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Rule-of-Thumb for Estimating Post-Retirement Expenses

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The first step in retirement planning is estimating how much money you'll need. A popular rule-of-thumb claims that you will only need about 70% of your pre-retirement income to maintain your lifestyle in retirement. While you will probably save some money currently being spent on work-related items (such as formal clothes and commuting), other costs go up in retirement (health care, hobbies, etc). 70% may be a useful rule of thumb, but some people find 50% is plenty while others feel they need 100%.
Source: http://www.investorguide.com/igu-article-745-retirement-planning-an-overview-of-retirement-planning.html