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Domain Name Validator

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Check if a domain name is correctly formatted — valid labels, no invalid characters, proper TLD. Instant format check in your browser, no signup needed.

Reviewed by the thecalcu.com team · Last updated June 25, 2026

What is a Domain?

A Domain Name Validator checks whether a domain name follows the formatting rules defined by the Domain Name System specifications, RFC 1035 (the original DNS RFC) and RFC 1123 (which updated the hostname syntax rules). Every domain name on the internet must conform to these rules to be resolvable. A name that violates the rules, a label starting with a hyphen, a label exceeding 63 characters, digits in the TLD, cannot function as a valid DNS hostname regardless of whether it is registered.

A domain name is structured as a series of labels separated by dots. The rightmost label is the top-level domain (TLD), .com, .in, .org, .io. Each label to the left adds specificity: thecalcu in thecalcu.com, or mail and thecalcu in mail.thecalcu.com. Every label must be 1–63 characters, contain only letters, digits, and hyphens, and start and end with a letter or digit.

This tool validates the format only. It does not perform a DNS lookup, does not check WHOIS registration data, and does not confirm whether the domain resolves to an IP address. Those checks require live network calls that a static, purely client-side tool cannot reliably make due to browser CORS restrictions.

Validation runs entirely in your browser, no domain name is transmitted or stored. For network-adjacent checks, see the URL Validator and IP Address Validator.

Why Use a Domain Name Validator?

A malformed domain name in a configuration file, environment variable, or API call causes a DNS resolution failure that can be hard to trace back to a typo. A label with a leading hyphen, an underscore where one is not permitted, or a TLD with digits is syntactically invalid and will be rejected by compliant DNS resolvers.

Common scenarios:

  • Entering a custom domain in a hosting platform or CDN configuration and wanting to confirm the format is correct before saving
  • Building a form that accepts user-supplied domain names and needing to validate the format at the point of entry
  • Writing server configuration (nginx virtual host, Apache ServerName, email MX record) where a malformed hostname will silently break routing
  • Checking a domain extracted from a text string (log file, CSV) to confirm it is a valid hostname before using it

Who Should Use This Validator?

Developers building forms or APIs that accept domain name input, validate the format at the boundary before passing the value to a DNS resolver or HTTP client.

DevOps and platform engineers configuring virtual hosts, TLS certificates, email records, or CDN domains who want to confirm the domain format is correct before applying the configuration.

System administrators adding domains to allowlists, firewall rules, or email filtering systems who want to catch format errors before they cause resolution failures.

Security teams analysing domain names from logs, threat intelligence feeds, or phishing reports who need to quickly confirm which entries are valid hostname formats.

What Insights Does the Domain Name Validator Give You?

When the domain is valid, the tool shows the individual non-TLD labels, the TLD, and the total character length, a quick breakdown of the domain's structure.

When the domain is invalid, the tool identifies the specific failure:

  • A protocol prefix was included (remove https://)
  • The domain exceeds 253 characters
  • A specific label contains an invalid character or starts/ends with a hyphen
  • The TLD contains digits or is too short

Scope: format only. Whether the domain is registered, has DNS records, or the certificate is valid are not checked.

How to use this Domain calculator

  1. Open the Domain Name Validator on this page.
  2. Type or paste the domain name into the Domain Name field, without a protocol (just thecalcu.com, not https://thecalcu.com).
  3. The result badge updates instantly. A green Valid badge confirms the domain follows RFC hostname naming rules.
  4. If the badge shows Invalid, read the error message, it specifies whether the problem is a protocol prefix, an invalid label character, a leading/trailing hyphen, or an invalid TLD.
  5. Fix the issue and the badge updates immediately.

Formula & Methodology

The validator applies these rules in sequence:

1. Strip a trailing dot (fully-qualified domain names may end with .)
2. Check total length ≤ 253 characters
3. Confirm no :// is present (protocol check)
4. Split on ., require at least 2 labels
5. For each non-TLD label: match against /^[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?$/ (1–63 chars, no leading/trailing hyphen)
6. For the TLD: match against /^[a-zA-Z]{2,}$/ (letters only, ≥ 2 chars)

Valid example: mail.thecalcu.com
Labels: mail, thecalcu. TLD: com. Length: 17 characters. All valid.

Invalid example (label starts with hyphen): -invalid.com
Label -invalid starts with a hyphen, not permitted by RFC 1123.

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Frequently Asked Questions

A Domain Name Validator checks whether a domain name follows the correct format defined by the DNS (Domain Name System) standards, RFC 1035 and RFC 1123. It confirms that each label (the parts separated by dots) contains only letters, digits, and hyphens, starts and ends with a letter or digit, is no longer than 63 characters, and that the top-level domain (TLD) contains only letters. It does not check whether the domain is registered or resolves in DNS.
A domain name is made up of labels separated by dots. In `thecalcu.com`, `thecalcu` and `com` are labels. Each label must be 1–63 characters long, may contain letters (A–Z, case-insensitive), digits (0–9), and hyphens, and must start and end with a letter or digit. A label may not start or end with a hyphen.
No. This tool checks format only, it confirms the domain name follows DNS naming rules. Whether a domain is registered, active, or resolves to an IP address requires a live DNS lookup, which cannot be done reliably from a browser-based tool due to CORS restrictions. To check domain registration, use a WHOIS tool or the registrar's domain search.
No. DNS resolution requires a network call to a DNS resolver. Making that call from a browser to arbitrary third-party domains is blocked by browser CORS security policies. This tool performs format validation only, purely synchronous, purely client-side, no network requests.
Enter just the domain name, without a protocol. Enter `thecalcu.com`, not `https://thecalcu.com`. If you include a protocol, the tool will tell you to remove it before validating.
A TLD (Top-Level Domain) is the rightmost label in a domain name, `.com`, `.in`, `.org`, `.io`, `.gov`, etc. For the purpose of this format check, a valid TLD must contain only letters (A–Z) and be at least 2 characters long. The tool does not check against the IANA list of registered TLDs, it checks format only. A domain like `example.xyz123` would fail because the TLD contains digits.
Yes. Subdomains like `mail.thecalcu.com` or `api.internal.company.co.uk` are valid and will be accepted. The tool validates each label separately, the subdomain labels follow the same 1–63 character, letter/digit/hyphen rules as the main domain labels.
A fully qualified domain name (including all labels and dots) must not exceed 253 characters. Individual labels must not exceed 63 characters. The validator checks both limits.
Internationalised domain names (IDNs), domains with non-ASCII characters like Hindi or Arabic script, are encoded in Punycode when they appear in DNS. For example, `भारत.भारत` becomes `xn--h2brj9c.xn--h2brj9c` in ASCII. This tool validates the ASCII/Punycode form. If you enter an IDN in its Unicode form (non-ASCII characters), it will likely fail the ASCII-only label check.
Underscores (`_`) are not permitted in DNS hostnames per RFC 1123. They are sometimes used in DNS record names (like `_dmarc.example.com` for DMARC TXT records) but not in standard hostnames. This tool follows the RFC hostname rules, so domains with underscores are flagged as invalid.
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