DNS Propagation Time Calculator
Work out the worst-case time a DNS change takes to reach every resolver, including the TTL you lowered beforehand.
About DNS Propagation Time Calculator
Lowering a TTL does nothing until the previously published TTL has expired in every cache, so the change window is driven by the old value unless you lowered it far enough in advance. Once the short TTL is live, the worst case is that TTL plus refresh skew plus the time the edit takes to reach all authoritative servers. Cutovers fail when the change window is planned from the new TTL instead of the one still cached, leaving a fraction of users pointed at a decommissioned endpoint. Keeping the old target alive for the straggler tail is what makes a DNS migration safe.
Formula
DNS Propagation Time
worst case = effective TTL + resolver refresh skew + zone sync time, where the effective TTL is the lowered value only once the previously published TTL has expired everywhere.
- effective TTL
- The TTL actually sitting in resolver caches right now
- zone sync
- Time for the edit to reach every authoritative server
- straggler tail
- Extra time for resolvers that enforce their own minimum TTL
Worked example
With the default values below, DNS Propagation Time Calculator returns Worst-Case Propagation of 420. Change any input above and every figure here is recalculated in your browser.
Inputs used
- TTL currently publishedaccepts 0–604,800 seconds
- 3,600 seconds
- TTL you lowered it toaccepts 0–604,800 seconds
- 300 seconds
- Lead time since lowering the TTLaccepts 0–720 hours
- 2 hours
- Zone sync to all authoritative serversaccepts 0–86,400 seconds
- 60 seconds
- Resolver refresh skewaccepts 0–86,400 seconds
- 60 seconds
- Resolvers that stretch short TTLsaccepts 0–100 %
- 5 %
Results
- Worst-Case Propagation
- 420 seconds
- Worst Case as a Duration
- 7m
- Typical Propagation
- 210 seconds
- TTL Actually in Caches Now
- 300 seconds
- Tail for Resolvers That Stretch TTLs
- 1,140 seconds
- Cutover Window to Plan
- 19.0 minutes
- Pre-Change Advice
- The lowered TTL is already in caches — the change can proceed
Step by step
- 1Values used
- 2DNS Propagation Time
- 3Worst-Case Propagation= 420 seconds
- 4Worst Case as a Duration= 7
- 5Typical Propagation= 210 seconds
- 6TTL Actually in Caches Now= 300 seconds
- 7Tail for Resolvers That Stretch TTLs= 1,140 seconds
- 8Cutover Window to Plan= 19.0 minutes
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is DNS Propagation Time calculated?
worst case = effective TTL + resolver refresh skew + zone sync time, where the effective TTL is the lowered value only once the previously published TTL has expired everywhere. Lowering a TTL does nothing until the previously published TTL has expired in every cache, so the change window is driven by the old value unless you lowered it far enough in advance. Once the short TTL is live, the worst case is that TTL plus refresh skew plus the time the edit takes to reach all authoritative servers.
Why does DNS Propagation Time matter?
Cutovers fail when the change window is planned from the new TTL instead of the one still cached, leaving a fraction of users pointed at a decommissioned endpoint. Keeping the old target alive for the straggler tail is what makes a DNS migration safe.
What values do I need to enter?
This calculator takes 6 inputs: TTL currently published, TTL you lowered it to, Lead time since lowering the TTL, Zone sync to all authoritative servers, Resolver refresh skew, Resolvers that stretch short TTLs. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.