DNS Failover Calculator

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.

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.

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.

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.

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.