The Best Bulk Domain Checker: What to Look For
Checking one domain is trivial. Checking four hundred is where tools quietly break, because registries rate-limit, timeouts get reported as available, and you find out at checkout.
September 22, 20266 min readDomain search · Bulk · RDAP
Check up to 500 domains at once without an account, or 2,000 with a free one.
Open the bulk domain checkerThe short answer. A bulk checker is only as good as what it does when it cannot get an answer. Tools that collapse timeouts into "available" look faster and are wrong in the most expensive direction. Ask three things: does it query the registry over RDAP, does it report unknown as a distinct state, and does it respect per-registry rate limits.
Why bulk is a different problem
Checking one domain is a single lookup. Checking four hundred is a distributed systems problem, and the failure modes are not obvious from the outside.
Registries rate-limit. Query too fast and you are throttled or blocked. A naive tool firing four hundred parallel requests gets rejected partway through.
Different extensions have different authorities. Each extension has its own RDAP server with its own limits. One slow registry should not stall the whole run.
Timeouts are not answers. This is the central issue. A request that times out means nothing was learned. Many tools record that as available, because it produces a clean-looking result set.
Results arrive at different speeds. Some registries answer in 50ms, some in several seconds. A tool that waits for everything before showing anything feels broken on large lists.
The three questions that separate tools
1. Does it ask the registry, or guess from DNS?
DNS is fast and free, which is why many tools use it. It is also wrong regularly.
A DNS lookup asks where a name points, not whether it exists in the registry. Registered domains with no DNS records are extremely common: investor portfolios, defensive registrations, names in the grace period after expiry. Query DNS for any of those and get nothing back, and a DNS-based checker reports them as available.
RDAP asks the registry directly and returns HTTP 404 when a name genuinely has no record. That is a definitive answer rather than an inference. We covered the difference in How to Check If a Domain Is Available.
2. Does it have an unknown state?
Any honest checker returns at least four outcomes:
| State | Meaning |
|---|---|
| Available | Registry has no record |
| Registered | Registry has a record |
| Unknown | The lookup did not complete |
| Error | Something failed and is worth showing |
Tools that report only available and registered are hiding the third case inside one of the first two, and the commercially damaging direction is hiding it inside available.
If a checker never shows you an unknown result across four hundred domains, it is not because every lookup succeeded.
3. Does it handle rate limits per registry?
Good bulk checking uses per-registry concurrency limits rather than one global setting, so a strict registry does not poison the whole run, and backs off properly when throttled instead of retrying immediately and being rejected harder.
What else matters at scale
Streaming results. Seeing answers arrive as they resolve rather than staring at a spinner for two minutes.
Input normalisation. Real lists are messy: full URLs, trailing slashes, uppercase, duplicates, blank lines, names with and without extensions. A good tool cleans that silently and tells you what it discarded.
Extension expansion. Paste a list of names without extensions, select the extensions, and check the whole matrix in one pass.
Export. CSV out, so results go into whatever you actually work in.
Pricing alongside availability. Available is only half the answer. A name available at $2 in year one and $20 on renewal is a different proposition, and that only shows up if the tool carries registrar pricing.
How our checker handles it
Since this is our own tool, the specifics rather than the adjectives.
RDAP first, routed through the IANA bootstrap registry to the authoritative server for each extension, rather than a third-party aggregator. DNS is used only as a secondary signal, and a missing DNS record is never treated as proof of availability.
Four distinct states, available, registered, unknown and error, reported separately rather than collapsed.
Per-registry concurrency and timeout handling, so one slow extension does not stall the run.
Streamed results, so rows appear as they resolve.
Registrar pricing inline, from a nightly sync covering over a thousand extensions, so you see renewal cost next to availability.
Volume limits by plan: 500 domains per check without an account, 2,000 on a free account, and 5,000 on Pro and Scale. No daily cap on the number of checks at any tier.
Try it on the checker.
Doing it yourself
If you would rather script it, the approach is:
- Read the IANA RDAP bootstrap file to find the authoritative server per extension
- Group your list by extension and process each group with its own concurrency limit
- Treat HTTP 404 as available and HTTP 200 as registered
- Treat anything else, including timeouts, as unknown rather than guessing
- Honour Retry-After headers rather than retrying immediately
That is genuinely all of it. The difficulty is not the protocol, it is the discipline about unknowns.
Common questions
What is the best way to check many domains at once?
Use a tool that queries registries over RDAP, handles per-registry rate limits, and reports unknown results separately. Paste your list, select extensions, and check the whole matrix in one run.
How many domains can I check at once?
Depends on the tool. Ours allows 500 per check without an account, 2,000 on a free account and 5,000 on paid plans, with no limit on how many checks you run.
Why do bulk checkers give different results?
Mostly because some infer availability from DNS rather than asking the registry, and some report failed lookups as available. Both produce more optimistic results than reality.
Is RDAP better than WHOIS for bulk checking?
Yes. RDAP returns structured JSON with meaningful HTTP status codes, so there is no text parsing. ICANN also removed the requirement to run WHOIS on port 43 in January 2025, so its availability is no longer guaranteed.
Can I check domains in bulk for free?
Yes. Our checker allows 500 domains per check with no account and no signup, and 2,000 with a free account.
Why does a domain show available in bulk but taken at checkout?
Usually because the bulk tool inferred availability from a missing DNS record, or reported a timeout as available. It can also mean the name is registry-reserved or premium-priced.
Put the research to work.
Check your shortlist across the extensions that matter, then save the strongest names to a project.
Check a domain