NEW YORK (CNN/Money) - I'm 56 years old and hoping to tap Social Security at 62. I'll also have a small pension. What would be considered a minimum amount of savings someone in my position should have to be able to retire comfortably -- and how should those savings be invested?

-- Anonymous, Cumming, Georgia

You've posed what, for anyone nearing retirement, amounts to the Mother of All Retirement Planning Questions. After all, few of us are going to be able to get by just on Social Security and a company pension (indeed, many of us won't even have the old-fashioned defined benefit company pension that deliver a monthly check for life.) Which means the amount we've accumulated in savings will determine in large part how well we'll live after we retire.

So how do you determine how much is enough? A truly thorough answer to that question would be enough to fill a book. (I speak from experience since my latest book, "We're Not In Kansas Anymore: Strategies for Retiring Rich in a Totally Changed World" deals extensively with this very issue, not just for people in or nearing retirement, but for those in the earlier phases of retirement planning as well.)

But here's the Cliff Notes version of the answer.

You begin by setting a target retirement income level -- that is, the amount of income you will need to live on in retirement. In the earlier stages of retirement planning, it may be okay to set this target by assuming you'll need a given percentage of your pre-retirement income (many planners say 75 to 80 percent, although that figure can easily go to 90 to 100 percent, depending on the lifestyle you want to live).

But once you're within five to 10 years of retirement, you really ought to do an actual budget in which you project your actual retirement living expenses. This may still involve some guesswork, but it should at least give you a fairly accurate idea of your spending, and you can make adjustments later on.

Once you've set your target income level -- for the purposes of an example, let's assume it's $50,000 a year -- you can then see how much of that target you can reach from sources other than investments.

First, go to the Social Security Web site and get an estimate of how much you'll receive from Social Security. If you'll qualify for, say, $12,000 in benefits, then you know you'll need $38,000 a year from other sources.

If you've got a company pension that will pay, say, $13,000 a year, you're now down to $25,000 that must come from your investments. If you have other sources of assured income -- disability benefits, other pensions, whatever -- factor them in as well. For the purposes of this exercise, though, let's assume that you'll receive just Social Security and the company pension.

So now the question is, how large a pile of savings do you need so you can pull out $25,000 a year during retirement without going through all your assets while you're still alive?

At this point, things can get a bit complicated. One issue is that while you may need $25,000 initially to cover your expenses, that figure will have to grow because living expenses tend to rise over time.

That means your savings will have to generate $25,000 in real terms -- that is, the amount you withdraw will grow over time with inflation. Actually, in this case, the amount you draw from your portfolio will have to be a bit more than $25,000 adjusted for inflation.

Why? Because, unlike Social Security, payments from most company pensions aren't adjusted for inflation. So as your living expenses grow over time, the portion of your expenses that your company pension pays for will decrease, requiring you to draw more from your portfolio.

You also have to factor in the way your savings are invested. If you invest very cautiously, as many retirees do -- that is, hold mostly bonds and/or CDs -- then you will likely earn low returns. That means you will need a larger pile of money to create the cash you need than you would if you included some stocks and stock funds in your portfolio to boost your returns. Of course, you don't want to go overboard with equities, since they tend to make your portfolio more volatile and leave you more vulnerable to market downturns.

All of which is to say, there is no single answer to the question of how much money you need in savings to retire comfortably. There is, however, a rule of thumb I believe can be somewhat useful. For every $1 of income you'll need to draw from your savings in retirement, I'd say you should have $20 to $25 socked away.

So for the scenario we've created here -- you need $25,000 initially that will be adjusted for inflation plus a bit more -- I'd say you should have somewhere between $500,000 and $625,000 socked away in investments.

I stress, however, that this rule of thumb is just a very rough guideline. If you retire early, are healthy and are likely to spend a long time in retirement, you may be drawing on your investments 35 years or longer. That argues for a $25-to-$1 rather than $20-to-$1 ratio ($625,000 in the case above rather than $500,000). If you're retiring later, then maybe you can get by with something closer to $20 to $1.

You've also got to factor in how you intend to invest, how much leeway you have in reducing your expenses if it seems you're going through your stash more quickly than you thought and just how much you're willing to risk running out of cash while you're still alive.

Ideally, you should do more than rely on a rule-of-thumb estimate. You can get a decent idea of how long different size nest eggs might last given different withdrawal rates and different investment strategies by going to the Retirement Income Calculator at the T. Rowe Price Web site.

A new service from Fidelity Investments also gets at this issue. (For details, see the story I wrote on it earlier this year). And, of course, there's always the option of having a financial planner crunch the numbers for you.

But the important thing is that, one way or another, you do some sort of decent analysis before you retire to see whether you'll have enough income and assets to live comfortably. Fail to do that, and you're essentially making luck and guesswork the foundations of your retirement security.

Walter Updegrave is a senior editor at MONEY Magazine and is the author of "We're Not in Kansas Anymore: Strategies for Retiring Rich in a Totally Changed World."