There are all kinds of advertisements promising a "free credit score" or a "free credit report," but if this sounds too good to be true, that's because it is.

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The majority of these services will indeed give you a credit report and some version of a credit score, but only after you've entered your credit card information and agreed to a trial subscription of whatever credit monitoring service the company is selling.

Plus, the credit score you receive might not be very useful. While there actually is a way to get a free credit report with no strings attached, you'll most likely need to pay for your "real" credit score.

Your "real" credit score and what makes it better
The FICO credit scoring formula is used in more than 90% of lending decisions, according to the Fair Isaac Co. There are several different versions of the FICO formula, as it is updated from time to time; however, the different versions are fairly similar.

There are several other scoring models, and odds are that your "free credit score" site of choice is giving you one of these. The Vantage Score is next in popularity to FICO, but there is a long list of other possibilities. Experian has the Plus Score, TransUnion has its TransRisk score, and various websites have their own scoring models, which are not used by any lenders. These are sarcastically known as "FAKO" scores by many consumers.

To be fair, these scores aren't totally "fake." On the contrary, they are based on the actual data in your credit report and will drop if you miss payments and increase as your credit history improves.

However, they can be somewhat misleading if you're looking to apply for credit. Some have different scoring scales than the FICO model, which ranges from 300 to 850, and some weigh things differently. It is not uncommon for a consumer to think his or her credit score (on a different scoring model) is 700, while a FICO check by a lender reveals it's just 650.

That's why the FICO score is better. It tells you what lenders see when they decide whether to approve your loan application. You can only get it from myFico.com, and it'll cost you.

You can get your report (but not your score) truly for free
If you're about to apply for credit and you know your lender is looking for a certain score (this is fairly common, particularly with mortgages), it's probably a good idea to know your FICO score. If that's not your situation, and you just want to know what your credit report says, there is a no-strings-attached way to get all three of your credit reports for free.

Thanks to the Fair Credit Reporting Act, consumers are entitled to one free credit report from each of the major credit bureaus (Experian, Equifax, and TransUnion) once per year at a time of your choosing. You can also obtain an additional copy if you're turned down for credit or believe you're a victim of identity theft.

However, just because a website advertises "free credit reports" doesn't mean they are really free. In fact, the only place to order your (truly) free credit reports through this program is at annualcreditreport.com.

You get what you pay for
There are a few exceptions to that rule. For example, if you have certain Discover credit cards, the company will provide your FICO score from one of the major credit bureaus as part of your "perks," just like some cards give you frequent flyer miles.

But other than that, it costs money to get your real FICO score. You can buy your FICO score and report from each bureau for about $20, or subscribe to a FICO monitoring service for as low as $14.95 per month. It's not uncommon for the three to be slightly different, so you might want to order each score.

However, the investment may be completely worth it. Before you go into a lender's office to apply for credit, don't you want to see the score the lender is looking at, not some other formula? The other scores can be useful as a guideline, but there is simply no substitution for the real thing.

Matthew Frankel has no position in any stocks mentioned. The Motley Fool owns shares of Discover Financial Services. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.