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The error message says what to type, not that something failed

By the end of this lesson, you'll know how to write, for every field in your form, the error message that shows up when something's wrong, one that says exactly what to fix.

By Yodai · Published September 6, 2026 · Updated October 13, 2026

Someone fills out your booking form. They type the phone number with extra spaces, or leave out a digit, or paste the email with a comma instead of a period. They hit submit and the field turns red. Next to it, one word appears: Error.

That person doesn't know what they did wrong. They don't know if their phone number is entered wrong, if there's something further down the form they still need to fill in, or if the mistake is yours, not theirs. They stare at the screen for a few seconds, and if they have another tab open with a competitor, they open it. This really happens: in user testing from the Baymard Institute, one person couldn't figure out why their phone number wasn't being accepted and said that, at that point, they'd just go to another website. The problem was a space.

The first part of the job is choosing which fields to ask for, in what order, and why. There's one more piece before the submit button: what you tell people exactly when they make a mistake, so they fix it instead of leaving. Check first whether your website lets you change these messages; if not, give whoever built it the list of messages you want.

An error isn't an alarm, it's an instruction

The word Error warns you, but it doesn't instruct. It does its one job, saying something went wrong, and stops there. It doesn't say what to write instead, which is why it's almost never enough on its own. The W3C's accessibility guidelines break this into two steps: say in text which field has the error, and when you know how to fix it, say that too. Their own example: "The email isn't valid" covers the first step; "Enter a valid email in the format name@domain.com" covers both.

A good error message answers one question: what do I need to write differently to make this work? If someone reads it and still doesn't know what key to press next, the message isn't doing its job, no matter how red the field is.

Tone matters too. Among the form guidelines tested with 65 people in a University of Basel study is this one: the error message is polite, uses words anyone can understand, and says both what's wrong and how to fix it. If your website talks to people in a friendly, straightforward way without raising its voice, the error message should do the same: enter the phone number with 9 digits fixes it; INVALID PHONE NUMBER, in all caps, neither says what to write nor sounds friendly to someone who was one click away from booking.

In data from 1,362 forms published by Zuko, a company that sells software for measuring how people fill out forms, email and phone rank just behind password, though at noticeably lower rates, as the two fields with the highest drop-off among the six most common ones. This is the company's own data, not an independent study, but it points to these two fields as the first ones worth writing carefully. The same rule applies later to name or date fields too, if those can also go wrong in your case.

The email fails because of a typo, not the format

The phone number fails because of formatting: a digit is missing, there's an extra country code, or the form doesn't accept spaces. The email fails because of a typo: the @ is missing, the period turns into a comma, the domain gets cut off halfway through. These are two different kinds of mistakes, and the message should tell them apart. For the phone number, you can even state the exact digit count: Spain's national telephone numbering plan sets the rule that phone numbers are always dialed with all 9 digits. What you shouldn't do is require it to be typed without spaces: in the Baymard Institute test mentioned earlier, that was exactly the problem, the website wouldn't accept a number with a space in it.

If someone types janesmithgmail.com, telling them "Invalid email" doesn't show them where the problem is. "Your email is missing the @, check how you typed it" does, because it points to the exact missing character and doesn't make them reread the whole thing letter by letter. The Baymard Institute calls this an adapted message: the form already knows which rule failed, so it can say so. In their comparison of online stores, 98% use generic messages.

If the mistake is at the end, like jane@gmail without the ".com", the same kind of message works: "Looks like your email is missing the ending, like '.com'." You don't need to write a different sentence for every variation of the same mistake, but it's worth having two or three ready for the mistakes that really do keep repeating, instead of one generic line that covers everything and says nothing.

How to write it, field by field

For phone, email, and any other field that might fail in your form, the same four rules apply. The fourth one has a study behind it: in a University of Basel test with 303 people, messages placed right next to the failing field performed better than ones placed above or below the form, and above was exactly where most websites put them.

Every message already says what to fix, not that something failed. There's one last piece of text to write: the button's, and why a plain "Submit" lets bookings slip away that could have closed.

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