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DNS Failover Calculator

Estimate Route 53 failover time from health check interval, failure threshold, record TTL and resolver propagation.

About DNS Failover Calculator

Route 53 marks an endpoint unhealthy only after the failure threshold is met, so detection takes interval multiplied by threshold — 90 seconds with the standard 30-second interval and a threshold of three. Recovery then waits for caches to expire, which is the record TTL plus an allowance for resolvers and clients that hold entries a little longer, and the two phases add up to the real user-visible outage. DNS failover cannot beat the TTL, so a service with a 300-second TTL has a five-minute floor on recovery no matter how aggressive the health checks are — which is why load balancer or Global Accelerator failover is used when seconds matter.

Formula

DNS Failover

failover time = interval × failure threshold + record TTL + propagation allowance.

interval
Seconds between Route 53 health check requests
failure threshold
Consecutive failures before the endpoint is marked unhealthy
TTL
Seconds resolvers may cache the record before asking again

Worked example

With the default values below, DNS Failover Calculator returns Estimated Failover Time of 180. Change any input above and every figure here is recalculated in your browser.

Inputs used

Health check request intervalone of Standard — every 30 seconds, Fast — every 10 seconds
Standard — every 30 seconds
Failure thresholdaccepts 1–10 checks
3 checks
Record TTLaccepts 0–172,800 seconds
60 seconds
Resolver and client propagation allowanceaccepts 0–600 seconds
30 seconds
Recovery time objectiveaccepts 1–3,600 seconds
120 seconds
Expected failover events per yearaccepts 0–365 events
4 events

Results

Estimated Failover Time
180 seconds
Fault Detection Time
90 seconds
DNS Convergence Time
90 seconds
Annual Downtime from Failovers
12.0 minutes
Availability Lost to Failovers
0.0023%
Recovery Objective Check
Misses the objective — shorten the TTL or switch to fast health checks

Step by step

  1. 1Values used
  2. 2DNS Failover
  3. 3Estimated Failover Time= 180 seconds
  4. 4Fault Detection Time= 90 seconds
  5. 5DNS Convergence Time= 90 seconds
  6. 6Annual Downtime from Failovers= 12.0 minutes
  7. 7Availability Lost to Failovers= 0.0023
  8. 8Recovery Objective Check= Misses the objective — shorten the TTL or switch to fast health checks

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 or fees or charges into account.
  • Figures are rounded for display to 4 decimal places, 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 Failover calculated?

failover time = interval × failure threshold + record TTL + propagation allowance. Route 53 marks an endpoint unhealthy only after the failure threshold is met, so detection takes interval multiplied by threshold — 90 seconds with the standard 30-second interval and a threshold of three. Recovery then waits for caches to expire, which is the record TTL plus an allowance for resolvers and clients that hold entries a little longer, and the two phases add up to the real user-visible outage.

Why does DNS Failover matter?

DNS failover cannot beat the TTL, so a service with a 300-second TTL has a five-minute floor on recovery no matter how aggressive the health checks are — which is why load balancer or Global Accelerator failover is used when seconds matter.

What values do I need to enter?

This calculator takes 6 inputs: Health check request interval, Failure threshold, Record TTL, Resolver and client propagation allowance, Recovery time objective, Expected failover events per year. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Why not set the TTL to 1 second?

Query volume, and therefore cost, scales inversely with TTL, and some resolvers enforce a floor regardless of what you publish. A TTL of 60 seconds is the common compromise; below that you pay for queries you cannot rely on being honoured.

How do fast health checks change the maths?

A 10-second interval with a threshold of three detects a fault in 30 seconds instead of 90, at a higher monthly price per health check. Confirm the current fast health check rate with AWS, since the premium can be significant across many endpoints.

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DNS Failover Calculator – Calculate Estimated Failover Time