SAN FRANCISCO — Google is about to change the way its influential search engine recommends websites on smartphones and tablets in a shift that’s expected to sway where millions of people shop, eat and find information.

The revised formula, scheduled to be released Tuesday, will favor websites that Google defines as “mobile-friendly.” Websites that don’t fit the description will be demoted in Google’s search results on smartphones and tablets while those meeting the criteria will be more likely to appear at the top of the rankings — a prized position that can translate into more visitors and money.

Although Google’s new formula won’t affect searches on desktop and laptop computers, it will have a huge influence on how and where people spend their money, given that more people are relying on their smartphones to compare products in stores and look for restaurants. That’s why Google’s new rating system is being billed by some search experts as “Mobile-geddon.”

“Some sites are going to be in for a big surprise when they find a drastic change in the amount of people visiting them from mobile devices,” said Itai Sadan, CEO of website-building service Duda.

It’s probably the most significant change that Google has ever made to its mobile search rankings, according to Matt McGee, editor in chief for Search Engine Land, a trade publication that follows every tweak that the company makes to its closely guarded algorithms.

To stay in Google’s good graces, websites must be designed so they load quickly on mobile devices. Content also must be easily accessible by scrolling up and down — without having to also swipe to the left or right. It also helps if all buttons for making purchases or taking other actions on the website can be easily seen and touched on smaller screens.

If a website has been designed only with PC users in mind, the graphics take longer to load on mobile devices and the columns of text don’t all fit on the smaller screens, to the aggravation of someone trying to read it.

Google has been urging websites to cater to mobile devices for years, mainly because that is where people are increasingly searching for information.