Work out read replica sizing instantly with clear inputs, formula shown and shareable results.
Replicas only help read-heavy workloads, because every write is still applied on every replica — so adding replicas increases total write work rather than reducing it. Sizing starts by subtracting write load from the primary's capacity to find what read capacity remains, then divides the excess across replicas at a safe 70 percent utilisation.
Replica count
primary read capacity = per-instance capacity - write rate; reads to offload = total reads - primary capacity; replicas = ceil(offload / (capacity x 0.7))
No, they make it slightly worse, since each replica must apply the full write stream. Write scaling requires sharding or a different storage engine.
Route reads that must see the just-written value to the primary, or use a session consistency token. Blindly sending all reads to replicas produces confusing missing-data bugs.