing baby formula are:

• Freshly boiled tap water that has cooled to about 70°C (158°F) or room temperature. Boiling kills harmful bacteria and is especially recommended for babies under 3 months, premature infants, or those with compromised immune systems.
• Distilled water or purified bottled water labeled as deionized, purified, demineralized, or distilled. These types of water have most impurities removed.
• Tap water is generally safe in the US if it meets local safety standards but should be boiled for very young or high-risk infants.
• Avoid using well water unless it has been tested and is known to be free from contaminants like nitrates.
• Spring water is not recommended for daily use due to its mineral content.
• Bottled water is safe only if it is boiled first because it is not sterile.
• Do not use artificially softened or repeatedly boiled water.

For infants under six months, the safest approach is to boil water and let it cool before mixing formula. For older babies with good water sources, plain tap water may be acceptable without boiling. Fluoridated water is safe but excessive fluoride exposure over a long time can cause mild dental fluorosis. Thus, the recommended water for formula preparation is either safely boiled tap water or distilled/purified water. Always ensure the water is safe and free from harmful substances before use for infant formula.