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Without a deep understanding of what it costs you to live, any discussion of retirement savings or income is pointless. Despite the oft-repeated advice that you will spend some standard percent (perhaps 60% on the low end to 100% or more on the high end) of your pre-retirement income in retirement, what it costs you to live is generally not a function of how much you make! There are millionaires who live like college students, and minimum-wage workers who live like millionaires — for a while — on credit. You’re probably somewhere in between. But do you really know? To get serious about retirement planning, you simply must have an accurate picture of the range of your monthly living expenses. You should understand your bare minimum or fixed expenses, your average expenses, and your ideal expenses — allowing for some luxuries.

The only truly accurate way to determine expenses, is to actually keep track of all your expenses for at least a year, while you are living a retirement-like lifestyle, but before actually retiring. Over the long haul, this is one of the most important few actions you can take to build wealth and retire comfortably! You can track expenses with a desktop tool such as Quicken or an online tool such as mint.com. But it’s still hard, and requires more discipline and attention to detail than many can muster.

Below I’ll offer a baseline number that you can adjust up or down for your situation. But you’ll need more accuracy to be confident. How do you get that if you aren’t the detail type? One approach is to sit down with a good set of budget categories, plus your checking account and credit card statements, and try to estimate a monthly or annual amount for each category. Don’t forget those less-frequent expenses such as home and auto repairs, vacations, and property taxes!

As a baseline, consider the living expenses of two real-life couples in their 50’s, no children at home, living on opposite sides of the country. Both have what I would call a “restrained upper-middle-class lifestyle.” Think smaller houses in upscale neighborhoods, gas-efficient vehicles, few big toys or fancy clothes, careful diets, but plenty of frugal fun: road trips, coffee bars, dining out, books and movies. In both cases, minimum monthly expenses sans mortgage (this assumes your house is paid for), come in at a bit over $4,000/month, or $48,000/year.

If you need more confirmation on the cost of a comfortable lifestyle, consider these data points: In his ground-breaking book Work Less, Live More on early retirement, Bob Clyatt writes that “average budgets for generally well-heeled early-retired couples are around $40,000 to $45,000 [a year]” — about $3,750/month on the high end. And, according to bundle.com — a web site created by a former banker using aggregated spending transactions to find out how people handle their money — the average expenses (again not including mortgage or rent) for higher-income ($125K+) couples age 50-65, no kids, are about $4,675/month or about $56,000/year.

If your lifestyle sounds different from these, you can scale up or down as needed. For example, if you’re willing to live in more modest surroundings, buy used, and eat lower on the food chain, you can probably live on quite a bit less. For example, changing the income to $40-$50K at bundle.com yields a budget of just $2,258/month. On the other hand, if manicured retirement communities, expensive vehicles, and international travel are your idea of retirement living, you could need quite a bit more.

Even more important for a secure retirement than your savings “number,” is your expense number. What is yours? How much will it cost you to live each month in retirement?

(For a couple of quick ideas on reducing expenses, see Wish Lists and Dining Cash.)