When to use: Use when sizing a BACnet /IP network segment. Estimate the polling traffic from a controller or supervisor reading points across many devices. Each poll is a ReadProperty request + response (2 packets), and every BACnet/IP packet carries roughly 50 bytes of BVLC/UDP/IP/Ethernet overhead on top of the APDU. The tool reports bandwidth, link utilisation on 100 Mbps and 10 Mbps links, and the smallest IPv4 subnet that fits your device count plus the BBMD/router.
This tool calculates the network bandwidth consumed by BACnet/IP polling traffic on an Ethernet segment, helping engineers verify that their control network can handle the message load without saturating the link. It is used when designing or auditing BACnet/IP networks per ASHRAE 135 Annex J.
BACnet/IP uses UDP port 47808 and wraps every application message in BVLC (4 bytes), UDP (8 bytes), IPv4 (20 bytes), and Ethernet (18 bytes) headers — approximately 50 bytes of overhead per packet. A ReadProperty poll cycle produces two packets: a request from the supervisor and a response from the device. Total bandwidth is therefore: Bandwidth (bps) = Devices × PollRate × 2 × (APDU + 50 bytes) × 8.
For a typical 100-device network polled at 0.2 Hz with 50-byte APDUs, this yields roughly 100 kbps — well under a 10 Mbps link. COV (Change of Value) subscriptions can dramatically reduce polling overhead by pushing updates only when a point changes, cutting bandwidth by 60–80% in stable conditions.
ASHRAE 135 (BACnet) Annex J defines the BACnet/IP binding, including the BVLL (BACnet Virtual Link Layer) framing and UDP encapsulation. The standard recommends keeping network utilization below 30–40% of link capacity to allow headroom for alarm bursts and COV floods. IPv4 subnet sizing should follow IETF RFC 1918 private address space conventions, with a /24 or /25 subnet common for building control networks. BACnet Broadcast Management Devices (BBMDs) are required to relay broadcast messages across IP subnets.
Keep link utilization below 30% under normal polling conditions; alarm storms and COV floods can transiently multiply traffic by 3–5x. A dedicated VLAN for BACnet/IP control traffic isolates it from IT traffic and prevents building automation traffic from being disrupted by large file transfers. Size the subnet with headroom: add at least 2 addresses for the BBMD/router and router interface, and prefer a /26 or larger to avoid fragmentation. Polling rate should be set to match the control update rate required by the sequence — 0.2 Hz (every 5 s) is typical for supervisory polling, while PID loop inputs may require 1 Hz.
Enter the number of BACnet/IP devices on the segment, the polling rate in polls per device per second (0.2 = every 5 seconds), and the average APDU payload size (default 50 bytes for ReadProperty). The calculator reports bandwidth in kbps and Mbps, link utilization on 10 Mbps and 100 Mbps links, and the smallest IPv4 subnet that fits your device count plus the BBMD/router. If utilization exceeds 30%, reduce polling rate, switch to COV subscriptions, or segment the network.
ASHRAE 135 assigns UDP port 47808 (0xBAC0) as the well-known BACnet/IP port. All BACnet/IP devices listen on this port for incoming messages.
A ReadProperty response APDU is typically 26–50 bytes depending on the data type. Analog values (floating point) take about 26 bytes; string properties can be longer. The tool defaults to 50 bytes, which is a conservative average.
Use COV subscriptions when most points are stable most of the time. A point that changes infrequently wastes bandwidth being polled at 0.2 Hz; with COV it only sends a message when the value changes by more than the COV increment. COV is ideal for setpoints, alarms, and binary status points.
Yes — BACnet broadcasts cannot traverse IP routers without a BBMD (BACnet Broadcast Management Device) on each subnet to relay broadcast messages. Each subnet must have at least one BBMD, and BBMDs on different subnets must be configured with each other's addresses in their broadcast distribution table (BDT).
A /26 (62 usable hosts) is typical for small panels, while a /25 (126 hosts) or /24 (254 hosts) suits larger floors or buildings. Reserve addresses for the BBMD, router interfaces, and future expansion. Avoid /28 or smaller as they leave little room for growth.
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