Two-thirds of US adults surveyed think at least sometimes about how much they need to save and invest for retirement, according to the January 2008 ING-commissioned "Retirement Number Study" conducted by Ipsos.

Although more than half of respondents said that they had calculated the money needed for retirement (also known as the "retirement number"), over one-third said all they could do is guess.

Nearly one out of two said that calculating the number is not easy, and that they wouldn't know where to start.

More than four out of 10 US adults said they did not like thinking about their retirement number, and nearly as many (39%) said it was boring.

Many respondents did not know what needed to be included when considering retirement, such as living expenses, life expectancy and age of retirement.

ING used the data to promote its online retirement calculator. Many retirement savings sites now offer such tools, which help users determine the amount they will need after retiring.