DNS TTL Impact Calculator

About the DNS TTL Impact Calculator

DNS TTL (Time to Live) controls how long resolvers cache a DNS response before querying the authoritative server again. Higher TTLs reduce DNS query volume and improve resolution speed from cache. Lower TTLs enable faster propagation of DNS changes but increase query load.

This calculator helps you understand the trade-offs of TTL values for your domains. It estimates cache hit rates, propagation times, and the query volume impact of different TTL settings. This is especially important when planning DNS migrations, failover configurations, or CDN integration.

Before a planned DNS change (migration, failover test), lower the TTL in advance by the current TTL duration. For example, if TTL is 3600 seconds (1 hour), lower it to 300 seconds at least 1 hour before the change. This ensures all cached entries expire before you make the change.

When This Page Helps

DNS TTL values directly affect how quickly your domain changes take effect and how many queries hit your DNS servers. This calculator helps choose the right TTL for different scenarios.

How to Use the Inputs

1. Enter the current DNS TTL value in seconds.
2. Enter the average requests per second to your domain.
3. Enter the number of unique recursive resolvers querying your domain.
4. Review the cache performance and propagation time analysis.

Example Calculation

Tips & Best Practices

• Use 300–600 seconds TTL for records that may need quick changes.
• Use 3600+ seconds TTL for stable records (mail servers, static infrastructure).
• Lower TTL at least 2x current TTL before a planned DNS change.
• After completing a migration, raise TTL back to reduce query load.
• CDN providers often manage their own TTL — check before overriding.
• Monitor authoritative DNS query volume to validate TTL effectiveness.

TTL Strategy for Different Use Cases

Static infrastructure (mail servers, name servers): TTL 3600–86400 seconds. These rarely change and benefit from aggressive caching. Web applications with potential failover: TTL 60–300 seconds. These need to change quickly during incidents.

Pre-Migration TTL Lowering

The pre-migration TTL dance: (1) Lower TTL to 60–300 seconds, (2) Wait for the old TTL to expire, (3) Make the DNS change, (4) Verify propagation, (5) Raise TTL back to normal. Skipping step 2 means some resolvers still have the old record cached at the high TTL.

DNS Query Cost Considerations

Cloud DNS services charge per million queries. At 1,000 resolvers with 60-second TTL, you generate 1,000/60 = 16.7 authoritative queries per second, or ~1.44 million per day. At 3600-second TTL, that drops to 24,000 per day — a 60x reduction.

Frequently Asked Questions

• What is a good default TTL?

  For most records: 300–600 seconds (5–10 minutes) balances change speed with cache efficiency. For stable records: 3600 seconds (1 hour) or more. For records requiring instant failover: 30–60 seconds (high query cost).

• How long does DNS propagation actually take?

  Maximum propagation time equals the previous TTL value. If TTL was 3600 seconds, all resolvers will have updated within 1 hour. In practice, many resolvers update sooner as their cached entries expire at different times.

• Does lowering TTL guarantee instant propagation?

  No. Some resolvers enforce minimum TTLs (e.g., 30 or 60 seconds). ISP resolvers may ignore low TTLs. Client operating systems cache DNS independently. Real-world propagation has a long tail even with low TTLs.

• What is the impact of very low TTL?

  TTL under 60 seconds significantly increases query volume to authoritative servers and can increase latency for initial requests (cache misses). Some cloud DNS providers charge per query, making very low TTLs expensive.

• Should I use TTL 0?

  TTL 0 means no caching, but many resolvers treat it as a minimum of 30–60 seconds. It dramatically increases authoritative query load. Use it only for testing or when instant record changes are critical and you can handle the query volume.

• How does DNS TTL affect failover?

  During a failover, DNS records change to point to backup infrastructure. The failover takes effect as resolver caches expire. With 300-second TTL, full failover takes up to 5 minutes. Health-check-based DNS (Route 53) can trigger changes faster.