This is a post from staff writer Robert Brokamp of The Motley Fool. Robert is a Certified Financial Planner and the adviser for The Motley Fool’s Rule Your Retirement service.

Whether you can retire, and whether your money will last after you retire, starts with a very simple maxim: spend less than you have. However, once you start actually crunching some numbers, you find that the equation of retirement is actually quite complicated, with many variables that have different consequences. And that’s a good thing, because it gives you options — different levers you can pull to shore up your retirement security.

What are those levers, and which will have the biggest impact on your retirement? As with many things regarding financial planning, the answers depend partially on your unique circumstances. However, in this article we’ll discuss the factors common to most retiree-wannabes, and quantify their results for two hypothetical workers – one 35-year-old and one 50-year-old — using the “Am I saving enough? What can I change?” calculator found on Fool.com. That calculator produces results in terms of the number of months your retirement will be fully funded. As the tool estimates the impact each variable has on our test subjects, we will report the results in terms of additional years of a fully funded retirement, just so you don’t have to divide the results by 12 in your head (not that we don’t trust your math skills — we just think it makes more sense to think in terms of years).

And now, let’s lay out the starting point for each of our guinea pigs, whom we’ll call Fergie and Madonna.

Name Fergie Madonna
Current Age 35 50
Income 50,000 75,000
Age at retirement 65 65
Monthly retirement income $4,167 $4,688
Current value of 401(k) $50,000 $250,000
Annual savings $5,000 $7,500
Years retirement is funded 11.4 16.9
Age at which savings is depleted 76.4 81.9

As we go through this exercise, don’t focus too much on their particular numbers or how much those profiles are similar to yours. What we’re investigating is how many years of fully funded retirement are added due to various changes. The magnitude of those effects will be similar regardless of where Fergie, Madonna, or you are starting.

And now, let’s pull some levers.

Strategy 1: Increase savings rate from 10 percent to 15 percent
Years added to Fergie’s retirement: 4.8
Years added to Madonna’s retirement: 3.0

Let’s start with the no-brainer: Saving more will boost your retirement security. The younger you are, the bigger the impact. Remember that your savings rate is the combination of your contributions to your investment accounts as well as an employer match, if you get one. So someone who saves 10 percent but also receives a match of 50 cents on the dollar up to a saving rate of 6 percent is actually saving a total of 13 percent.

Strategy 2: Retire later
Years added to Fergie’s retirement: 2.8 at age 67, 10.0 at age 70
Years added to Madonna’s retirement: 5.0 at age 67, 8.1 at age 70

Retiring later can be very powerful, for three reasons: additional years of saving, additional years for portfolio to grow, and higher Social Security benefits. Also, while not captured in our analysis, another factor in your retirement security is how long your retirement will last, which is determined by when you’ll retire and when you’ll expire. You can control only one (assuming we don’t want to get macabre here), and the later you retire, the shorter your retirement will be. The benefits of retiring later also apply — though not as large — to working part-time in the first few years of retirement. All that said, a strategy of working a few years later is contingent upon being physically able to keep punching the clock.

Strategy 3: Require less retirement income
Years added to Fergie’s retirement: 10.3
Years added to Madonna’s retirement: 6.9

Our original scenario assumed that Madonna could live on 75 percent of her preretirement income, and Fergie would require 100 percent (since she’s not reached the ideally higher income she’ll have right before retirement). If we lower those percentages to 65 percent and 75 percent, respectively, then their retirements look much better. Some retirees can make it on $35,000 to $45,000 a year, especially if the mortgage has been paid off. But that income may not be enough to provide the retirement you aspire to. Also, there’s a big wild card when it comes to income needs, and that’s health care. The costs are rising, and they’re being covered less and less by Uncle Sam and former employers. Projected income needs should build in the possibility of unexpected increases in expenses.

Strategy 4: Get a lump sum due to downsizing, inheritance or other source
Years added to Fergie’s retirement: 1.8
Years added to Madonna’s retirement: 3.2

This is tricky to quantify since the benefit depends on the size of the lump sum and when it’s invested. For our calculations, we assumed each Fergie and Madonna received a $50,000 windfall at age 50. The most likely source of such a chunk of change would be downsizing, which might be a good strategy for those who bought a big house many years ago in order to raise kids who have since left the nest. As for inheritances, they are big question marks since you don’t know what someone else’s estate will be worth or how much of it you’ll inherit. But those who are confident they’ll get something from someone might include a conservative estimate in their calculations.

Many other important factors

While those four are significant variables in your retirement equation that you might be able to control, several other factors will play a part. Here are just a few:

  • Investment returns: We assumed a 6 percent annual return for our calculations. Whether that turns out too pessimistic (as we hope) or optimistic, time will tell. But had Fergie and Madonna earned 8 percent a year, their retirements would essentially be fully funded. While that sounds oh-so-promising, don’t bet on getting bailed out by markets.
  • When to take Social Security, and what the program will look like: The decision about when to begin receiving benefits is not simple, especially if you’re married. Choosing the right strategy for your situation can provide higher benefits for the rest of your life. Of course, given the financial challenges facing the program and the country as a whole, it makes sense for younger workers to assume they’ll get three-quarters or less of what they’re currently projected to receive.
  • Income growth: Our analysis assumed that Fergie’s and Madonna’s income would grow at the rate of inflation, yet for most professionals, income actually grows faster. If our hypothetical workers were real go-getters and earned raises that exceeded inflation by two percentage points, that would fund another one to three years of retirement, due to bigger contributions to investment accounts and higher Social Security benefits.

Calculate, monitor, repeat

As you can see, your retirement has a lot of moving parts — some you can control, many you cannot. The good news is that a few tweaks here and there can have a large collective impact – and the sooner you begin tweaking, the better. No financial tool can predict the future, but some number-crunching can determine if you’re headed in the right direction, and the potential consequences of changing one or a few variables. Once you’ve done the analysis and taken action, monitor regularly — at least once a year. The road to retirement will take many twists and turns, but keeping your hand on the steering wheel and checking the map every once in a while will increase the chances that you’ll get there safe and sound.

This article is about Investing, Planning, Retirement