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What, exactly, is a "lifestyle"? We’re all chasing a better one, but what does that mean on a day-to-day basis? Well, in financial terms, your lifestyle is reflected most clearly in your recurring expenses: the financial obligations that you commit to each month and that seem necessary to support your way of living.

We’re talking here about essentials like housing, groceries, transportation, insurance, utilities, and taxes. And we're also talking about a variety of discretionary expenses such as entertainment, memberships, subscriptions, maintenance plans, and personal services.

Look at the list above: Can you cut back in any of these areas without affecting the quality of your life? Whether your goal is building wealth, retiring early, or just making your dollars go farther, controlling your living expenses will pay huge dividends.

Recurring expenses are especially important—and insidious—for a number of reasons. For starters, these expenses are often automatic. They hit your bank account like clockwork every month while your attention is elsewhere. Unless you track your expenses or balance your account, you may not even notice them. But each one costs you, and unless you take action, they will go on forever.

Businesses love recurring charges, which represent steady income at very low cost. So companies are skilled at making these expenses easy for you to add on impulse—often requiring a simple consent or web form—but hard to stop unless you pick up the phone or send a written cancellation. Even the most ethical companies have little incentive to help you minimize your monthly charges. Their policies and procedures are necessarily oriented to persuading you to tack on new ones. So it's up to you to be vigilant.

Relying on the Rule of 300

Recurring expenses may seem small or insignificant, but, from the perspective of retirement or financial independence, they are all substantial. Why? Because of what I call the Rule of 300: "The amount of money you must save to meet a monthly expense in retirement is approximately 300 times that expense."

Where does that factor of 300 come from? It stems, simply, from two multipliers. The first, 12, is easy to understand: To convert a monthly expense to an annual one, you must multiply by the 12 months in a year. The second multiplier comes from the well-known “4% rule” for withdrawal from retirement savings. (That rule is under attack as possibly too optimistic, but that only makes the need to control recurring expenses even stronger.) The 4% rule is another way of saying you need to save 25 times your annual expenses to retire safely. So 25 is the second multiplier. Combine these two multipliers, 12 times 25, and you get my “Rule of 300” for the amount you must save to cover a monthly expense in retirement.

For example: Say you commit to a seemingly insignificant $30-per-month membership. A dollar a day sounds cheap, and you think you’ll enjoy the convenience. But, once you stop working, you'll need to have saved $30 times 300—or $9,000—to pay for that membership from your investments! Yes, believe it or not, a mere "dollar a day" expense actually represents about $9,000 in required retirement savings. How long will it take you to save that much? And is it worth it?

Don't get me wrong. It's really important to enjoy life. I'm a big fan of occasional splurges, fun treats along the road to financial independence. I'd be the last one to deny the simple joy of an occasional latte, the delight of opening a new book, the excitement of an evening on the town. But these are all one-time expenses: They don't inflate your lifestyle. And you can easily reduce or eliminate them, if needed. No phone calls, negotiations, or transaction costs required.

Recurring expenses, even small ones, deserve serious consideration before you sign on the bottom line. I set a very high bar for committing to any new recurring expenses and recommend you do the same. Before you decide that a regular financial commitment sounds "cheap," multiply it by 300, then picture how much work it will take for you to save that number.

Darrow Kirkpatrick is a software engineer and author who lived frugally, invested successfully, and retired in 2011 at age 50. He writes regularly about saving, investing and retiring on his blog CanIRetireYet.com. This column appears monthly.

More from Darrow Kirkpatrick:
The Single Most Important Thing You Can Do to Achieve Financial Success
The One Retirement Question You Must Get Right
How to Figure Out Your Real Cost of Living in Retirement