Probably less than you think, and you would not know, because you have only ever looked at it on the machine it was designed on. The people deciding whether to call you are holding a phone, standing somewhere, with one thumb free.
Here is how to check it properly in about ten minutes, and what to do about what you find.
Why does a site that looks fine on a laptop fail on a phone?
Because "it fits" and "it works" are different tests, and most sites only pass the first one.
Almost every site built in the last decade is responsive, meaning the layout reflows to the width of the screen. That part is usually fine. What breaks is everything that depended on a mouse, a wide screen, or a fast connection: menus that need hovering, price tables that were three columns and are now unreadable, forms that were short on a laptop and feel endless on a phone, and images sized for a desktop hero that a phone now has to download in full over a weak signal.
None of this shows up in the design review. It shows up as people leaving.
How do you check without buying anything?
Three passes. Do them in this order.
Pass one: your own phone, on cellular data, not your office Wi-Fi. Turn Wi-Fi off. Load your homepage. Then do the thing a customer would do: find your hours, find a price, get to the contact form, and send yourself a test enquiry. Time it. If any of those takes more than three taps or you have to zoom in to read something, that is the finding.
Pass two: a phone that is not yours. Ideally someone older, on a different network, who has never seen the site. Hand it to them and say nothing. Watch where they hesitate. You will learn more in ninety seconds than from any report.
Pass three: your analytics. Do not guess at how many of your visitors are on mobile. Open Google Analytics, look at device category, and compare the bounce and conversion figures between desktop and mobile. If mobile traffic is high and mobile enquiries are not, you have found where the money is going. That is the same diagnosis behind traffic that never turns into leads.
What actually breaks first?
In rough order of how often we find it.
Tap targets that are too small or too close together. Buttons, menu items and links sized for a cursor. WCAG 2.2 sets the minimum at 24 by 24 CSS pixels for pointer targets under success criterion 2.5.8 (W3C, Understanding SC 2.5.8 Target Size (Minimum)), and that is a floor rather than a goal. Two links stacked a few pixels apart in a footer means somebody keeps hitting the wrong one and giving up.
The phone number that is not a link. If your number is typed as plain text, or worse, sitting inside an image, tapping it does nothing. On a phone it should dial. This is a five minute fix that people leave for years.
Forms that ask too much. Every field is a reason to abandon on a small screen. Name, contact, one message box. Set the right keyboard type for each field, so an email field brings up the email keyboard and a phone field brings up the number pad. And confirm the thing actually sends, because forms that silently fail are the single most expensive fault a small site can have.
The hidden menu with everything in it. A hamburger icon is fine. A hamburger icon hiding fourteen items, three of which matter, is not. The two or three actions people came for should be visible without opening anything.
Sticky headers that eat the screen. A fixed bar looks tidy on a laptop and swallows a third of a phone screen. Worse, it can sit on top of the field you just tapped, so you cannot see what you are typing.
Text baked into an image. Banners with the offer written into the JPEG. On a narrow screen that text shrinks to nothing, and it cannot be selected, translated or read aloud.
Tables. Price lists and specification tables built for a wide screen either overflow sideways or squash into columns nobody can read. On a phone they usually need to be stacked as cards.
Does any of this affect whether you show up on Google?
Yes, and more directly than most owners expect.
Google states plainly that it "uses the mobile version of a site's content, crawled with the smartphone agent, for indexing and ranking" (Google Search Central, Mobile-first indexing). The version being judged is the phone version. Google's guidance also warns that if the mobile site carries less content than the desktop one, you can expect to lose traffic, because there is less for it to work with.
The practical version: if a chunk of your service copy is hidden on mobile to keep the layout clean, that copy is effectively not on your website. Put it in an accordion or a tab rather than removing it. This sits underneath showing up in local search, where the searcher is almost always on a phone in the first place.
What about speed on a phone?
Test it on the connection your customers have, not the one in your office.
Google's Core Web Vitals treat a Largest Contentful Paint above 2.5 seconds as a problem, and on a phone that largest element is nearly always the hero image. A desktop-sized image being sent to a phone is the most common cause, and the fix is on the developer's side: correct dimensions per device, modern formats, and nothing loading before it is needed. There is more on the whole chain in why your website is slow.
Do you need a separate mobile site?
No. That approach died years ago and left behind a lot of stranded m. subdomains. One site, one set of content, laid out differently at different widths.
Page builders complicate this. Most let you tune the mobile view separately, which is useful, and also means someone has to actually do it. A template that was never adjusted for phones is not the tool's fault. It is an unfinished job.
Fix it, or rebuild it?
Fix it, in most cases. Tap targets, a dialable phone number, a shorter form, a trimmed menu and properly sized images are hours of work on an existing site, not a project.
Rebuild when the mobile layout is broken because the underlying build is old enough that responsive behaviour was bolted on afterwards, when the site is slow for structural reasons rather than image reasons, or when you were going to redesign anyway. If that is where you are, do it without throwing away the rankings you already have. Our site work runs from $1,200 to $8,000 depending on scope, and how we scope it is on web design, with the full ranges on pricing.
Either way, run pass one today. Phone in hand, Wi-Fi off, try to hire yourself.
If you would rather someone else did it, send us your URL for a free teardown. We will go through your site on a real phone and tell you what breaks, what it is costing you, and which fix to do first, whether or not you work with us.