Size Aurora read replicas for a target query rate and utilisation, check the 15-replica limit and price the reader fleet.
Aurora replicas read the same shared storage volume as the writer, so read scaling is close to linear and the sizing is a capacity division rather than a replication-throughput problem. The spare matters because a fleet sized at exactly 100% of capacity is one failover away from overload, and lag is estimated from the writer's change rate since replicas apply redo from storage rather than replaying a binlog. The replica hourly rate is an editable input with a realistic us-east-1 default, so confirm the current price with AWS for your region and instance family. Aurora replica lag is typically well under a second, which makes the reader endpoint safe for most read traffic — but only if the fleet has enough headroom that one reader disappearing does not cascade.
Aurora Replica
replicas = ceil(read QPS ÷ (per-instance QPS × target utilisation)) + failover spare, capped at the 15-replica cluster limit.
replicas = ceil(read QPS ÷ (per-instance QPS × target utilisation)) + failover spare, capped at the 15-replica cluster limit. Aurora replicas read the same shared storage volume as the writer, so read scaling is close to linear and the sizing is a capacity division rather than a replication-throughput problem. The spare matters because a fleet sized at exactly 100% of capacity is one failover away from overload, and lag is estimated from the writer's change rate since replicas apply redo from storage rather than replaying a binlog. The replica hourly rate is an editable input with a realistic us-east-1 default, so confirm the current price with AWS for your region and instance family.
Aurora replica lag is typically well under a second, which makes the reader endpoint safe for most read traffic — but only if the fleet has enough headroom that one reader disappearing does not cascade.
This calculator takes 7 inputs: Peak read queries, Read capacity of one instance, Target utilisation per replica, Keep one spare replica for failover, Replica instance price, Billed hours per month, Writer change rate. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
RDS replicas replay the binary log, which is a single-threaded logical operation on the replica. Aurora replicas read the same storage volume as the writer and only apply redo to their buffer cache, so lag is usually tens of milliseconds rather than seconds.