Work out mqtt message rate instantly with clear inputs, formula shown and shareable results.
Broker load is devices times message rate. Bandwidth must include MQTT fixed and variable header overhead (roughly 20-40 bytes) and the acknowledgement traffic of QoS 1 or 2, which adds 50-100% to the byte count for small payloads.
MQTT load
messages/s = devices x messages per hour / 3600; bandwidth = messages/s x (payload + header) x QoS overhead
A PUBACK is a whole extra packet. When the payload is only 180 bytes, acknowledgement traffic is a large fraction of the total.
Concurrent connections and per-connection state more often than raw message rate. Keepalive traffic from many idle devices can dominate.