A cross-sectional examination of the accuracy of volume markers on infant feeding bottles available for sale in Australia between December 2013 and February 2014 was carried out. Ninety-one bottles representing 28 different brands were examined. Eighty-eight bottles were hard sided. Volumes in these bottles were marked in a combination of milliliters and ounces. Thirty-six (41%) bottles claimed compliance with the European standard EN14350, five (6%) with non-existent Australian standards, and forty-seven (54%) bottles had no standard claim. Nineteen bottles (22%) had at least one measured marking outside the tolerance of EN14350. Bottles claiming compliance with EN14350 were not less likely to have inaccurate markings than those that made no claim. More expensive bottles did not have fewer inaccurate markings. Three bottles were disposable liner systems and had particularly large volume inaccuracies (up to 43% outside the marked volume). Inaccurate volume markers on infant feeding bottles are a previously neglected but potentially important source of error in the reconstitution of infant formula. Over-concentrated and under-concentrated infant formula can cause serious illness or malnutrition. Over-concentrated infant formula may contribute to obesity. Bottles with inaccurate volume markers are unfit for purpose; disposable liner bottles are particularly poor in this regard and should be prohibited from having volume markers on the bottle casing. To avoid individual or public harms, well-enforced standards are needed. Guidance for parents, carers, and health professionals is needed to ensure that infant formula is accurately reconstituted.