How to check domain availability in bulk, and read the answer right.
A bulk domain checker answers, for every name on a list, whether it is available, for sale or registered. This guide explains how 5,000 names are answered in one run, what each result really means, and where the answers come from.
Updated September 202611 minute readSources: ICANN, IETF
- names in one run on Pro
- 5,000names in one run on Pro
- extensions checked live
- 591extensions checked live
- answered first from daily zone files
- 349answered first from daily zone files
- the longest an available answer is cached
- 15 minthe longest an available answer is cached
How do you check thousands of domains at once?
In three passes, each one smaller than the last. Most of your list never reaches a registry. Names already known to be taken are answered from zone data on disk, DNS clears most of the rest, and only the remainder is sent to the registry for the authoritative word.
One run, 5,000 names
thickness = names still unanswered
Zone index
No network. Answered from data already on disk.
DNS
One live lookup. A name that answers is in use.
Registry
The authoritative word, for what is genuinely left.
What is a zone file, and why does it make checking faster?
A zone file is the registry’s own daily list of every name in an extension that has nameservers. For 349 extensions a snapshot is indexed locally, so a name that is in it is answered as taken before any network request. It can prove a name is taken, never that it is free.
It can prove
This name is taken.
A domain listed in today’s zone has nameservers in the registry, which only a registered domain can have. That is an answer, and it arrives without a single network request.
It cannot prove
This name is free.
Plenty of registered domains point nowhere and never appear in a zone file. So absence is not evidence here, and those names go on to DNS and the registry like any other.
Extensions covered by a daily snapshot
349
It only ever removes work
The zone pass confirms taken names and nothing else, so every name it cannot vouch for is checked exactly as it would have been without any zone data. A snapshot that is missing, stale or mid-rebuild simply stops being consulted. How big those zones are, and how they move every day, is on the domain analytics page.
What do available, for sale and registered mean?
Every result tells you what to do next. Available comes with the cheapest first-year price across registrars, for sale with the owner’s asking price, and registered with the date it expires. Likely available means DNS found nothing but the registry has not confirmed it yet.
northvale.com
$9.48 first year
cheapest of 8 registrars
Confirmed by the registry, priced across the registrars we track, and one click from being yours.
northvale.co
$2,495
asking price, listed now
Taken, and buyable today. The owner has already named a number, so this one is a decision, not a dead end.
launchpad.com
Expires Mar 14, 2027
the date to watch
Held by someone else, with the expiry date where the registry publishes one. Watch it and you will know the day that changes.
The first answers land immediately
Results appear as they settle, so you are reading and starring the top of your list while the tail is still running, and each card sharpens in place as a better answer arrives.
And when the registry will not say
A handful of extensions publish no usable registration data, and now and then a lookup genuinely fails. Those results say so instead of being rounded up to available.
Can a registered domain still be bought?
Often, yes. Once your results are on screen, registered names are matched against public marketplace listings, and a name whose owner has published an asking price reads as for sale with that price. A taken name with no listing shows none, because its price is whatever the owner would accept.
After the check settles
northvale.co
Registered
Before: a name you would have crossed off the list.
northvale.co
For sale · $2,495After: the owner has already named a price, and the listing is one click away.
Not listed, but you still want it? Add it to Domain Hub, which watches it until it changes hands or drops.
What is RDAP, and how is it different from WHOIS?
RDAP is the Registration Data Access Protocol, the structured replacement for WHOIS. It answers in JSON with standard status codes instead of free text. Since 28 January 2025 it is the definitive source for generic domain registration data, and most registries no longer have to run WHOIS at all.
WHOIS · text, worded by each registry
One registry
No match for "NORTHVALE.COM".
Another
NOT FOUND
A third
%% No entries found for the selected source(s).
RDAP · one shape everywhere
HTTP/1.1 404 Not Found{ "errorCode": 404, "title": "Not Found"}
A status code a program can read, instead of a sentence it has to recognise.
| Aspect | WHOIS | RDAP |
|---|---|---|
| Response format | Free-form text, different at every registry | Structured JSON with a documented shape |
| Reading a result | Scrape and pattern-match the prose | Read a field and an HTTP status code |
| Not-found signal | A sentence that varies by registry | A standard status code |
| Internationalisation | Inconsistent character handling | Unicode throughout |
| Standardised | By convention only | By IETF specification |
| Bulk suitability | Fragile at scale | Built for programmatic access |
The long version is in RDAP vs WHOIS: what replaced WHOIS and why it matters.
Why do some extensions answer faster than others?
Because registries publish different rate limits, and they differ by an order of magnitude. Each registry is paced to its own documented ceiling and backs off if it pushes back, so a large run is never throttled into errors that look like answers.
Request pacing, by registry
Who sees the names I check?
By default, nobody but the registry: DNS and registry lookups go straight from your browser to the source. The one step that reaches our server is the zone pass, which reads a local index and keeps nothing.
Directly from your browser
Default path
DNS and registry lookups go straight from your browser to the source. The name you are checking never reaches our server on this path.
Our zone index
Zone pass
For the 349 extensions with a snapshot, the names are sent to our server to be read against the local index. The request is answered without being stored or cached.
Through our server
Fallback path
Used only when a registry refuses browser requests or a lookup is inconclusive, so the answer can still be completed.
How fresh are the results?
Registered answers are cached for 24 hours and available ones for only 15 minutes, because anyone can register a free name at any moment and a stale available is the expensive mistake. Inconclusive lookups are never cached.
Registered
24h
Registrations rarely change hour to hour, so a repeated check returns instantly instead of spending a registry request.
Available
15m
Anyone can register a free name at any moment. A stale available is the expensive mistake, so it expires quickly.
How many domains can I check for free?
500 per run without an account, 2,000 with a free account, and 5,000 on Pro. Check sessions are unlimited on every tier; the tier only changes how much fits into one run.
500
domains per run
5 extensions
2,000
domains per run
10 extensions
5,000
domains per run
20 extensions (25 on Scale)
Coverage runs to 591 extensions, from the common ones to country codes and niche new gTLDs, of which 349 also have a daily zone snapshot behind them.
Domain checker questions
Short answers to what people ask most about checking domain availability.
How many domains can I check at once?
Visitors can check 500 domains in one run without an account, free accounts 2,000, and Pro accounts 5,000. Results stream in as they resolve rather than appearing all at once at the end, so you can start reading the first answers while the rest are still running. Check sessions themselves are unlimited on every tier.
What is a zone file, and why does it make a check faster?
A zone file is the list a registry publishes of every domain in its extension that has nameservers, the same data that makes the internet resolve. Through ICANN's Centralized Zone Data Service we hold a daily snapshot for 349 extensions, indexed locally. When your list includes one of them, those names are answered from the index before any network lookup happens, which is why a long run of .com names settles in a fraction of the time it used to.
If a domain is not in the zone file, is it free?
No, and the checker never says so. A zone file only lists names that have nameservers, so a domain can be registered and simply never pointed anywhere, which keeps it out of the file entirely. The zone pass is therefore allowed to prove that a name is taken and nothing else. Anything it does not recognise goes on to DNS. A name with no DNS either is shown as likely available, never as available, and in small checks the registry is asked to confirm it.
What is RDAP, and how is it different from WHOIS?
RDAP is the Registration Data Access Protocol, the modern replacement for WHOIS. Both answer the same question, but WHOIS returns free-form text that differs between registries and has to be scraped, while RDAP returns structured JSON with a documented shape and standard HTTP status codes. That means a definitive answer can be read reliably rather than guessed at from prose, which is what makes checking thousands of names at once practical.
What does "likely available" mean?
It means DNS found no delegation for the name, but the registry has not confirmed that it is unregistered. A domain can be registered without ever being pointed at a server, so absence of DNS is strong evidence but not proof. Rather than showing those as available and being wrong some of the time, they are marked as likely available until an authoritative registry answer arrives.
Do you show registered domains that are for sale?
Yes. After a check settles, registered results are compared against public marketplace listings, and a name whose owner has published an asking price reads as for sale with that price rather than as plain registered. A taken name with no listing shows no price, because what it would cost is whatever its owner would accept.
Are results cached, and how fresh are they?
Yes, with two different lifetimes because the two answers age differently. A registered domain is cached for 24 hours, since registrations rarely change from one hour to the next. An available domain is cached for only 15 minutes, because anyone can register it at any moment and a stale "available" is the more damaging error. Inconclusive results are never cached.
Do I need an API key or an account?
No. Checking is free and needs no key and no sign-up. An account raises how many domains and extensions you can include in a single run and lets your history, shortlist and projects sync, but the checker itself works without one.
Further reading
Sources
- ICANN: Launching RDAP, sunsetting WHOIS (2025)
- IETF RFC 9083: RDAP JSON responses
- ICANN: Centralized Zone Data Service
Short of names to check? Ideas generates them, and Brand Check tests the one you like beyond the domain.