Estimating your retirement expenses ahead of time isn't always the easiest thing to do. Until you're actually in retirement, you may not realize how much you'll ultimately spend on things like transportation, entertainment, and healthcare. But one thing's for sure: If you hope to live comfortably as a senior, you should plan on spending at least 70% of what your expenses amounted to during your working years. And to pull that off, you'll need a healthy level of income.

Yet some seniors may be limited to just $18,168 a year in income. And if you're not careful, you could easily wind up in the same boat.

Relying too heavily on Social Security is a mistake

Many people neglect their retirement savings and assume they can fall back on Social Security instead. But that's a mistake that could leave you completely cash-strapped as a senior.

Older man holding pen over document

Image source: Getty Images.

Social Security is only designed to replace about 40% of your former wages, assuming you earn an average wage during your career. But as we said earlier, you'll generally need at least 70% of your previous wages to maintain a decent lifestyle.

In fact, the average senior on Social Security today collets a mere $1,514 a month , which amounts to an annual income of $18,168. If that doesn't sound like a lot of money to live on, you're right -- it's not. And if you want to put yourself in a position where your retirement income is higher, you'll need to take savings matters into your own hands.

Secure your own future

Not only does Social Security pay the average recipient just $18,168 a year, but there's a distinct possibility that benefits will be cut in the not-so-distant future. As such, it really pays to build your own nest egg so that you're not overly reliant on those benefits during your senior years.

If your employer offers a 401(k) plan, signing up is a smart move. That way, your contributions will be deducted automatically from your paychecks, and you may be entitled to free money in the form of an employer match. If you don't have access to a 401(k), open an IRA through any bank or financial institution that offers them and save on your own. The good news is that if you contribute to either account consistently throughout your career, you'll put yourself in a strong position to enter retirement with enough savings to enjoy your senior years.

Case in point: If you manage to contribute $400 a month to either account over 40 years, you'll wind up with a total of $958,000 if your investments in your 401(k) or IRA generate an average annual 7% return. That's a bit below the stock market's average, and so it's a reasonable assumption to work with.

If you don't have 40 working years ahead of you, you may need to save more money on a monthly basis. But if you sock away $600 a month for 30 years, you'll still end up with $680,000 in savings, assuming that same 7% return.

While Social Security plays a crucial role in keeping millions of seniors afloat, it's not enough money to live comfortably on. If you're having a hard time motivating yourself to save for retirement, ask yourself this: Does living on $18,168 seem pleasant, or even doable? If the answer is a resounding no, then you know what you need to do.