01
Unix Timestamp Converter
Converts Unix epoch timestamps to human-readable dates in any time zone, and dates back to…
DNS Propagation Time Calculator
Works out how long a DNS change takes to be seen everywhere, from the TTL that was published before you made the change.
Developer
Nothing uploaded
No sign-up
Free, unlimited
Enter your figures and press Calculate.
In short
How long a DNS change takes is decided by the TTL that was published <b>before</b> the change, not the new one. Authoritative servers update the moment you save; what you are waiting for is every resolver that cached the old answer to have its copy expire, and each holds it for as long as the TTL said when it fetched it. So a record that had a 24-hour TTL can show the old answer for up to 24 hours no matter how low you set the new value. The fix has to happen before the change: lower the TTL to about 300 seconds, wait one full old-TTL period so caches pick up the short value, then make the real change and see it take effect in minutes.
How to use the DNS Propagation Time Calculator
- Enter the TTL that was on the record before you edited it.
- Enter the new TTL.
- Read the worst case, which is the old TTL.
About the DNS Propagation Time Calculator
The DNS Propagation Time Calculator works out how long a DNS change takes to be seen everywhere, from the TTL that was published before you made the change. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.
What people use it for
- Telling a client how long a migration will take.
- Planning a mail or hosting move with no downtime.
- Explaining why one person still sees the old site.
Details
| What decides the wait | The old TTL, published before the change |
|---|---|
| Authoritative servers | Update instantly — nothing propagates |
| Average wait | About half the old TTL |
| Before a planned change | Lower the TTL, wait one old-TTL period, then change |
| Typical low TTL | 300 seconds |
The maths behind it
This tool uses the longest anyone can see the old answer equals the TTL that was published before the change; on average it is about half that, because caches expire at different moments. Every intermediate value is shown on screen, so the result can be checked by hand.
The single most useful habit in DNS is lowering the TTL before a change rather than after. It costs nothing, takes one extra day of planning, and turns a migration from an unpredictable wait into a five-minute cutover.
Where this tool stops
Assumes resolvers honour the TTL, and most do. Browser and operating-system caches sit on top of that and follow their own rules, which is why a single stubborn machine is rarely a DNS problem at all.
Frequently asked questions
Why does my new low TTL not help?
Because no resolver has seen it yet. A cache that fetched the record yesterday was told to hold it for the old TTL and will not ask again until that expires — and only then does it learn the new, shorter value. The new TTL governs the <i>next</i> change, never this one.
Is "propagation" even the right word?
Not really, and the name causes the confusion. Nothing spreads outward and there is no queue. Authoritative servers change the instant you save; what you are waiting for is caches expiring, independently, all over the world. "Cache expiry" describes it accurately and makes the TTL rule obvious.
How do I make a change take effect quickly?
Plan it a day ahead. Lower the TTL to 300 seconds, wait a full old-TTL period so every cache has picked up the short value, then make the real change — it will be visible in about five minutes. Put the TTL back up afterwards, because a permanently short TTL means every visitor pays for a fresh lookup.
Why does one person still see the old site?
Something in their path is not honouring the TTL. Some ISP resolvers round it up, some corporate ones hold records far longer, and the browser and operating system each cache separately on top. Flushing the OS cache and trying a different network usually settles whether it is them or you.
Does the DNS Propagation Time Calculator cost anything?
No charge and no sign-up. There is no free tier to run out of because there is no paid tier — inputtesthub.com is funded by advertising rather than by metering the page.
Is the DNS Propagation Time Calculator safe to use with private information?
No. The DNS Propagation Time Calculator runs entirely in your browser using JavaScript. Nothing is transmitted, logged or stored, which you can verify yourself in the Network tab of your browser's developer tools.
Can I check the maths myself?
It uses the longest anyone can see the old answer equals the TTL that was published before the change; on average it is about half that, because caches expire at different moments. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What problem does the DNS Propagation Time Calculator solve?
In practice it gets used for three things: telling a client how long a migration will take. Planning a mail or hosting move with no downtime. Explaining why one person still sees the old site. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Where does the DNS Propagation Time Calculator stop being reliable?
Assumes resolvers honour the TTL, and most do. Browser and operating-system caches sit on top of that and follow their own rules, which is why a single stubborn machine is rarely a DNS problem at all. Knowing where a tool stops being trustworthy matters more than the cases where it works.
What are the key numbers for the DNS Propagation Time Calculator?
What decides the wait is The <b>old</b> TTL, published before the change. Authoritative servers is Update instantly — nothing propagates. Average wait is About half the old TTL. These are printed on the page itself, so you do not have to take them on trust.
Can I rely on this for exact work?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.
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