Most of your customers will view your website from their phone, and therefore your website must work on a small screen.

In short
A mobile friendly website has text you can read without zooming, buttons big enough to tap, pages that load quickly, a simple single-column layout and no pop-ups that cover the screen. Getting these basics right helps keep visitors on your site for longer.
Readable Text Without Zooming
Body text should be set at a minimum size of 16px and include sufficient space between lines to clearly read the sentences without becoming indistinct. Don’t make your customers have to pinch and zoom to read your content.
Tap-Friendly Buttons and Links
Buttons and links on mobile should be big enough to press with your thumb. We recommend a minimum size of 44px x 44px for buttons and links, and adequate space between similarly sized links.
Fast Load Times on Mobile
Compress your images prior to upload and refrain from loading superfluous scripts. Slow pages are one of the biggest reasons that users will abandon a site before they have even looked at its content. These core principles of mobile-friendly design translate very well to site load performance as well.
A Single-Column Layout
A single column is often best for mobile, as multi-column layouts can break up and cause horizontal scrolling on smaller screens. For more on layouts that adapt to the screen, read how responsive web design could help your business.
No Intrusive Pop-Ups
Pop-ups covering the screen of mobile phone users are considered a Google ranking problem and should be avoided. Overlays should be small and easily closable.
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By ensuring these basics are in place you can see the impact they have on keeping visitors on your site for longer and generating more leads.
FAQs
How can I check if my website is mobile friendly?
Open it on your own phone and try the things a customer would do: read a page, use the menu and fill in a form. Free tools such as Google PageSpeed Insights also report on mobile speed and usability problems.
Is responsive design the same as mobile friendly?
Responsive design is the most common way to get there, because the same page rearranges itself to fit the screen. A site can still be responsive but awkward on a phone if the text is tiny or the buttons are cramped, so check the details too.
Does a mobile friendly site help with Google rankings?
Google mainly uses the mobile version of a site for indexing and ranking, so a poor mobile experience can hold a site back in search results.
Should the mobile version of a site have less content?
Generally not. Because Google looks at the mobile version, content hidden or removed on phones may not count. Keep the same content and present it in a layout that suits a small screen.