More than 50% of people underestimate their average daily personal water usage by half, according to a survey carried out for Irish Water.
The Commission for Regulation of Utilities says the average person uses 129 litres of water a day.
However, 56% of respondents to the survey said they believed that the average person uses less than 50 litres a day.
20% of people believe they use between 51 and 100 litres a day, 14% believe it is between 101 and 200 litres and 10% said the average daily usage is more than 200 litres.
The survey of 1,000 people was carried out by Behaviour & Attitudes for Irish Water between 11 and 19 July.
It also found that 65% of people had made a "concerted effort" to conserve water.
The survey period coincided with the recent heatwave that has seen water restrictions imposed on many homes, as well as the introduction of a national hosepipe ban.
When asked for their reasons for conserving water, 34% said the most compelling reason was to future-proof and safeguard water supplies.
28% of people said they wanted to minimise the environmental impact and 22% believed that saving money on water treatment meant it could be used elsewhere in the economy.