First, a correction to most of the advice on this subject: Google's Mobile-Friendly Test no longer exists. It was retired in December 2023, along with the mobile usability report in Search Console. If a guide tells you to start there, it was written before that and has not been touched since.
What replaced it is better anyway, because it was always the right answer: pick up a phone and use your own website.
Why this matters more than it used to
Google indexes the mobile version of your site. Not the desktop one. If a section of content is hidden on phones, or a page is simpler on mobile, that simpler version is what gets ranked — the desktop version is not consulted as a fallback.
So a mobile problem is not a penalty applied to your real site. The mobile version is your real site, as far as search is concerned. And it is what the majority of your visitors see.
The five-minute test
Do this on an actual phone, on mobile data rather than your office wi-fi, holding it in one hand. Not the phone simulator in your browser — that tells you about layout and nothing about what it is like to use.
1. Can you scroll sideways?
Load your homepage and try to push the page left and right. It should not move.
If it does, something on the page is wider than the screen. It is nearly always one of four things: an image with a fixed width, a table, an embedded map or video, or a long unbroken string like an email address in a heading.
This is the single most common mobile fault and one of the most damaging, because it makes every part of the page feel broken rather than just the guilty element.
Fix: a developer job, but a small one — usually one line telling images to shrink to fit and putting the wide table in its own scrolling container.
2. Can you read the text without pinching?
Body text should be comfortable at arm's length without zooming. Around 16 pixels is the floor.
Then check whether you can zoom. Some sites disable pinch-to-zoom entirely, which looks tidy and locks out anyone with less than perfect eyesight. It is an accessibility failure and there is essentially no good reason for it.
3. Can you hit the buttons?
Try to tap every link in your main navigation, your phone number, and your primary call to action — with a thumb, in one hand, not a fingertip while looking closely.
Targets want to be about 48 by 48 pixels with a gap between them. Footer link lists and inline links in a paragraph are where this usually fails.
4. Does tapping your phone number start a call?
This one is worth money and it is missed constantly.
A phone number typed as plain text is just characters. Tapping it does nothing. The visitor has to memorise it, leave your site, open the dialler and type it in — and a meaningful number of them will not.
A number marked up as a link opens the dialler with the number filled in. It is one small change to the markup, it costs nothing, and on a site where the phone is how customers buy, it is one of the highest-return fixes available.
Test: tap your own number on your own phone. Either the dialler opens or it does not.
5. Does anything block the screen?
Watch the first few seconds. Count what appears on top of the content: a cookie banner, a newsletter popup, a chat bubble, a discount wheel.
On a desktop these are irritating. On a phone, two of them stacked can cover the entire screen, and the close button is often the smallest tap target on the page. Google specifically treats intrusive interstitials on mobile as a problem.
Fix: keep the one you are legally required to have, make its dismiss button large, and delay or delete the rest.
Three more, if you have another five minutes
Fill in your own contact form
On the phone. All the way through, and send it. Watch for fields too narrow to see what you typed, a keyboard covering the submit button, and the wrong keyboard appearing — an email field should bring up the keyboard with the @ symbol, a phone field the number pad.
Then check the message actually arrived. Broken contact forms are more common than anyone expects and completely invisible until somebody tests one.
Check the same content is there
Compare a service page on desktop and on mobile. If the mobile version is missing sections, that content is missing from what Google ranks.
Content inside collapsed accordions is fine — it is in the page and gets indexed normally. Content genuinely removed on small screens is not.
Time it on mobile data
Turn wi-fi off. A homepage that loads instantly in the office and takes eight seconds on a train is a homepage most people never see. Phones have slower processors and worse connections, and they are the majority of your traffic.
What to fix first
- Sideways scrolling. Makes the whole site feel broken.
- Tap-to-call. Cheapest fix on the list with the most direct connection to revenue.
- Anything covering the screen. Free to fix — usually deleting something.
- Tap target sizes.
- Text size and zoom.
- Speed on mobile data. Bigger job, see why websites are slow.
Nearly all of this is checkable in five minutes with a device you already own, and most owners have never done it — because they built the site on a desktop, they test it on a desktop, and their customers do not use one.