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DNS TTL Calculator

Turn a DNS TTL into plain time, the worst-case propagation window, how many origin lookups it causes per day and the resulting cache hit rate.

Record TTL

Lookups per day

Distinct resolvers / regions

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How it works

origin lookups ≈ min(86400 ÷ TTL, requests ÷ resolvers) × resolvers · cache hit rate = 1 − origin lookups ÷ total lookups · propagation ≤ TTL

A record’s TTL is how long any resolver may cache the answer before asking your authoritative servers again, so it directly sets the worst-case propagation delay: after a change, some users can keep seeing the old value for up to one full TTL. It also governs load — each independent resolver refreshes at most once per TTL, so a longer TTL means fewer origin queries and a higher cache hit rate, but slower propagation. The usual playbook is to lower the TTL well before a planned change (so it propagates fast), then raise it again afterwards to cut load and improve resilience.

Worked example

A 3,600-second (1-hour) TTL means a change can take up to an hour to reach everyone. If 20 resolvers each refresh once an hour, that is 20 × 24 = 480 origin lookups a day. Against 500,000 user lookups, only 480 miss the cache, for a 99.90% hit rate.

Sources: Time to live — DNS — Wikipedia

Frequently asked questions

How long before a change do I lower the TTL?

Lower it at least one old-TTL period ahead. If the current TTL is 24 hours, drop it to a few minutes a full day before the migration so the short value has propagated by the time you make the switch.

Does a longer TTL reduce DNS load?

Yes. With a longer TTL each resolver asks your authoritative servers less often, so origin queries fall and the cache hit rate rises — at the cost of slower propagation when records change.

Why might propagation take longer than the TTL?

Some resolvers ignore low TTLs and enforce a minimum, and stale entries or negative caching can linger. The TTL is the intended ceiling, but real-world propagation sometimes overshoots it.

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This calculator is for educational purposes. Double-check important results before acting on them.