How do I pick an average payload size?

Sample 100 messages from a representative client and average the size. Sparkplug B with delta-encoding usually lands at 0.1 to 0.5 KB per message; JSON without compression can be 1 to 5 KB.

What is the billed-network share?

The share of MQTT traffic that crosses a metered link. A plant-local broker on the same LAN is 0 percent billed. A cloud broker over fiber is often 100 percent billed by egress per GB.

Should I include retained messages and Last Will?

Retained messages and LWT are part of the broker count if your broker reports them. Most broker bills count message events, so they will be in the numerator already.

Why include a fixed broker line?

Hosted brokers (AWS IoT Core, Azure IoT Hub, HiveMQ Cloud, EMQX) usually have a tiered subscription on top of message volume. The fixed line keeps that visible separate from variable network spend.

IIoT, SCADA & Edge Connectivity calculator

MQTT Data Cost Calculator

Estimate monthly MQTT data cost. Enter the number of MQTT messages published per month across all edge clients, the average payload size in KB (after compression and Sparkplug encoding if used), the share of traffic that crosses a billed network (cellular link or cloud egress), and a fixed broker subscription or hosting cost. The calculator returns the variable data cost and the loaded total.

What this calculator does

Estimate monthly MQTT or Sparkplug B data spend from messages published per month, average payload size in KB, the share that actually travels over billed network (cellular or cloud egress), and a fixed broker subscription or hosting cost.
Use it when an OT or cloud architect is sizing a cellular or cloud MQTT bill before signing the platform contract, or when a CFO asks for the variable network cost per connected machine.
It returns the monthly MQTT or Sparkplug B data cost, broken into variable network spend and a fixed broker subscription or hosting line.

Formula used

Variable MQTT data cost per month = messages per month × average payload size in KB × share over billed network (the engine treats average payload size as the per-message cost weight)
Total MQTT data cost per month = variable data cost + fixed broker subscription or hosting cost
Note: this calculator weights spend by messages × KB × share. To convert a $/GB cellular or egress rate into the KB-weighted basis used here, divide $/GB by 1,048,576 (KB per GB) and enter that value as average payload size; or use a known $/million-messages rate if your provider bills that way.

Inputs explained

MQTT messages per month: Use the total publish-and-subscribe count across all edge clients per month, from the broker monthly report.
Average payload size: Use the average MQTT payload size after Sparkplug B encoding or JSON compression (typical 0.1 to 2 KB per message).
Share of traffic over billed network: Use the share that crosses a metered link (cellular SIM, satellite, or cloud egress); LAN-to-broker traffic is usually 0 percent billed.
Fixed broker subscription or hosting cost: Use the monthly fixed cost for HiveMQ, EMQX, Mosquitto support, AWS IoT Core, Azure IoT Hub, or your broker host.

How to use the result

Run it before signing a cellular SIM pool contract or a cloud broker agreement, or when the operations team is debating whether to keep the broker on-prem or move it to the cloud.
It is a planning estimate. Real cellular bills include SIM rental, overage, and SMS heartbeat fees; pair with the cellular invoice for true cost.

Common questions

How do I pick an average payload size? Sample 100 messages from a representative client and average the size. Sparkplug B with delta-encoding usually lands at 0.1 to 0.5 KB per message; JSON without compression can be 1 to 5 KB.
What is the billed-network share? The share of MQTT traffic that crosses a metered link. A plant-local broker on the same LAN is 0 percent billed. A cloud broker over fiber is often 100 percent billed by egress per GB.
Should I include retained messages and Last Will? Retained messages and LWT are part of the broker count if your broker reports them. Most broker bills count message events, so they will be in the numerator already.
Why include a fixed broker line? Hosted brokers (AWS IoT Core, Azure IoT Hub, HiveMQ Cloud, EMQX) usually have a tiered subscription on top of message volume. The fixed line keeps that visible separate from variable network spend.

Last reviewed 2026-05-12.