It's hot. Really hot. The humidity is awful. The kids hate this. And I don't have time to take them to the beach.

We should put a swimming pool in. That spot in the side yard would be a perfect place for one. But a decent in-ground pool is expensive, and money's kind of tight. I don't think I can get a HELOC right now with property values where they are. And it would be insane to put something like that on a credit card.

But wow, it's hot out.

Oh hey, I could take a loan from my 401(k). I'd just be borrowing my own money, right?

Stop. Stop right there.

It's easy, it's common, and it's tempting
Look, I know that many people think of 401(k) loans as no big deal, and I know how common they are. Years ago, I spent some time answering calls in a center run by a big 401(k) plan provider. People called in with all kinds of requests -- investing questions, paperwork requests, all kinds of stuff -- but far and away, the most common request was for a loan.

According to recent figures from Fidelity, 23% of plan participants between the ages of 30 and 49 have at least one retirement plan loan outstanding. Now, I'm sure that some of those loans reflect genuine economic hardship -- the economy has been through the ringer recently, after all -- but I'm certain that plenty of them also reflect someone's "need" for a fancier car, or a new kitchen, or a swimming pool, or whatever.

For some folks, it's just another credit line to draw on. Of course, the 401(k) industry hasn't helped any, making things like 401(k) debit cards (yes, really) available. Taking a loan from your retirement plan is easier than ever these days. And since it's your money you're borrowing, it's no big deal, right?

Wrong.

Why a 401(k) loan is (usually) a bad idea
See, here's the problem. When you take a loan from your 401(k), that money is no longer invested. It's out of the market. It's not growing. And -- the last year's market drama notwithstanding -- the big, actively managed funds that power most 401(k)s have seen solid growth in recent years, powered by stocks like these:

Stock

Missed Gain per $1,000 Borrowed 5 Years Ago

Apple (NASDAQ:AAPL)

$9,056

Gilead Sciences (NASDAQ:GILD)

$1,675

Google (NASDAQ:GOOG)

$3,434

McDonald's (NYSE:MCD)

$1,354

ExxonMobil (NYSE:XOM)

$638

Goldman Sachs (NYSE:GS)

$878

Chevron (NYSE:CVX)

$655

Source: Yahoo! Finance. Based on five-year returns as of Aug. 17.

These stocks are all current holdings of either Fidelity Contrafund (FCNTX) or Fidelity Large Cap Value (FSLVX), both of which are 401(k) stalwarts in many employer plans. They can give you a good idea of what you'd be missing by taking out a loan. The chart shows the gains you would have seen over the last five years -- yes, through last year's crash -- on $1,000 invested in each.

And of course, the compounding makes it worse -- if you're still 20 years from retirement, and you're able to get a market-average 10% return between now and then, every $1,000 you lose out on now is about $6,700 you'll be missing in 20 years.

How much do you really want that swimming pool?

Is there any good time to take a 401(k) loan?
If you're in genuinely dire financial straits, a 401(k) loan may be the best option available. If it's what you've gotta do, take it and don't worry about it. (Although you should consider these other options before you make the call.)

But my experience tells me that the majority of 401(k) loans fund consumer stuff, not the necessities of life. It's just more optional debt, really. And as we've hopefully all learned by now, unnecessary debt is what keeps most not-yet-rich people from ever acquiring real wealth.

Read Rule Your Retirement and learn how to get the most out of what your 401(k) offers. A free trial gets you 30 days of full access with no catch, obligation, or secret gotchas. Try it now.

Fool contributor John Rosevear owns shares of Apple. Google is a Motley Fool Rule Breakers recommendation. Apple is a Motley Fool Stock Advisor selection. You can try any of our Foolish newsletters today, free for 30 days. The Motley Fool has a disclosure policy.