Those who are considering whether to buy a home have no doubt heard that current mortgage rates make it an advantageous time for such a purchase. The typical rate for a 30-year fixed rate mortgage is under 4 percent, allowing borrowers to get a more affordable monthly payment and put less money toward interest on the loan.

However, rates are likely to fluctuate over time. Since the home buying process usually lasts several weeks, the rate that was offered when you started shopping for a property might not be available when you are approved for a loan. If this situation occurs, you'll find yourself making a higher monthly payment to cover the extra interest.

One way to ensure that you can benefit from a low mortgage rate is to lock it in. This step will let you get a loan with an agreed-upon rate, even if rates increase by the time you secure a loan. However, locking in a rate also means you'll have to time the decision correctly and possibly pay a little extra for the benefit.

The rate you lock in will only last for a certain period of time. Broderick Perkins, writing for the legal site Nolo, says locks usually last for an average of 30 days. Some may be good for only 15 days, while others can apply for 60 days or longer.

Locking a rate for a longer period of time will help protect it from expiring if it takes longer than expected to close on a home. However, this longer period is also likely to come at a higher expense. Lisa Smith, writing for the financial site Investopedia, says the charges for locking in a rate tend to climb incrementally every 30 days. The longer the period, the more expensive it is.

The lender may charge you a slightly higher interest rate than what is currently available in exchange for locking it in. They may also require that you pay points in order to secure the rate. A borrower using points to lower an interest rate will pay a certain up-front cost for lowering a fraction of the interest percentage.

Naturally, you'll want to make sure the cost you pay at this point will be worth it in the long run. Dale Robyn Siegel, writing for the financial site Bankrate, says you should calculate how much the lower interest rate will save you each month and multiply this by the amount of time you expect to spend in the home. If the extra fees for locking in do not exceed the savings from the lower interest rate, it's a good decision.

You should also be careful about when you decide to lock in a rate. Since the lock will only apply for a certain period of time, it doesn't make sense to take this action when you are still house hunting. The earlier in the process you secure a rate, the more likely it is that it will expire before you can finalize the loan. You'll then have to pay extra to extend the lock for a longer period of time.

The best time to lock in a rate depends on the borrower's circumstances. Michele Lerner, also writing for Bankrate, says an increase in rates may be enough to jeopardize a borrower's ability to afford a home. In that case, they may want to lock in a good rate as soon as possible.

However, this will also put more pressure on you while you are looking for a home. Waiting until after an offer has been accepted allows you to lock in a rate when you know you will be taking out a loan. It also lets you benefit from a shorter, less expensive lock-in period.

In some circumstances, your lender may not charge you for locking in a rate. The real estate site Zillow says short-term locks may not have a charge, or a flat fee of a few hundred dollars. The cost for locking in may also be refundable.

One risk of locking in a mortgage rate is that lower rates may be available by the time you finalize the loan. Smith says some lenders offer a "float down" option that lets you exchange your locked-in rate for a new one. However, this option often comes at an extra cost. Rewriting your lock is another option, but it also usually requires added expense.

Lenders may be willing to work with you to preserve a rate in order to keep your business. Lerner says there are usually options for extending a lock if it expires before the closing or if the closing occurs later than expected. She says lenders typically charge one-quarter of a point for every 15 extra days.

Compare the costs of different lenders. Siegel says this process will allow you to compare different interest rates as well as costs and options for locking in a rate.

Make sure the lender provides you with all pertinent information on a lock. Perkins says the documentation should include the effective date, any charges, the expiration date, and any options such as floating down or extending the lock.