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You shop online, you save money. Just about everybody believes that and comparison shops online. And these days, mortgage rates are easy to shop for online, too.

It鈥檚 convenient. Immediate. We鈥檙e programmed to believe it. But are online mortgage lenders consistently offering听?

Comparison shopping mortgage rates

Manish Grover, a marketing consultant in the New York City area, certainly seems to believe so. He sent NerdWallet an email telling us just that.

鈥淚鈥檝e taken out two mortgages now from online lenders advertising on Zillow, and both times I鈥檝e got a rate which is at least 0.50% lower than my local banks or my primary bank,鈥� he says. 鈥淎nd I am considering the rate after all costs are considered (APR).鈥�

Grover听seems to know what he鈥檚 doing. Comparing annual percentage rates is definitely the way to go, because that鈥檚 an interest rate that considers all of the estimated fees and expenses that are built into a loan.

鈥淚n addition, the processing complexity is much lower as well,鈥� Grover writes. 鈥淪o my question is: What are some of the factors that help these online lenders to undercut banks? Is it because they don鈥檛 have physical infrastructure and do most business on phone and email?鈥�

Good question. But before we determine how they do it, we鈥檝e got to find out if online lenders really do offer the lowest rates on a day-in and day-out basis.

NerdWallet tracks the daily rates of national lenders for our Mortgage Rate Index. So we pulled three of the biggest banks from our daily survey to represent the brick-and-mortar constituency, as well as听, the largest online mortgage lender.

To even the field, we asked听听补苍诲听, arguably the two biggest runners-up in online home loan lending, to provide us their published rates for the same time frame. We heard back from Guaranteed Rate but not LoanDepot.

We considered all of these published rates for 30-year fixed-rate loans from June 16 through July 19, just before, during and after Britain voted to leave the European Union, sending mortgage rates falling dramatically. Here鈥檚 what we found:

Mortgage rates head-to-head

Lenders price their mortgages based on a variety of parameters:

• The bond market, which impacts their costs and how much they earn on their money.
• Their 鈥渋nventory鈥�: how much money they have to lend.
• Their target profit, called a margin.
• Competitive factors.
• A customer鈥檚 credit profile: the risk the lender is assuming.

There are other layers to the process, but that鈥檚 the foundation.

Our mortgage rate comparison 听below comes with a big asterisk. Here, hold on to it and we鈥檒l talk about that shortly.

In our initial analysis of four lenders (Bank of America, Chase, Quicken and Wells Fargo), Chase was clearly the low-rate winner. Then we added in Guaranteed Rate.

One thing we noticed is that Guaranteed Rate doesn鈥檛 move its听rates as much as the other lenders surveyed. Guaranteed Rate听names a rate and sticks with it for days, sometimes weeks at a time. And for a handful of days, Guaranteed Rate really nailed the lowest rate. The rest of the time, it was pretty much neck-and-neck with Chase.

Meanwhile, Quicken, our other online contender, is basically at the top of the rate pack with Wells Fargo.

Here are the rate averages for the period for each lender (from lowest to highest):

Guaranteed Rate: 3.40%
Chase: 3.44%
Bank of America: 3.65%
Quicken: 3.69%
Wells Fargo: 3.70%

And now the asterisk

Shopping mortgage rates online has some serious drawbacks 鈥� and it鈥檚 all part of that big asterisk. You see, each lender has its own assumptions for the published rates it quotes. And the differences can be pretty significant.

Consider some of the assumptions made by each lender:

• Bank of America: $200,000 loan, 20% down payment, discount points up to 1%, 80% loan to value, single-family home in California.
• Chase: Loan amount of $215,000, 20% down, discount points vary, 60-day rate lock, 鈥渆xcellent鈥� credit, 鈥渘ot available in all states.鈥�
• Guaranteed Rate: $300,000 home, down payment of 25%, 740 FICO score or better, 55-day rate lock, based on a single-family home in Illinois.
• Quicken: $200,000 loan, 75% loan to value, debt-to-income ratio less than 30% and credit score over 740.
• Wells Fargo: $200,000 loan, 25% down payment, loan with an escrow account and a 740 credit score.

It鈥檚 all in the details: The location and value of the property, your credit score, your down payment and especially the discount points charged.

All of the lenders say that the rates quoted are for 鈥渋nformational purposes only,鈥� 鈥渁re subject to change without notice鈥� and 鈥測our loan鈥檚 interest rate will depend鈥� on a whole mess of stuff.

In other words, don鈥檛 count on this rate.

How to shop mortgage rates

Online lenders may not necessarily offer the consistently lowest mortgage rates. Sorry, Manish. But to his credit, Manish shopped around and got the best rate he could find, online or otherwise. And that鈥檚 the real lesson.

It鈥檚 also important to note that in shopping for a rate online, you鈥檙e strolling through a big, dimly lit store. We compared a bunch of mortgage rates, all of which depend on varying factors. To really know what your rate is going to be, you have to submit an actual application 鈥� to several lenders. Because if there鈥檚 one thing apparent, it鈥檚 that every lender is pricing its loans a little differently.

But say you can save just 0.30% on a $300,000 home loan. The difference between scoring a 30-year mortgage at 3.40% rather than one at 3.70% saves you a little more than $50 a month 鈥� and over $18,000 in interest over the life of the loan. And shopping multiple lenders is likely to听.

With thousands of dollars in play, over more than just a few years, small differences can add up to big money.

Hal Bundrick is a staff writer at NerdWallet, a personal finance website. Email:听hal@nerdwallet.com. Twitter:.

This article first appeared in 听